INSTALLATION

E 2010 Lennox Industries Inc.

INSTRUCTIONS
Dallas, Texas, USA

G51MP SERIES UNITS

GAS UNITS
506508−01
07/2010
Supersedes 505,065M
Litho U.S.A.

Table of Contents
Unit Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
G51MP Parts Identification . . . . . . . . . . . . . . . . . . . . . . 3
Shipping and Packing List . . . . . . . . . . . . . . . . . . . . . . . 4
Safety Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Use of Furnace as a Construction Heater . . . . . . . . . . 5
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Combustion, Dilution & Ventilation Air . . . . . . . . . . . . . 6
Installation − Setting Equipment . . . . . . . . . . . . . . . . . . 9
Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Duct System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Pipe & Fittings Specifications . . . . . . . . . . . . . . . . . . . 16
Vent Piping Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . 18
Joint Cementing Procedure . . . . . . . . . . . . . . . . . . . . . 19
Venting Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Gas Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Electrical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Unit Start−Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Gas Pressure Adjustment . . . . . . . . . . . . . . . . . . . . . . 41
High Altitude Information . . . . . . . . . . . . . . . . . . . . . . . 43
Other Unit Adjustments . . . . . . . . . . . . . . . . . . . . . . . . 45
Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Planned Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Repair Parts List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
THIS MANUAL MUST BE LEFT WITH THE Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
HOMEOWNER FOR FUTURE REFERENCE Ignition Control Board Diagnostic Codes . . . . . . . . . 54
Vent Pipe Sizing Worksheet . . . . . . . . . . . . . . . . . . . . 55
Start−Up & Performance Check List . . . . . . . . . . . . . . 55

WARNING
FIRE OR EXPLOSION HAZARD.
Failure to follow safety warnings exact-
ly could result in serious injury, death,
or property damage.

Do not store or use gasoline or other

WHAT TO DO IF YOU SMELL GAS:
flammable vapors and liquids in the D Do not try to light any appliance.
vicinity of this or any other ap- D Do not touch any electrical switch; do not
pliance. use any phone in your building.
D Leave the building immediately.
Installation and service must be D Immediately call your gas supplier from a
performed by a qualified installer, neighbor’s phone. Follow the gas supplier’s
service agency or the gas supplier. instructions.
D If you cannot reach your gas supplier, call
the fire department.

07/10 506508−01
*2P0710* Page 1 *P506508-01*
G51MP Unit Dimensions − inches (mm)

*NOTE − 60C and 60D size units installed in upflow applica-

tions that require air volumes over 1800 cfm (850 L/s) must
have one of the following:
1. Single side return air with transition, to accommodate
SUPPLY AIR
20 x 25 x 1 in. (508 x 635 x 25 mm) air filter. OPENING
Required to maintain proper air velocity.
2. Single side return air with optional RAB Return Air Base
3. Bottom return air.
4. Return air from both sides.
5. Bottom and one side return air.
Refer to Engineering Handbook for additional information.
1
OPTIONAL
**Consider sizing requirements for optional IAQ equipment 23-3/4 (603)
before cutting side return opening. EXTERNAL
SIDE RETURN
AIR FILTER KIT 25 (635)
1 Optional External Side Return Air Filter Kit is not for use (Either Side) TOP VIEW
with the optional RAB Return Air Base.
28−1/2
A (724)
9/16
B 9/16 (14) 19−7/16 (14)
5−1/2 6−1/2 (494)
(140) (165)

COMBUSTION AIR INTAKE

6−1/2 (Either Side)
11−5/8 (295) (165) 3−1/8
(79)
Right EXHAUST AIR OUTLET
9−3/4 (248) 4−1/8 6−3/4 (Either Side)
1 OPTIONAL
Left (103) (171) GAS PIPING INLET
EXTERNAL (Either Side)
SIDE RETURN
AIR FILTER KIT CONDENSATE
(Either Side) 4−7/8 (124) Right TRAP CONNECTION
(Either Side)
40 2−1/4 (57) Left
(1016) ELECTRICAL INLET
(Either Side)
2−1/2
(64)

14−3/4 4 23
(375) (102) (584)
18-3/4 (476) 14**
16 *OPTIONAL
(406) RETURN CUTOUT (356)
(Either Side)
AIR FLOW
1−15/16 (49)
5/8 (16)
23−1/2
C 3/4 (19) (597)
3/4 (19) *Bottom Return 4−1/4 *Bottom Return
Air Opening (108) Air Opening
FRONT VIEW SIDE VIEW

A B C
Model No.
in. mm in. mm in. mm
G51MP−24B−045
G51MP−36B−045 17−1/2 446 16−3/8 416 16 406
G51MP−36B−070
G51MP−36C−090
G51MP−48C−090
G51MP−60C−090 21 533 19−7/8 504 19−1/2 495
G51MP−48C−110
G51MP−60C−110
G51MP−60D−135 24−1/2 622 23−3/8 546 23 584

Page 2
G51MP Parts Arrangement

G51MP PARTS IDENTIFICATION

TOP CAP

DuralokPlusTM
HEAT EXCHANGER
ASSEMBLY

CABINET

BURNER BOX
ASSEMBLY

GAS VALVE
AND MANIFOLD

FLUE
COLLAR

COMBUSTION
AIR PRESSURE WARM HEADER
PROVE SWITCHES (COLLECTOR)
BOX
COMBUSTION AIR
INDUCER
CONDENSER COIL

BURNER
ACCESS PRIMARY LIMIT
PANEL

COLD HEADER
(COLLECTOR)
BOX

BLOWER
ACCESS DOOR
DOOR INTERLOCK
SWITCH
SECONDARY
LIMITS (2)

SIGHT BLOWER
GLASS ASSEMBLY

CONTROL BOX
FIGURE 1

Page 3
G51MP Gas Furnace NOTE − G51MP−48C−110 and G51MP−60C−110 units also
include a 2" diameter PVC street elbow, which is shipped
The G51MP Category IV gas furnace is shipped ready for on the blower deck in the heating compartment.
installation in the upflow, downflow, horizontal left air dis- G51MP−60D−135 units are shipped with a 3" to 2" PVC re-
charge or horizontal right air discharge position. The fur- ducing elbow.
nace is shipped with the bottom panel in place. The bot- Canadian Installations Only − Replace the provided 2"
tom panel must be removed if the unit is to be installed in street elbow shipped with the G51MP−48C−110 and
upflow applications with bottom return air. The bottom G51MP−60C−110 units with 2" street elbow Y0162. Re-
panel must also be removed and discarded in all downflow place the provided 3" X 2" reducing elbow shipped with the
or horizontal applications. G51MP−60D−135 units with 3" X 2" reducing elbow 40W22.
Y0162 and 40W22 meet B149 vent requirements.
The furnace is equipped for installation in natural gas ap-
plications. A conversion kit (ordered separately) is re- Check equipment for shipping damage. If you find any
quired for use in propane/LP gas applications. damage, immediately contact the last carrier.
The G51MP can be installed as either a Direct Vent or a The following items may also be ordered separately:
Non−Direct Vent gas central furnace. 1 − Thermostat
NOTE − In Direct Vent installations, combustion air is taken 1 − Propane/LP changeover kit
from outdoors and flue gases are discharged outdoors. In
Non−Direct Vent installations, combustion air is taken from
indoors and flue gases are discharged outdoors. See figure
DANGER
2 for applications involving roof termination. Danger of explosion.
There are circumstances in which odorant used with
DIRECT VENT INSTALLATION NON−DIRECT VENT
COMBUSTION INSTALLATION LP/propane gas can lose its scent. In case of a leak,
EXHAUST
AIR INTAKE
OUTLET EXHAUST
LP/propane gas will settle close to the floor and may
OUTSIDE OF be difficult to smell. An LP/propane leak detector
OUTLET
HOUSE
should be installed in all LP applications.

COMBUSTION Safety Information

AIR INTAKE INSIDE
OF HOUSE

WARNING
Improper installation, adjustment, alteration, service
or maintenance can cause property damage, person-
al injury or loss of life. Installation and service must
G51MP G51MP
be performed by a licensed professional installer (or
CONDENSATE
DRAIN equivalent), service agency or the gas supplier.
CONDENSATE
DRAIN

FIGURE 2 CAUTION
Shipping and Packing List As with any mechanical equipment, personal injury
can result from contact with sharp sheet metal
Package 1 of 1 contains edges. Be careful when you handle this equipment.
1 − Assembled G51MP unit
Use only the type of gas approved for use with this furnace.
1 − Bag assembly containing the following: Refer to unit nameplate.
3 − Screws G51MP units are CSA International certified to ANSI
3 − Wire nuts Z21.47 and CSA 2.3 standards.
1 − Snap bushing In the USA, installation of gas furnaces must conform with lo-
1 − Snap plug cal building codes. In the absence of local codes, units must
1 − Wire tie be installed according to the current National Fuel Gas Code
1 − Condensate trap (ANSI-Z223.1/NFPA 54). The National Fuel Gas Code is
1 − Condensate trap cap available from the following address:
2 − 2" diameter vent / intake plugs American National Standards Institute, Inc.
1 − 3" diameter cabinet plug (intake) 11 West 42nd Street
1 − 2" diameter debris screen New York, NY 10036

Page 4
In Canada, installation must conform with current National When installed, this furnace must be electrically grounded
Standard of Canada CSA-B149 Natural Gas and Propane according to local codes. In addition, in the United States,
Installation Codes, local plumbing or waste water codes installation must conform with the current National Electric
and other applicable local codes. Code, ANSI/NFPA No. 70. The National Electric Code
In order to ensure proper unit operation in non−direct vent (ANSI/NFPA No. 70) is available from the following ad-
applications, combustion and ventilation air supply must be dress:
provided according to the current National Fuel Gas Code National Fire Protection Association
or CSA-B149 standard. 1 Battery March Park
This furnace is CSA International certified for installation Quincy, MA 02269
clearances to combustible material as listed on the unit In Canada, all electrical wiring and grounding for the unit
nameplate and in the tables in figures 8, 13 and 17. Accessi- must be installed according to the current regulations of the
bility and service clearances must take precedence over fire Canadian Electrical Code Part I (CSA Standard C22.1)
protection clearances. and/or local codes.
NOTE − For installation on combustible floors, the furnace NOTE − This furnace is designed for a minimum continu-
shall not be installed directly on carpeting, tile, or other ous return air temperature of 60°F (16°C) or an intermit-
combustible material other than wood flooring. tent operation down to 55°F (13°C) dry bulb for cases
where a night setback thermostat is used. Return air tem-
For installation in a residential garage, the furnace must
perature must not exceed 85°F (29°C) dry bulb.
be installed so that the burner(s) and the ignition source
The G51MP furnace may be installed in alcoves, closets,
are located no less than 18 inches (457 mm) above the
attics, basements, garages, and utility rooms.
floor. The furnace must be located or protected to avoid
physical damage by vehicles. When a furnace is installed This furnace design has not been CSA certified for installa-
in a public garage, hangar, or other building that has a haz- tion in mobile homes, recreational vehicles, or outdoors.
ardous atmosphere, the furnace must be installed accord- Never use an open flame to test for gas leaks. Check all
ing to recommended good practice requirements and cur- connections using a commercially available soap solution
rent National Fuel Gas Code or CSA B149 standards. made specifically for leak detection.
NOTE − Furnace must be adjusted to obtain a temperature Use of Furnace as Construction Heater
rise within the range specified on the unit nameplate. Failure
to do so may cause erratic limit operation and premature heat Lennox does not recommend the use of G51MP units as a
exchanger failure. construction heater during any phase of construction. Very
This G51MP furnace may be used as a high−static unit low return air temperatures, harmful vapors and operation
heater. The G51MP may also be installed in an aircraft han- of the unit with clogged or misplaced filters will damage the
gar in accordance with the Standard for Aircraft Hangars unit.
(ANSI/NFPA No. 408−1990). G51MP units may be used for heating of buildings or struc-
tures under construction, if the following conditions are
Installation in parking structures must be in accordance
met:
with the Standard for Parking Structures (ANSI/NFPA No.
88A−1991). Installation in repair garages must be in accor- D The vent system must be permanently installed per
dance with the Standard for Repair Garages (ANSI/NFPA these installation instructions.
No. 88B−1991). D A room thermostat must control the furnace. The use of
fixed jumpers that will provide continuous heating is not
This G51MP furnace must be installed so that its electrical allowed.
components are protected from water.
D The return air duct must be provided and sealed to the
When this furnace is used with cooling units, it shall be furnace.
installed in parallel with, or on the upstream side of, cooling D Return air temperature range between 60°F (16°C) and
units to avoid condensation in the heating compartment. 80°F (27°C) must be maintained.
With a parallel flow arrangement, a damper (or other means D Air filters must be installed in the system and must be
to control the flow of air) must adequately prevent chilled air maintained during construction.
from entering the furnace. If the damper is manually oper- D Air filters must be replaced upon construction comple-
ated, it must be equipped to prevent operation of either the tion.
heating or the cooling unit, unless it is in the full HEAT or D The input rate and temperature rise must be set per the
COOL setting. furnace rating plate.

Page 5
 D One hundred percent (100%) outdoor air must be pro-
vided for combustion air requirements during construc-
tion. Temporary ducting may supply outdoor air to the
CAUTION
furnace. Do not connect duct directly to the furnace. G51MP unit should not be installed in areas normally
subject to freezing temperatures.
Size the temporary duct following these instructions in
section for Combustion, Dilution and Ventilation Air in a
confined space with air from outside. Combustion, Dilution & Ventilation Air
D The furnace heat exchanger, components, duct system, If the G51MP is installed as a Non−Direct Vent Fur-
air filters and evaporator coils must be thoroughly nace, follow the guidelines in this section.
cleaned following final construction clean−up.
NOTE − In Non−Direct Vent installations, combustion air
D All furnace operating conditions (including ignition, in-
is taken from indoors and flue gases are discharged out−
put rate, temperature rise and venting) must be verified
according to these installation instructions. doors.

General
WARNING
These instructions are intended as a general guide and do
Insufficient combustion air can cause headaches,
not supersede local codes in any way. Consult authorities
nausea, dizziness or asphyxiation. It will also cause
having jurisdiction before installation. excess water in the heat exchanger resulting in rust-
In addition to the requirements outlined previously, the fol- ing and premature heat exchanger failure. Excessive
lowing general recommendations must be considered exposure to contaminated combustion air will result
when installing a G51MP furnace: in safety and performance related problems. Avoid
exposure to the following substances in the com-
• Place the furnace as close to the center of the air dis- bustion air supply:
tribution system as possible. The furnace should also be Permanent wave solutions
located close to the chimney or vent termination point. Chlorinated waxes and cleaners
• When the furnace is installed in non−direct vent applica- Chlorine base swimming pool chemicals
tions, do not install the furnace where drafts might blow Water softening chemicals
directly into it. This could cause improper combustion De−icing salts or chemicals
and unsafe operation. Carbon tetrachloride
• When the furnace is installed in non−direct vent applica- Halogen type refrigerants
Cleaning solvents (such as perchloroethylene)
tions, do not block the furnace combustion air opening
Printing inks, paint removers, varnishes, etc.
with clothing, boxes, doors, etc. Air is needed for proper Hydrochloric acid
combustion and safe unit operation. Cements and glues
• When the furnace is installed in an attic or other insu- Antistatic fabric softeners for clothes dryers
lated space, keep insulation away from the furnace. Masonry acid washing materials
• When the furnace is installed in an unconditioned
In the past, there was no problem in bringing in sufficient
space, consider provisions required to prevent freezing
outdoor air for combustion. Infiltration provided all the air
of condensate drain system.
that was needed. In today’s homes, tight construction
practices make it necessary to bring in air from outside
WARNING for combustion. Take into account that exhaust fans, ap-
Product contains fiberglass wool. pliance vents, chimneys, and fireplaces force additional
Disturbing the insulation in this product during air that could be used for combustion out of the house.
installation, maintenance, or repair will expose you Unless outside air is brought into the house for combus-
to fiberglass wool. Breathing this may cause lung tion, negative pressure (outside pressure is greater than
cancer. (Fiberglass wool is known to the State of Cal- inside pressure) will build to the point that a downdraft
ifornia to cause cancer.)
can occur in the furnace vent pipe or chimney. As a result,
Fiberglass wool may also cause respiratory, skin,
combustion gases enter the living space creating a po-
and eye irritation.
tentially dangerous situation.
To reduce exposure to this substance or for further
information, consult material safety data sheets
available from address shown below, or contact your
supervisor.
Lennox Industries Inc.
P.O. Box 799900
Dallas, TX 75379−9900

Page 6
In the absence of local codes concerning air for combus− construction with weather stripping and caulking around
tion and ventilation, use the guidelines and procedures in the windows and doors, follow the procedures in the air
this section to install G51MP furnaces to ensure efficient from outside section.
and safe operation. You must consider combustion air Confined Space
needs and requirements for exhaust vents and gas pip- A confined space is an area with a volume less than 50 cubic
ing. A portion of this information has been reprinted with feet (1.42 m3) per 1,000 Btu (.29 kW) per hour of the com−
permission from the National Fuel Gas Code (ANSI− bined input rating of all appliances installed in that space. This
Z223.1/NFPA 54). This reprinted material is not the com- definition includes furnace closets or small equipment rooms.
plete and official position of the ANSI on the referenced
When the furnace is installed so that supply ducts carry air
subject, which is represented only by the standard in its
circulated by the furnace to areas outside the space con-
entirety.
taining the furnace, the return air must be handled by ducts
In Canada, refer to the CSA B149 installation codes. which are sealed to the furnace casing and which terminate
outside the space containing the furnace. This is especially
CAUTION important when the furnace is mounted on a platform in a
confined space such as a closet or small equipment room.
Do not install the furnace in a corrosive or contami- Even a small leak around the base of the unit at the platform
nated atmosphere. Meet all combustion and ventila- or at the return air duct connection can cause a potentially
tion air requirements, as well as all local codes.
dangerous negative pressure condition. Air for combustion
and ventilation can be brought into the confined space ei-
All gas-fired appliances require air for the combustion pro- ther from inside the building or from outside.
cess. If sufficient combustion air is not available, the fur-
Air from Inside
nace or other appliance will operate inefficiently and un-
safely. Enough air must be provided to meet the needs of all If the confined space that houses the furnace adjoins a
fuel−burning appliances and appliances such as exhaust space categorized as unconfined, air can be brought in by
fans which force air out of the house. When fireplaces, ex- providing two permanent openings between the two
haust fans, or clothes dryers are used at the same time as spaces. Each opening must have a minimum free area of 1
the furnace, much more air is required to ensure proper square inch (645 mm2) per 1,000 Btu (.29 kW) per hour of
combustion and to prevent a downdraft. Insufficient air total input rating of all gas−fired equipment in the confined
causes incomplete combustion which can result in carbon space. Each opening must be at least 100 square inches
monoxide. (64516 mm2). One opening shall be within 12 inches (305
mm) of the top of the enclosure and one opening within 12
In addition to providing combustion air, fresh outdoor air di- inches (305 mm) of the bottom. See figure 3.
lutes contaminants in the indoor air. These contaminants
EQUIPMENT IN CONFINED SPACE − ALL AIR FROM INSIDE
may include bleaches, adhesives, detergents, solvents
and other contaminants which can corrode furnace compo-
ROOF TERMINATED
nents. EXHAUST PIPE

The requirements for providing air for combustion and ven-

tilation depend largely on whether the furnace is installed in
an unconfined or a confined space.
Unconfined Space
OPENINGS
(To Adjacent
An unconfined space is an area such as a basement or SIDE WALL
TERMINATED EX-
Unconfined
Space)
large equipment room with a volume greater than 50 cubic HAUST PIPE (ALTER-
NATE LOCATION) G51MP
feet (1.42 m3) per 1,000 Btu (.29 kW) per hour of the com-
bined input rating of all appliances installed in that space.
This space also includes adjacent rooms which are not
NOTE − Each opening shall have a free area of at least one square
separated by a door. Though an area may appear to be un- inch per 1,000 Btu (645mm 2 per .29kW) per hour of the total input
confined, it might be necessary to bring in outdoor air for rating of all equipment in the enclosure, but not less than 100 square
inches (64516mm. 2).
combustion if the structure does not provide enough air by
infiltration. If the furnace is located in a building of tight FIGURE 3

Page 7
Air from Outside When ducts are used, they shall be of the same cross−sec-
If air from outside is brought in for combustion and ventila- tional area as the free area of the openings to which they
tion, the confined space shall be provided with two connect. The minimum dimension of rectangular air ducts
permanent openings. One opening shall be within 12" shall be no less than 3 inches (75 mm). In calculating free
(305mm) of the top of the enclosure and one within 12" area, the blocking effect of louvers, grilles, or screens
(305mm) of the bottom. These openings must communi- must be considered. If the design and free area of protec-
cate directly or by ducts with the outdoors or spaces (crawl tive covering is not known for calculating the size opening
or attic) that freely communicate with the outdoors or indi- required, it may be assumed that wood louvers will have
rectly through vertical ducts. Each opening shall have a 20 to 25 percent free area and metal louvers and grilles
minimum free area of 1 square inch per 4,000 Btu will have 60 to 75 percent free area. Louvers and grilles
(645mm2 per 1.17kW) per hour of total input rating of all must be fixed in the open position or interlocked with the
equipment in the enclosure. When communicating with equipment so that they are opened automatically during
the outdoors through horizontal ducts, each opening shall equipment operation.
have a minimum free area of 1 square inch per 2,000 Btu
EQUIPMENT IN CONFINED SPACE − ALL AIR FROM OUTSIDE
(645mm2 per .59kW) per total input rating of all equipment (All Air Through Ventilated Attic)
in the enclosure (See figure 4).

EQUIPMENT IN CONFINED SPACE − ALL AIR FROM OUTSIDE ROOF TERMINATED VENTILATION LOUVERS
(Inlet Air from Crawl Space and Outlet Air to Ventilated Attic) EXHAUST PIPE (Each end of attic)

OUTLET
VENTILATION LOUVERS AIR
ROOF TERMINATED (Each end of attic)
EXHAUST PIPE

SIDE WALL INLET AIR

(Ends 12" above
TERMINATED bottom)
OUTLET EXHAUST PIPE
AIR
(ALTERNATE
G51MP
LOCATION)

SIDE WALL
TERMINATED NOTE−The inlet and outlet air openings shall each have a free area of
VENTILATION
EXHAUST PIPE
LOUVERS at least one square inch per 4,000 Btu (645mm 2 per 1.17kW) per hour
(ALTERNATE G51MP
INLET (For unheated of the total input rating of all equipment in the enclosure.
LOCATION) AIR crawl space)

FIGURE 5
EQUIPMENT IN CONFINED SPACE −
ALL AIR FROM OUTSIDE
NOTE−The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645mm 2 per 1.17kW) per
hour of the total input rating of all equipment in the enclosure. ROOF TERMINATED
EXHAUST PIPE

FIGURE 4

If air from outside is brought in for combustion and ventila- OUTLET AIR

tion, the confined space must have two permanent open-

ings. One opening shall be within 12 inches (305 mm) of
the top of the enclosure and one opening within 12 inches SIDE WALL
TERMINATED
(305 mm) of the bottom. These openings must communi- EXHAUST PIPE
G51MP
cate directly or by ducts with the outdoors or spaces (crawl (ALTERNATE
LOCATION)
INLET AIR

or attic) that freely communicate with the outdoors or indi-

rectly through vertical ducts. Each opening shall have a
minimum free area of 1 square inch (645 mm2) per 4,000 NOTE−Each air duct opening shall have a free area of at least one
square inch per 2,000 Btu (645mm2 per .59kW) per hour of the total
Btu (1.17 kW) per hour of total input rating of all equipment input rating of all equipment in the enclosure. If the equipment room
in the enclosure. See figures 4 and 5. When communicat- is located against an outside wall and the air openings communi-
ing with the outdoors through horizontal ducts, each open- cate directly with the outdoors, each opening shall have a free area
of at least 1 square inch per 4,000 Btu (645mm 2 per 1.17kW) per
ing shall have a minimum free area of 1 square inch (645 hour of the total input rating of all other equipment in the enclosure.
mm2) per 2,000 Btu (.56 kW) per total input rating of all
equipment in the enclosure. See figure 6. FIGURE 6

Page 8
Installation − Setting Equipment NOTE − 1/3 hp blower motors are equipped with four flex-
ible mounting legs, and 1/2 hp blower motors are equipped
WARNING with three flexible legs and one rigid leg. The rigid leg is
equipped with a shipping bolt and a flat white plastic wash-
Do not install the furnace on its front or its back. Do
er (rather than the rubber mounting grommet used with a
not connect the return air ducts to the back of the fur-
nace. Doing so will adversely affect the operation of flexible mounting leg). The bolt and washer must be re-
the safety control devices, which could result in per- moved before the furnace is placed into operation. Af-
sonal injury or death. ter the bolt and washer have been removed, the rigid leg
Select a location that allows for the required clearances will not touch the blower housing.
that are listed on the unit nameplate. Also consider gas NOTE − The G51MP−60D−135 units are equipped with a
supply connections, electrical supply, vent connection, shipping pad under the blower housing. Remove the ship-
condensate trap and drain connections, and installation ping pad prior to operation.
and service clearances [24 inches (610 mm) at unit Allow for clearances to combustible materials as indicated
front]. The unit must be level from front to back and side on the unit nameplate. Minimum clearances for closet or al-
to side. The unit may be tilted slightly (maximum 1/2 in.) cove installations are shown in figures 8, 13 and 17.
from back to front to aid in the draining of the heat ex-
changer. See figure 7.

SETTING EQUIPMENT
UPFLOW APPLICATION DOWNFLOW APPLICATION
UNIT MUST BE LEVEL SIDE−TO−SIDE IN ALL APPLICATIONS.

AIR FLOW
HORIZONTAL APPLICATION

AIR FLOW

AIR FLOW

FRONT VIEW FRONT VIEW

FRONT VIEW

AIR FLOW

UNIT
UNIT FRONT UNIT
FRONT FRONT
1/2"
max.
AIR FLOW
END VIEW
1/2" 1/2"
max. max.
UNIT SHOULD BE LEVEL FROM LEFT TO RIGHT BUT MAY BE
SIDE VIEW TILTED SLIGHTLY (MAX. 1/2") FROM BACK TO FRONT SIDE VIEW
TO AID IN THE DRAINING OF THE HEAT EXCHANGER.

FIGURE 7

Page 9
 Return Air −− Upflow Units
WARNING Return air can be brought in through the bottom or either
Blower access panel must be securely in place when side of the furnace installed in an upflow application. If the
blower and burners are operating. Gas fumes, which furnace is installed on a platform with bottom return, make
could contain carbon monoxide, can be drawn into an airtight seal between the bottom of the furnace and the
living space resulting in personal injury or death. platform to ensure that the furnace operates properly and
safely. The furnace is equipped with a removable bottom
panel to facilitate installation.
WARNING Markings are provided on both sides of the furnace cabinet
Improper installation of the furnace can result in per- for installations that require side return air. Cut the furnace
sonal injury or death. Combustion and flue products cabinet at the maximum dimensions shown on page 2.
must never be allowed to enter the return air system
or air in the living space. Use sheet metal screws and NOTE − When air volumes over 1800 cfm (850 L/s) are
joint tape to seal return air system to furnace. required with 60C or 60D models in an upflow applica-
In platform installations with furnace return, the fur- tion, the following return air options are available:
nace should be sealed airtight to the return air ple- 1 − Return air from single side with transition which will
num. A door must never be used as a portion of the
accommodate 20 x 25 x 1 in. (508 x 635 x 25 mm) air filter.
return air duct system. The base must provide a
(Required to maintain proper air velocity.) See figure 9.
stable support and an airtight seal to the furnace. Al-
low absolutely no sagging, cracks, gaps, etc. 2 − Return air from single side with optional RAB Return
For no reason should return and supply air duct sys- Air Base. See figure 11.
tems ever be connected to or from other heating de- 3 − Return air from bottom.
vices such as a fireplace or stove, etc. Fire, explo- 4 − Return air from both sides.
sion, carbon monoxide poisoning, personal injury
5 − Return air from bottom and one side.
and/or property damage could result.
Refer to Engineering Handbook for additional information.
Upflow Applications G51MP applications which include side return air and
The G51MP gas furnace can be installed as shipped in a condensate trap installed on the same side of the
the upflow position. Refer to figure 8 for clearances. cabinet require either a return air base or field−fabri-
cated transition to accommodate an optional IAQ ac-
Installation Clearances cessory taller than 14.2".
Top
Side Return Air
(with transition and filter)

Left Side Right Side

20" X 25" X 1"
(508mm X635mm X 25mm)
Air Filter

Bottom (Floor)
Top/Plenum 1 in. (25 mm)

*Front 0
Return
Back 0 Air
Sides 0† Plenum
Vent 0

Floor 0‡ Transition
*Front clearance in alcove installation must be 24 in. (610 mm). FIGURE 9
Maintain a minimum of 24 in. (610 mm) for front service access.
†Allow proper clearances to accommodate condensate trap and Removing the Bottom Panel
vent pipe installation. Remove the two screws that secure the bottom cap to the
‡For installations on a combustible floor, do not install the furnace
directly on carpeting, tile or other combustible materials other furnace. Pivot the bottom cap down to release the bottom
than wood flooring. panel. Once the bottom panel has been removed, reinstall
FIGURE 8 the bottom cap. See figure 10.

Page 10
 Setting an Upflow Unit
Removing the Bottom Panel
When the side return air inlets are used in an upflow ap-
plication, it may be necessary to install leveling bolts on the
bottom of the furnace. Use field−supplied corrosion−resist-
ant 5/16 inch machine bolts (4) and nuts (8). See figure 12.
NOTE − The maximum length of the bolt is 1−1/2 inches.
1 − Lie the furnace on its back and drill a 5/16 inch diame-
ter hole in each corner of the furnace’s bottom. See fig-
ure 12 for the correct location of the holes. Drill through
the bottom panel and the bottom flange of the cabinet.
2 − Install one bolt and two nuts into each hole. Screw the
first nut onto a bolt and then insert the bolt into a hole. A
Screw flat washer may be added between the nut and the bot-
tom of the unit.
3 − Screw another nut onto the bolt on the inside of the fur-
Bottom Cap nace base. A flat washer may be added between the
Bottom Panel nut and the bottom of the unit.

FIGURE 10
Optional Return Air Base
(Upflow Applications Only −− For use with B, C and D cabinets only)

1
23 (584)
4
(102) Overall 1 Minimum
14 (Maximum) 11 (279)
(356) 2 1 22−7−16
1 Unit side return air Maximum
14 (356) (570)
AIR FLOW Opening Overall
FURNACE
(Maximum)
FRONT
5−5/8 SIDE RETURN
7−1/4 (184) (143) AIR OPENINGS
(Either Side)

OPTIONAL RAB 23 (584) 3/4

17−1/2 (446) RAB−B (98M60) RETURN AIR BASE (19)
21 (533) RAB−C (98M58) 7/8
27−5/8 (702)
24−1/2 (622) RAB−D (98M59) (22)
SIDE VIEW
FRONT VIEW
NOTE− Optional Side Return Air Filter Kits are not for use with RAB Return Air Base.
1 Both the unit return air opening and the base return air opening must be covered by a single plenum or IAQ cabinet.
Minimum unit side return air opening dimensions for units requiring 1800 cfm or more of air (W x H): 23 x 11 in.
(584 x 279 mm).
The opening can be cut as needed to accommodate plenum or IAQ cabinet while maintaining dimensions shown.
Side return air openings must be cut in the field. There are cutting guides stenciled on the cabinet for the side return
air opening. The size of the opening must not extend beyond the markings on the furnace cabinet..
2 To minimize pressure drop, the largest opening height possible (up to 14 inches) is preferred.
NOTE− Optional Side Return Air Filter Kits are not for use with RAB Return Air Base.

FIGURE 11

Page 11
4 − Adjust the outside nut to the appropriate height and
Downflow Application Installation Clearances
tighten the inside nut to secure the arrangement.
Top
NOTE − The unit may be tilted back−to−front a maximum of
1/2". This will ensure proper draining of the heat exchang-
er.
Leveling Bolt Installation
Left Side Right Side
Inches (mm)
3/8
(10)
Furnace Front
3/8 1−3/4
(10) (44)
Bottom
Top 0
1−3/4 *Front 0
(44) Leveling Bolt
Locations Back 0
Furnace
Bottom Sides 0†
Leveling Bolt 3/8
(10) Vent 0
Locations
Floor NC‡

*Front clearance in alcove installation must be 24 in. (610 mm).

Maintain a minimum of 24 in. (610 mm) for front service access.
†Allow proper clearances to accommodate condensate trap and
3/8 1−3/4 vent pipe installation.
(10) (44) ‡The furnace may be installed on a combustible wood floor if an op-
tional additive base is installed between the furnace and the com-
1−3/4 (44) bustible floor.
FIGURE 13
FIGURE 12
Installation on Non−Combustible Flooring
Downflow Applications
1− Cut floor opening keeping in mind clearances listed on
The unit may be installed three ways in downflow applica- unit rating plate. Also keep in mind gas supply connec-
tions: on non−combustible flooring, on combustible flooring tions, electrical supply, flue and air intake connections
using an additive base, or on a reverse−flow cooling cabi-
and sufficient installation and servicing clearances.
net. Do not drag the unit across the floor in the down-
flow position. Flange damage will result. See table 1 for correct floor opening size.
Refer to figure 13 for clearances in downflow applica- 2− Flange warm air plenum and lower the plenum into the
tions. opening.
3− Set the unit over the plenum and seal the plenum to
the unit.
3− Ensure that the seal is adequate.
TABLE 1
NON−COMBUSTIBLE FLOOR OPENING SIZE
Front to Rear Side to Side
Cabinet Width
in. mm in. mm

B Cabinet (17.5") 19 − 3/4 502 16 − 5/8 422

C Cabinet (21") 19 − 3/4 502 20−1/8 511

D Cabinet (24.5") 19 − 3/4 502 23 − 5/8 600

Page 12
 Installation on Combustible Flooring Installation on Cooling Cabinet
(Using an Additive Base) 1 − Refer to reverse−flow coil installation instructions for
1 − When unit is installed on a combustible floor, an addi- correctly sized opening in floor and installation of cabi-
tive base must be installed between the furnace and net.
the floor. The base must be ordered separately. See 2 − When cooling cabinet is in place, set and secure the
table 2 for opening size to cut in floor. furnace according to the instructions that are provided
with the cooling coil. Secure the furnace to the cabinet.
CAUTION 3 − Seal the cabinet and check for air leaks.
The furnace and additive base shall not be installed Return Air Opening −− Downflow Units
directly on carpeting, tile, or other combustible ma-
terial other than wood flooring. The following steps should be taken when installing ple-
num:
TABLE 2 1 − Bottom edge of plenum should be flanged with a
ADDITIVE BASE FLOOR OPENING SIZE
hemmed edge (See figure 15 or 16).
Front to Rear Side to Side
Cabinet Catalog 2 − Sealing strips should be used to ensure an airtight seal
Width Number in. mm in. mm between the cabinet and the plenum.
B Cabinet 3 − In all cases, plenum should be secured to top of fur-
11M60 22 559 18 − 3/4 476
(17.5")
nace using sheet metal screws.
C Cabinet 4 − Make certain that an adequate seal is made.
11M61 22 559 22 − 3/4 578
(21")

D Cabinet PLENUM
11M62 22 559 25 − 3/4 654 SECURE FROM
(24.5") (Field Provided)
OUTSIDE CABINET
2 − After opening is cut, set additive base into opening.
3 − Check fiberglass strips on additive base to make sure
they are properly glued and positioned. ÉÉÉÉ CABINET
4 − Lower supply air plenum into additive base until ple- SEALING STRIP SIDE PANEL
num flanges seal against fiberglass strips. (Field Provided)
Side View
NOTE − Be careful not to damage fiberglass strips.
Check for a tight seal. FIGURE 15
5 − Set the furnace over the plenum.
6 − Ensure that the seal between the furnace and plenum PLENUM
(Field Provided)
is adequate. SEALING STRIP
(Field Provided)

G51MP UNIT ÉÉÉÉ CABINET

SIDE PANEL
SECURE FROM
INSIDE CABINET Side View
SUPPLY AIR PLENUM
FIGURE 16
PROPERLY
SIZED FLOOR
OPENING ADDITIVE BASE

FIGURE 14

Page 13
 Horizontal Applications Suspended Installation of Horizontal Unit
The G51MP furnace can be installed in horizontal applica- NOTE − If unit is suspended, horizontal support kit (Cat No.
tions with either right− or left−hand air discharge. 56J18 ordered separately) or equivalent is recommended
to ensure proper unit support and coil drainage.
Horizontal Application
1 − Select location for unit keeping in mind service and
Installation Clearances
other necessary clearances. See figure 17.
Right−Hand Discharge 2 − Provide service platform in front of unit.
Top 3 − If unit is installed above finished space, fabricate a
drain pan fitted with a 1/2 inch or 3/4 inch N.P.T. fitting.
Left End 4 − Using 3/8 inch rods and support frame kit (ordered
Right End
separately), fabricate suspension hangers for unit
keeping in mind front service access clearances.
Bottom (Floor)** 5 − Mount unit on support frame as shown in figure 18.
6 − Continue with exhaust, condensate and intake line
Left−Hand Discharge piping instructions.
Top
BLOWER ACCESS PANEL 3/8 in. RODS
INTAKE/EXHAUST
Left End Right End CONNECTION

Bottom (Floor)**

Top 0

Front* 0

Back 0

Ends 0

Vent 0

Floor 0‡ DRAIN PAN

SUPPORT FRAME (to protect finished space)
*Front clearance in alcove installation must be 24 in. (610 mm).
Maintain a minimum of 24 in. (610 mm) for front service access. FIGURE 18
**A 5−1/2 service clearance must be maintained below the unit to 7− If unit is suspended above finished space, hang the
provide for servicing of the condensate trap.
‡For installations on a combustible floor, do not install the furnace field−provided drain pan below the support frame as
directly on carpeting, tile or other combustible materials other shown in figure 18. Leave 5−1/2 inches for service
than wood flooring.
clearance below unit for condensate trap.
FIGURE 17
8− Route auxiliary drain line so that water draining from
The G51MP may also be installed as a unit heater. Either this outlet will be easily noticed by the homeowner. If
suspend the furnace as shown in figure 18, or install the
necessary, run the condensate line into a condensate
furnace on a field−fabricated raised platform. The unit
pump to meet drain line slope requirements. The
must be supported at both ends and beneath the blower
deck to prevent sagging. pump must be rated for use with condensing furnaces.
Protect the condensate discharge line from the pump
Refer to figure 17 for clearances in horizontal applications.
to the outside to avoid freezing.
Platform Installation of Horizontal Unit
1− Select location for unit keeping in mind service and
other necessary clearances. See figure 17.
2− Construct a raised wooden frame and cover frame
with a plywood sheet. Provide a service platform in
front of unit. If unit is installed above finished space,
fabricate a drain pan to be installed under unit.
When installing the unit in a crawl space, a proper sup-
port platform may be created using cement blocks and
the horizontal support frame kit (ordered separately,

Page 14
 Lennox part number 56J18). Position the support Unit Heater Discharge Duct Guidelines
frame on top of the blocks and install the unit on the A field−fabricated and installed discharge air duct and grille
frame. Leave 5−1/2 inches for service clearance for cabinet is suitable for use with the G51MP heater. See fig-
condensate trap. ure 20. Keep the following items in mind when constructing
the cabinet.
EXHAUST PIPE
INTAKE PIPE 1 − Outer dimensions of cabinet should match those of the
*GAS
unit heater, so the duct/grille cabinet installs flush with
CONNECTOR
the unit heater cabinet. See figure 20.
*Gas connector may be 2 − Flange both ends of duct/grille cabinet so that screws
used for Canadian
installation if accept- can be used to secure cabinet to discharge end of unit
able by local authority
having jurisdiction. heater.
3 − To ensure proper operation, the duct/grille cabinet
must be at least 18 inches long.
4 − Use #10−16 x 1/2 inch sheet metal screws to secure
RAISED
PLATFORM duct/grille cabinet to unit, taking care not to damage in-
ternal components of unit heater when drilling holes or
CONDENSATE LINE
SERVICE PLATFORM installing screws. See figure 20.
FIGURE 19 5 − Use adjustable, double−deflection grille(s) to distribute
3 − Route auxiliary drain line so that water draining from discharge air. Adjust static pressure to be in the 0.06
this outlet will be easily noticed by the homeowner. If inch to 0.10 inch w.c. range.
necessary, run the condensate line into a condensate
pump to meet drain line slope requirements. The
pump must be rated for use with condensing furnaces. 18 in.
Protect the condensate discharge line from the pump MIN.

to the outside to avoid freezing.

4 − Set unit in drain pan as shown in figure 19. Leave 5−1/2
inches for service clearance below unit for condensate
trap.
5 − Continue with exhaust, condensate and intake piping DRAIN PAN
(to protect
installation according to instructions.
finished
G51MP Installed in Unit Heater Applications space)
Horizontal unit heaters may be installed either suspended
NOTE − When installing duct/grille cabinet, take care not to damage internal unit
from the ceiling using the support frame kit or mounted on a heater components when drilling holes or installing screws.
field−fabricated raised platform. The condensate trap must FIGURE 20
be installed where it can be serviced at a later date.
The G51MP may also be installed as a unit heater. Either
suspend the furnace from roof rafters or floor joists, as Return Air −− Horizontal Applications
shown in figure 18, or install the furnace on a field−fabri- Return air may be brought in only through the end of a fur-
cated raised platform, as shown in figure 19. The unit must nace installed in the horizontal position. The furnace is
be supported at both ends and beneath the blower deck to equipped with a removable bottom panel to facilitate
prevent sagging. installation. See figure 10.

Page 15
 Filters Pipe & Fittings Specifications
This unit is not equipped with a filter or rack. A field−pro- All pipe, fittings, primer and solvent cement must conform
vided filter is required for the unit to operate properly. Table with American National Standard Institute and the Ameri-
3 lists recommended filter sizes. can Society for Testing and Materials (ANSI/ASTM) stan-
A filter must be in place whenever the unit is operating. dards. The solvent shall be free flowing and contain no
TABLE 3 lumps, undissolved particles or any foreign matter that ad-
versely affects the joint strength or chemical resistance of
Furnace Filter Size
the cement. The cement shall show no gelation, stratifica-
Cabinet Width Side Return Bottom Return tion, or separation that cannot be removed by stirring. Re-
17−1/2" 16 X 25 X 1 (1) 16 X 25 X 1 (1) fer to the table 4 below for approved piping and fitting mate-
21" 16 X 25 X 1 (1) 20 X 25 X 1 (1) rials.
24−1/2" 16 X 25 X 1 (2) 24 X 25 X 1 (1) TABLE 4
PIPING AND FITTINGS SPECIFICATIONS
Duct System PIPE & FITTING MATERIAL
ASTM
SPECIFICATION
Use industry-approved standards to size and install the
Schedule 40 PVC (Pipe) D1785
supply and return air duct system. This will result in a quiet
Schedule 40 PVC (Cellular Core Pipe) F891
and low-static system that has uniform air distribution.
Schedule 40 PVC (Fittings) D2466
NOTE − Operation of this furnace in heating mode (indoor Schedule 40 CPVC (Pipe) F441
blower operating at selected heating speed) with an exter- Schedule 40 CPVC (Fittings) F438
nal static pressure which exceeds 0.5 inches w.c. may re- SDR−21 PVC or SDR−26 PVC (Pipe) D2241
sult in erratic limit operation. SDR−21 CPVC or SDR−26 CPVC (Pipe) F442
Supply Air Plenum Schedule 40 ABS Cellular Core DWV (Pipe) F628
If the furnace is installed without a cooling coil, a removable Schedule 40 ABS (Pipe) D1527
access panel should be installed in the supply air duct. The Schedule 40 ABS (Fittings) D2468
access panel should be large enough to permit inspection ABS−DWV (Drain Waste & Vent)
D2661
(Pipe & Fittings)
(by reflected light) of the heat exchanger for leaks after the
PVC−DWV (Drain Waste & Vent)
furnace is installed. The furnace access panel must always D2665
Pipe & Fittings)
be in place when the furnace is operating and it must not ASTM
PRIMER & SOLVENT CEMENT
allow leaks into the supply air duct system. SPECIFICATION

Return Air Plenum PVC & CPVC Primer F656

PVC Solvent Cement D2564
NOTE − Return air must not be drawn from a room
CPVC Solvent Cement F493
where this furnace, or any other gas−fueled ap-
ABS Solvent Cement D2235
pliance (i.e., water heater), or carbon monoxide−
PVC/CPVC/ABS All Purpose Cement For Fit-
producing device (i.e., wood fireplace) is installed. tings & Pipe of the same material D2564, D2235,
F493
When return air is drawn from a room, a negative pres- ABS to PVC or CPVC Transition Solvent
Cement D3138
sure is created in the room. If a gas appliance is operating
in a room with negative pressure, the flue products can CANADA PIPE & FITTING & SOLVENT
MARKING
CEMENT
be pulled back down the vent pipe and into the room. This
reverse flow of the flue gas may result in incomplete com- PVC & CPVC Pipe and Fittings
bustion and the formation of carbon monoxide gas. This PVC & CPVC Solvent Cement ULCS636
toxic gas might then be distributed throughout the house ABS to PVC or CPVC Transition Cement

by the furnace duct system.

Return air can be brought in through the bottom or either CAUTION
side of the furnace. If a furnace with bottom return air is
Solvent cements for plastic pipe are flammable liq-
installed on a platform, make an airtight seal between the uids and should be kept away from all sources of
bottom of the furnace and the platform to ensure that the ignition. Do not use excessive amounts of solvent
unit operates properly and safely. Use fiberglass sealing cement when making joints. Good ventilation should
strips, caulking, or equivalent sealing method between the be maintained to reduce fire hazard and to minimize
plenum and the furnace cabinet to ensure a tight seal. If a breathing of solvent vapors. Avoid contact of cement
filter is installed, size the return air duct to fit the filter frame. with skin and eyes.

Page 16
Use PVC primer and solvent cement or ABS solvent cement Canadian Applications Only − Pipe, fittings, primer
meeting ASTM specifications, refer to Table 4. As an alter- and solvent cement used to vent (exhaust) this ap-
nate, use all purpose cement, to bond ABS, PVC, or CPVC pliance must be certified to ULC S636 and supplied by a
pipe when using fittings and pipe made of the same materi- single manufacturer as part of an approved vent (ex-
als. Use transition solvent cement when bonding ABS to ei- haust) system. When bonding the vent system to the fur-
ther PVC or CPVC. nace, use ULC S636 approved One−Step Transition Ce-
Low temperature solvent cement is recommended. Metal or ment to bond the pipe to the flue collar, or to bond the 90°
plastic strapping may be used for vent pipe hangers. Uni- elbow or reducing 90° elbow to the flue collar. In addi-
formly apply a liberal coat of PVC primer for PVC or use a tion, the first three feet of vent pipe from the furnace flue
clean dry cloth for ABS to clean inside socket surface of fit- collar must be accessible for inspection.
ting and male end of pipe to depth of fitting socket. Table 5 lists the available exhaust termination kits, as well
as vent pipe equivalencies which must be used when siz-
ing vent pipe. All Lennox vent terminations are PVC.
TABLE 5
OUTDOOR TERMINATION KITS AND CORRESPONDING EQUIVALENCIES
Vent Pipe Length Equivalency (feet)
Outdoor
Outdoor Exhaust
Exhaust 1−1/2" 2" 3"
VENT Accelerator 2" Wall Plate 3" Wall Plate 2" Wall
Accelerator Concentric Concentric Concentric
UNIT PIPE (Dia. X Kit Kit Ring Kit
(Dia. X Kit Kit Kit
MODEL DIA. Length)
(in.) Length)
71M80 69M29
60L46 22G44 44J40
1−1/2" X 12" 2" X 12" or or 15F74
or 44W93 or 30G28 or 81J20
44W92 44W92
2 4 Not 12 Not Not 4 4* 4
Allowed Allowed Allowed
2−1/2 5 Not 15 Not Not 5 5* 5
24B−045 Allowed Allowed Allowed
36B−045 Not Not Not
3 7 21 7 7* 7
Allowed Allowed Allowed
4 14 Not 42 Not Not 14 14* 14
Allowed Allowed Allowed
2 4 Not 12 Not Not 4 4* 4
Allowed Allowed Allowed
2−1/2 5 Not 15 Not Not 5 5* 5
36B−070 Allowed Allowed Allowed
3 8 Not 24 Not Not 8 8* 8
Allowed Allowed Allowed
4 14 Not 42 Not Not 14 14* 14
Allowed Allowed Allowed
2 Not 1 Not 3 3 Not 1 1**
Allowed Allowed Allowed
36C−090 2−1/2 Not 2 Not 6 6 Not 2 2**
48C−090 Allowed Allowed Allowed
60C−090 3 Not 2 Not 6 6 Not 2 2**
Allowed Allowed Allowed
4 Not 4 Not 12 12 Not 4 4**
Allowed Allowed Allowed
2 Not 1 Not 3 3 Not 1 1**
Allowed Allowed Allowed
2−1/2 Not 2 Not 6 6 Not 2 2***
48C−110 Allowed Allowed Allowed
60C−110 Not Not Not
3 2 6 6 2 2***
Allowed Allowed Allowed
4 Not 4 Not 12 12 Not 4 4***
Allowed Allowed Allowed
3 Not 6 Not Not 15 Not 6 6***
60D−135 Allowed Allowed Allowed Allowed
4 Not 10 Not Not 25 Not 10 10***
Allowed Allowed Allowed Allowed
*Requires field−provided and installed 1−1/2" exhaust accelerator.  Termination kits 44W92, 44W93, 30G28 & 81J20 are ULC−636 ap-
**Requires field−provided and installed 2" exhaust accelerator. proved for Canadian Installation.
***For use only in non−direct vent applications, when snow riser is not re-  The 44W92 Concentric kit is provided with a 1−1/2" accelerator which
quired. Requires field−provided and installed 2" exhaust accelerator. must be installed on the exhaust outlet when this kit is used with the
G51MP−24B−045, G51MP−36B−045 and G51MP−36B−070 furnaces.

Page 17
Vent Piping Guidelines
The G51MP can be installed as either a Non−Direct Vent TABLE 6
or a Direct Vent gas central furnace. Type Elbow Equivalent ft.
NOTE − In Non-Direct Vent installations, combustion air is 90° 5
taken from indoors and flue gases are discharged outdoors.
45° 2 1/2
In Direct Vent installations, combustion air is taken from out-
doors and flue gases are discharged outdoors.
Intake and exhaust pipe sizing in Direct Vent applications NOTE − Elbow and pipe must be of same diameter. Two 45°
and exhaust pipe sizing in Non-Direct Vent applications −− elbows are equivalent to one 90° elbow of the same diame-
Size pipe according to tables 7 and 8. Table 7 lists the mini- ter.
mum equivalent vent pipe lengths permitted. Table 8 lists NOTE − The flue collar on all models is sized to accommo-
the maximum equivalent pipe lengths permitted. date 2" Schedule 40 flue pipe. When vent pipe which is
Maximum vent length is defined as: larger than 2" must be used in an upflow application, a 2"
Total length (linear feet) of pipe, elbow must be applied at the flue collar in order to proper-
Plus Equivalent length (feet) of fittings, ly transition to the larger diameter flue pipe. This elbow
Plus Equivalent length (feet) of termination. must be added to the elbow count used to determine ac-
NOTE − Include ALL pipe and ALL fittings, both in ceptable vent lengths. Assign an equivalent feet value to
doors and outdoors. this elbow according to the larger size pipe being used.
Regardless of the diameter of pipe used, the standard roof Contact the Application Department for more information
and wall terminations described in section Exhaust Piping concerning sizing of vent systems which include multiple
Terminations should be used. Exhaust vent termination pipe sizes.
pipe is sized to optimize the velocity of the exhaust gas as Use the following steps to correctly size vent pipe diameter.
it exits the termination. Refer to table 9. Refer to Vent Pipe Size Determination Worksheet on
*NOTE − The exhaust pipe should be offset a minimum of Page 55.
12 inches to avoid the possibility of water droplets being re- 1 − Determine the vent termination and its corresponding
leased from the exhaust termination. The minimum ex- equivalent feet value according to table 5.
haust vent length is 15 ft. Shorter exhaust vent lengths may 2 − Determine the number of 90° elbows required for both
result in the discharge of water droplets from the exhaust indoor and outdoor (e.g. snow riser) use. Calculate the
termination, in spite of the 12−inch vertical offset. See figure corresponding equivalent feet of vent pipe.
21. 3 − Determine the number of 45° elbows required for both
indoor and outdoor use. Calculate the corresponding
Each 45° and 90° elbow (including those provided with the
equivalent feet of vent pipe.
furnace) of any diameter is equivalent to pipe length of the 4 − Determine the length of straight pipe required.
same diameter. Table 6 shows the equivalent number of 5 − Add the total equivalent feet calculated in steps 1
feet. through 4 and compare that length to the maximum
In some applications which permit the use of several differ- values given in table 8 for the proposed vent pipe di-
ent sizes of vent pipe, a combination vent pipe may be ameter. If the total equivalent length required exceeds
used. Contact Lennox’ Application Department for assis- the maximum equivalent length listed in the appropri-
tance in sizing vent pipe in these applications. ate table, evaluate the next larger size pipe.

Exhaust Pipe Offset

12" Min.

12" Min.*
Upflow Application
Upflow Application Side Wall Termination
Rooftop Termination
12" Min.*
12" Min.*

Horizontal Application
Rooftop Termination Horizontal Application
Side Wall Termination

*A minimum of 1/4" (6mm) drop for each 12" (305mm) of horizontal run is mandatory for drainage.

FIGURE 21

Page 18
 IMPORTANT WARNING
Do not use screens or perforated metal in exhaust DANGER OF EXPLOSION!
terminations. Doing so will cause freeze−ups and Fumes from PVC glue may ignite during system
may block the terminations. check. Allow fumes to dissipate for at least 5 minutes
before placing unit into operation.
TABLE 7
MINIMUM VENT PIPE LENGTHS 1 − Measure and cut vent pipe to desired length.
G51MP MIN. EQUIV. 2 − Debur and chamfer end of pipe, removing any ridges
MODEL VENT LENGTH EXAMPLE
or rough edges. If end is not chamfered, edge of pipe
045, 070, 5 ft. plus 2 elbows of 2", 2−1/2", 3" may remove cement from fitting socket and result in a
090 or 4" diameter pipe
leaking joint.
5 ft. plus 2 elbows of 2", 2−1/2", 3" or
110** 15 ft.* 3 − Clean and dry surfaces to be joined.
4" diameter pipe
5 ft. plus 2 elbows of 3" or 4" 4 − Test fit joint and mark depth of fitting on outside of pipe.
135*** diameter pipe
5 − Uniformly apply a liberal coat of PVC primer for PVC or
*Any approved termination may be added to the minimum equivalent length use a clean dry cloth for ABS to clean inside socket
listed. surface of fitting and male end of pipe to depth of fitting
**G51MP−48C−110 and G51MP−60C−110 must have 90° street ell (supplied or socket.
field replacement Canadian kit) installed directly into unit flue collar. NOTE − Furnace flue collar and air inlet fitting are both
***G51MP−60D−135 must have 3" to 2" reducing ell (supplied or field replace- made of ABS material. Use transition solvent cement
ment Canadian kit) installed directly into unit flue collar. when joining ABS fittings to PVC pipe.
TABLE 8 6 − Promptly apply solvent cement to end of pipe and in-
MAXIMUM VENT PIPE LENGTHS
side socket surface of fitting. Cement should be ap-
DIRECT (2 PIPE) AND NON−DIRECT (1 PIPE) APPLICATIONS
plied lightly but uniformly to inside of socket. Take
MAXIMUM EQUIVALENT VENT
G51MP LENGTH FEET care to keep excess cement out of socket. Apply sec-
ALTITUDE
MODEL
2" dia. 2−1/2" dia. 3" dia. 4" dia. ond coat to end of pipe.
045 110 135 160 250 NOTE − Time is critical at this stage. Do not allow prim-
070 70 135 160 250
0 − 2000
090 50 100 125 225
er to dry before applying cement.
(0 − 609 m)
110* 30 70 125 200 7 − Immediately after applying last coat of cement to pipe,
135** n/a n/a ***125 180 and while both inside socket surface and end of pipe
045 110 135 160 250
are wet with cement, forcefully insert end of pipe into
070 70 135 160 250
2001 − 4500 socket until it bottoms out. Turn PVC pipe 1/4 turn dur-
(610 − 1371 m) 090 50 100 125 225
110* 20 70 125 200 ing assembly (but not after pipe is fully inserted) to dis-
135** n/a n/a ***90 180 tribute cement evenly. DO NOT turn ABS or cellular
045 110 135 160 250
core pipe.
070 70 135 160 250
4501−7500 NOTE − Assembly should be completed within 20 sec-
(1372−2286 m) 090 30 100 125 225
110* n/a 70 125 200 onds after last application of cement. Hammer blows
135** n/a n/a ***90 180 should not be used when inserting pipe.
045 110 135 160 250
070 70 135 160 250 8 − After assembly, wipe excess cement from pipe at end
7501 − 10000 of fitting socket. A properly made joint will show a
090 n/a 100 125 225
(2287 − 3048 m)
110* n/a 70 125 200 bead around its entire perimeter. Any gaps may indi-
135** n/a n/a ***90 180 cate a defective assembly due to insufficient solvent.
n/a −− Not allowed.
*G51MP−48C−110 and G51MP−60C−110 must have 90° street ell (supplied or 9 − Handle joints carefully until completely set.
field replacement Canadian kit) installed directly into unit flue collar.
**G51MP−60D−135 must have 3" to 2" reducing ell (supplied or field replace- Venting Practices
ment Canadian kit) installed directly into unit flue collar.
***90° elbows used in configuration of G51MP−60D−135 vent, must be lim- The thickness of construction through which vent pipes
ited to 3" sweep elbows. may be installed is 24" (610mm) maximum and 3/4"
(19mm) minimum.
Joint Cementing Procedure 1 − Use recommended piping materials for exhaust pip-
ing.
All cementing of joints should be done according to the 2 − Secure all joints, including drip leg, gas-tight using ap-
specifications outlined in ASTM D 2855. proved cement.

Page 19
 Suspend piping using hangers at a minimum of every 5 Exhaust Piping
feet (1.52m) for schedule NOTE − A 2" diameter PVC street ell is located on the
40 PVC and every 3 feet STRAPPING blower deck of the 48C−110 and 60C−110 units. In upflow
(.91m) for ABS−DWV, PVC− (metal, plastic or downflow applications, the street ell must be glued
or large wire
DWV, SPR−21 PVC, and ties) using transition solvent cement directly into the unit flue
SDR−26 PVC piping. A suit- collar. See figure 23. A 3" to 2" PVC reducing ell is located
able hanger can be fabri- on the blower deck of the 60D−135 units. In upflow or
cated by using metal or downflow applications, the reducing ell must be glued
FIGURE 22 using transition solvent cement directly into the unit flue
plastic strapping or a large
collar.
wire tie.
1 − Choose the appropriate side for venting in upflow or
3 − In areas where piping penetrates joists or interior
downflow positions. Exhaust piping exits from the top
walls, hole must be large enough to allow clearance on
of the unit in horizontal air discharge applications.
all sides of pipe through center of hole using a hanger.
Glue the field−provided exhaust vent pipe (or provided
4 − Secure piping at the point where it exits the outside street ell or reducing ell in upflow or downflow applica-
wall or roof in order to prevent transmission of vibra- tions) to the flue collar. All PVC cement joints should
tion to the structure. be made according to the specifications outlined in
5 − When furnace is installed in a residence where unit is ASTM D 2855. Refer to pipe and fittings specifications
shut down for an extended period of time, such as a and gluing procedures.
vacation home, make provisions for draining conden-
sate collection trap and lines.

TYPICAL EXHAUST PIPE CONNECTIONS AND CONDENSATE TRAP INSTALLATION

IN UPFLOW OR DOWNFLOW DIRECT OR NON−DIRECT VENT APPLICATIONS
(Right−Hand Exit in Upflow Application Shown) 45° 45°
MAX MAX
2−1/2",
3", OR
4"
PLUG 2"
PLUG
TRANSITION
or
2" −045, −070, or
2" −090 with
VENT PLUG
(Must be 2" **2" 2−1/2", 3", or 4" SIDE VIEW
glued in 2" vent pipe
place)

PLUG 2−1/2",
4"
3", OR
4" TRANSITION
CONDENSATE (use only if 4"
TRAP TRANSITION pipe is
(Must be installed required)
on same side as 2"
2"
exhaust piping)

*2"
3" to 2" REDUCING ELBOW
(provided)
*2" diameter street elbow provided. −110 with
2−1/2", 3", OR 4" −135 with
** Street elbow may be used on −045, −070 and −090.
vent pipe 3" OR 4" vent pipe

FIGURE 23

Page 20
 4 − Route piping to outside of structure. Continue with
TYPICAL EXHAUST PIPE CONNECTIONS
HORIZONTAL DIRECT OR installation following instructions given in piping ter-
NON−DIRECT VENT APPLICATIONS mination section.
(Horizontal Right−Hand Air Discharge Application Shown)

*Limit pipe length to 2"

2−1/2",
−24B−045 CAUTION
in −135 applications. 3", OR
−36B−045
−36B−070 Do not discharge exhaust into an existing stack or
4"
−36C−090 stack that also serves another gas appliance. If verti-
−48C−090 cal discharge through an existing unused stack is re-
TRANSITION −60C−090
−48C−110
quired, insert PVC pipe inside the stack until the end
−60C−110 is even with the top or outlet end of the metal stack.
*2"
−60D−135*

2"
2" CAUTION
−24B−045 The exhaust vent pipe operates under positive pres-
−36B−045 sure and must be completely sealed to prevent leak-
DO NOT transition from 2" −36B−070
−36C−090
age of combustion products into the living space.
smaller to larger pipe
size in horizontal runs. −48C−090
−48C−110 Intake Piping
−60C−090 The G51MP furnace may be installed in either direct vent
−60D−110
or non−direct vent applications. In non−direct vent applica-
tions, when intake air will be drawn into the furnace from the
surrounding space, the indoor air quality must be consid-
ered and guidelines listed in Combustion, Dilution and Ven-
tilation Air section must be followed.
The G51MP unit is designed for either left−side or right−side
air intake connections in either upflow or downflow applica-
tions. In horizontal applications, air intake must be brought
in through the top. Intake air piping is independent of ex-
haust piping.
FIGURE 24
Follow the next four steps when installing the unit in Direct
Vent applications, where combustion air is taken from out-
doors and flue gases are discharged outdoors. The pro-
vided air intake screen must not be used in direct vent ap-
plications.
IMPORTANT 1 − Use transition solvent cement to connect PVC pipe to
Exhaust piping and condensate trap must be the ABS slip connector located on the side of the burn-
installed on the same side of the unit in upflow and
er box.
dowflow applications or use alternate drain kit
76M20. 2 − Use a sheet metal screw to secure the intake pipe to the
connector, if desired. A pilot indentation is provided in the
2 − All horizontal runs of exhaust pipe must slope back to- slip connector to assist in locating and starting the fasten-
ward unit. A minimum of 1/4" (6mm) drop for each 12" er.
(305mm) of horizontal run is mandatory for drainage.
3 − Glue the provided 2" ABS plug into the unused ABS air
Horizontal runs of exhaust piping must be supported ev-
intake connector on the opposite side of the cabinet with
ery 5 feet (1.52m) using hangers.
ABS or all pupose solvent cement.
NOTE − Exhaust piping should be checked carefully to 4 − Route piping to outside of structure. Continue with instal-
make sure there are no sags or low spots. lation following instructions given in general guide lines for
3 − On the opposite side of the cabinet, glue the provided piping terminations and in intake and exhaust piping ter-
2" ABS vent plug into the unused ABS flue collar with minations for direct vent sections. Refer to figure 25 for
ABS or all purpose solvent cement. pipe sizes.

Page 21
 TYPICAL AIR INTAKE PIPE CONNECTIONS
UPFLOW OR DOWNFLOW DIRECT VENT APPLICATIONS 2−1/2",
(Right−Hand Exit in Upflow Application Shown) 3" OR
4

TRANSITION 2−1/2",
2 3" OR
2 4
2 TRANSITION
2
2 *2"
2
−24B−045
PLUG −36B−045
(Must be −24B−045
−36B−070 −36B−045 −24B−045
glued in
place) −36C−090 −36B−070 −36B−045
−48C−090 −36C−090 −36B−070
−48C−110 −48C−090 −36C−090
−60C−090 −60C−090 −48C−090
−60C−110 −48C−110 −60C−090
−60C−110 −48C−110
60C−110
−60D−135*

*Limit pipe length to 4"

in −135 applications.

FIGURE 25
Follow the next three steps when installing the unit in Non-
TYPICAL AIR INTAKE PIPE CONNECTIONS
HORIZONTAL DIRECT VENT APPLICATIONS
Direct Vent applications where combustion air is taken
(Horizontal Right−Hand Air Discharge Application Shown) from indoors and flue gases are discharged outdoors.

−24B−045
−36B−045
−36B−070 TYPICAL AIR INTAKE PIPE CONNECTIONS
−36C−090 UPFLOW OR HORIZONTAL NON−DIRECT
*Limit pipe −48C−090 2−1/2”, VENT APPLICATIONS
length to 4" in −60C−090 3” OR 4”
−135 applications. 48C−110
(Right−Hand Exit in Upflow Application Shown)
−60C−110
−60D−135*
TRANSITION 6 in. Max.

−24B−045 *2”
−36B−045 2−1/2”,
−36B−070 3” OR 4”
−36C−090 2” PLUG
−48C−090 2” (Must be
−60C−090 glued in
48C−110
place)
TRANSITION INTAKE
−60C−110
2” DEBRIS
−24B−045 SCREEN
−36B−045 2” (Provided)
−36B−070 2”
−36C−090
−48C−090
48C−110
−60C−090 2”
60C−110

NOTE − Debris screen and elbow may be rotated, so that

screen may be positioned to face forward, backward or
downward.
FIGURE 27

FIGURE 26

Page 22
 TYPICAL AIR INTAKE PIPE CONNECTIONS
DOWNFLOW NON−DIRECT VENT APPLICATIONS
(Right−Hand Exit in Downflow Applications Shown)

2" SWEEP
ELL
INTAKE
DEBRIS
SCREEN
(Provided)

6 in. Max.
PLUG
(Must be PLUG
glued in (Must be
place) glued in
place) 2"
18 in.

INTAKE DEBRIS 2" SWEEP ELL

SCREEN
(Provided)
Downflow Additive Flloor Base
Downflow
Evaporator
Coil NOTE − Debris screen and sweep ell may be rotated, so that
screen may be positioned to face forward, backward or to the side.

FIGURE 28
1 − Use field−provided materials and the factory−provided 2− Use a sheet metal screw to secure the intake pipe to the
air intake screen to route the intake piping as shown in connector, if desired. A pilot indentation is provided in the
figures 27 and 28. Maintain a minimum clearance of 3" slip connector to assist in locating and starting the fasten-
(76mm) around the air intake opening. The air intake er.
opening (with the protective screen) should always be 3− Glue the provided 2" ABS plug into the unused ABS air
directed either downward or straight out. Use 2" pipe intake connector on the opposite side of the cabinet with
and fittings only and make sure that the air intake does ABS or all purpose solvent cement.
not extend more than 6" beyond the G51MP cabinet.
The air intake connector must not be located near
the floor. To avoid this complication in downflow
applications which do not include a downflow
evaporator coil, the intake air routing should be modi-
fied as shown in figure 28.

Page 23
General Guidelines for Vent Terminations (13mm) Armaflex or equivalent. In extreme cold climate
In Non-Direct Vent applications, combustion air is taken areas, 3/4" (19mm) Armaflex or equivalent may be neces-
from indoors and the flue gases are discharged to the out- sary. Insulation on outside runs of exhaust pipe must be
painted or wrapped to protect insulation from deterioration.
doors. The G51MP is then classified as a non-direct vent,
Exhaust pipe insulation may not be necessary in some
Category IV gas furnace.
specific applications.
In Direct Vent applications, combustion air is taken from NOTE − During extremely cold temperatures, below
outdoors and the flue gases are discharged to the out- approximately 20°F (6.7°C), units with long runs of vent
doors. The G51MP is then classified as a direct vent, pipe through unconditioned space, even when insulated,
Category IV gas furnace. may form ice in the exhaust termination that prevents the
In both Non-Direct Vent and Direct Vent applications, the unit from operating properly. Longer run times of at least 5
vent termination is limited by local building codes. In the minutes will alleviate most icing problems. Also, a heating
absence of local codes, refer to the current National Fuel cable may be installed on exhaust piping and termination to
prevent freeze−ups. Heating cable installation kit is avail-
Gas Code ANSI Z223−1/NFPA 54 in U.S.A., and current
able from Lennox. See Condensate Piping section for part
CSA−B149 Natural Gas and Propane Installation Codes in
numbers.
Canada for details.
Position termination according to location given in figure 29
or 30. In addition, position termination so it is free from any
obstructions and 12" above the average snow accumula-
IMPORTANT
tion. Do not use screens or perforated metal in exhaust
terminations. Doing so will cause freeze−ups and
At vent termination, care must be taken to maintain may block the terminations.
protective coatings over building materials (prolonged
exposure to exhaust condensate can destroy protective
coatings). It is recommended that the exhaust outlet not be
located within 6 feet (1.8m) of a condensing unit because IMPORTANT
the condensate can damage the painted coating.
For Canadian Installations Only:
NOTE − If winter design temperature is below 32°F (0°C), In accordance to CSA International B149 installation
exhaust piping should be insulated with 1/2" (13mm), Ar- codes, the minimum allowed distance between the
maflex or equivalent when run through unheated space. combustion air intake inlet and the exhaust outlet of
Do not leave any surface area of exhaust pipe open to out- other appliances shall not be less than 12 inches
side air; exterior exhaust pipe should be insulated with 1/2" (305mm).

Page 24
 VENT TERMINATION CLEARANCES
FOR NON−DIRECT VENT INSTALLATIONS IN THE USA AND CANADA

INSIDE CORNER
DETAIL

H
D A

E
B

L C B B I
Fixed
Fixed Operable Closed M
Closed
F
Operable
B K
J
A
B

AREA WHERE TERMINAL

VENT TERMINAL AIR SUPPLY INLET IS NOT PERMITTED

US Installations1 Canadian Installations2

A= Clearance above grade, veranda, 12 inches (305mm) or 12 in. 305mm) 12 inches (305mm) or 12 in. 305mm)
porch, deck or balcony above average snow accumulation. above average snow accumulation.

B= Clearance to window or 6 inches (152mm) for appliances <10,000

door that may be opened 4 feet (1.2 m) below or to side of opening; Btuh (3kw), 12 inches (305mm) for
1 foot (30 cm) above opening appliances > 10,000 Btuh (3kw) and
<100,000 Btuh (30kw), 36 inches (.9m)
for appliances > 100,000 Btuh (30kw)
C= Clearance to permanently * 12" * 12"
closed window
D= Vertical clearance to ventilated soffit
located above the terminal within a * Equal to or greater than soffit depth. * Equal to or greater than soffit depth.
horizontal distance of 2 feet (mm)
from the center line of the terminal
E= Clearance to unventilated soffit * Equal to or greater than soffit depth. * Equal to or greater than soffit depth.
F= Clearance to outside corner * No minimum to outside corner * No minimum to outside corner
G= Clearance to inside corner * 3 feet (.9m) * 3 feet (.9m)
H= Clearance to each side of center line ex- * 3 feet (.9m) within a height 15 feet (4.5m) 3 feet (.9m) within a height 15 feet (4.5m)
tended above meter / regulator assembly above the meter / regulator assembly above the meter / regulator assembly
I= Clearance to service regulator * 3 feet (.9m) 3 feet (.9m)
vent outlet
J= Clearance to non−mechanical air 6 inches (152mm) for appliances <10,000
supply inlet to building or the com- 4 feet (1.2 m) below or to side of opening; Btuh (3kw), 12 inches (305mm) for
bustion air inlet to any other ap- 1 foot (30 cm) above opening appliances > 10,000 Btuh (3kw) and
pliance <100,000 Btuh (30kw), 36 inches (.9m)
for appliances > 100,000 Btuh (30kw)
K= Clearance to mechanical air sup- 3 feet (.9m) above if within 10 feet 6 feet (1.8m)
ply inlet (3m) horizontally
L= Clearance above paved sidewalk or 7 feet (2.1m) 7 feet (2.1m)
paved driveway located on public property
M= Clearance under veranda, porch, *12 inches (305mm) 12 inches (305mm)
deck or balcony
1 In accordance with the current ANSI Z223.1/NFPA 54 Natural Fuel Gas Code
2 In accordance with the current CSA B149.1, Natural Gas and Propane Installation Code *For clearances not specified in ANSI Z223.1/NFPA 54 or CSA B149.1, clear-
 A vent shall not terminate directly above a sidewalk or paved driveway that is located ance will be in accordance with local installation codes and the requirements
between two single family dwellings and serves both dwellings. of the gas supplier and these installation instructions."
 Permitted only if veranda, porch, deck or balcony is fully open on a minimum of two
sides beneath the floor. Lennox recommends avoiding this location if possible.

FIGURE 29

Page 25
 VENT TERMINATION CLEARANCES
FOR DIRECT VENT INSTALLATIONS IN THE USA AND CANADA

INSIDE CORNER
DETAIL

H
D A

E
B

L C B B I
Fixed
Fixed Operable Closed M
Closed
F
Operable
B K
J
A
B

AREA WHERE TERMINAL

VENT TERMINAL AIR SUPPLY INLET IS NOT PERMITTED

US Installations1 Canadian Installations2

A= Clearance above grade, veranda, 12 inches (305mm) or 12 in. 305mm) 12 inches (305mm) or 12 in. 305mm)
porch, deck or balcony above average snow accumulation. above average snow accumulation.

B= Clearance to window or 6 inches (152mm) for appliances <10,000 6 inches (152mm) for appliances <10,000
door that may be opened Btuh (3kw), 9 inches (mm) for appliances Btuh (3kw), 12 inches (305mm) for
> 10,000 Btuh (3kw) and <50,000 Btuh appliances > 10,000 Btuh (3kw) and
(15 kw), 12 inches (305mm) for ap- <100,000 Btuh (30kw), 36 inches (.9m)
pliances > 50,000 Btuh (15kw) for appliances > 100,000 Btuh (30kw)
C= Clearance to permanently * 12" * 12"
closed window
D= Vertical clearance to ventilated soffit
located above the terminal within a * Equal to or greater than soffit depth * Equal to or greater than soffit depth
horizontal distance of 2 feet (mm)
from the center line of the terminal
E= Clearance to unventilated soffit * Equal to or greater than soffit depth * Equal to or greater than soffit depth
F= Clearance to outside corner * No minimum to outside corner * No minimum to outside corner
G= Clearance to inside corner * 3 feet (.9m) * 3 feet (.9m)
H= Clearance to each side of center line ex- 3 feet (.9m) within a height 15 feet (4.5m) 3 feet (.9m) within a height 15 feet (4.5m)
tended above meter / regulator assembly above the meter / regulator assembly above the meter / regulator assembly
I= Clearance to service regulator * 3 feet (.9m) 3 feet (.9m)
vent outlet
J= Clearance to non−mechanical air 6 inches (152mm) for appliances <10,000 6 inches (152mm) for appliances <10,000
supply inlet to building or the com- Btuh (3kw), 9 inches (mm) for appliances Btuh (3kw), 12 inches (305mm) for
bustion air inlet to any other ap- > 10,000 Btuh (3kw) and <50,000 Btuh appliances > 10,000 Btuh (3kw) and
pliance (15 kw), 12 inches (305mm) for ap- <100,000 Btuh (30kw), 36 inches (.9m)
pliances > 50,000 Btuh (15kw) for appliances > 100,000 Btuh (30kw)
K= Clearance to mechanical air sup- 3 feet (.9m) above if within 10 feet 6 feet (1.8m)
ply inlet (3m) horizontally
L= Clearance above paved sidewalk or * 7 feet (2.1m) 7 feet (2.1m)
paved driveway located on public property
M= Clearance under veranda, porch, *12 inches (305mm) 12 inches (305mm)
deck or balcony
1 In accordance with the current ANSI Z223.1/NFPA 54 Natural Fuel Gas Code
2 In
*For clearances not specified in ANSI Z223.1/NFPA 54 or CSA B149.1, clear-
accordance with the current CSA B149.1, Natural Gas and Propane Installation Code ances will be in accordance with local installation codes and the requirements
 A vent shall not terminate directly above a sidewalk or paved driveway that is located of the gas supplier and these installation instructions."
between two single family dwellings and serves both dwellings.
 Permitted only if veranda, porch, deck or balcony is fully open on a minimum of two
sides beneath the floor. Lennox recommends avoiding this location if possible.

FIGURE 30

Page 26
Details of Intake and Exhaust Piping Terminations for FIELD SUPPLIED WALL TERMINATION OR
Direct Vent Installation (15F74) WALL RING TERMINATION KIT
NOTE − In Direct Vent installations, combustion air is tak- NOTE − FIELD PROVIDED
en from outdoors and flue gases are discharged to out- REDUCER MAY BE
1/2" (13mm) ARMAFLEX
doors. REQUIRED TO ADAPT
INSULATION IN UN-
LARGER VENT PIPE SIZE
Intake and exhaust pipes may be routed either horizontally TO TERMINATION CONDITIONED SPACE
through an outside wall or vertically through the roof. In attic
or closet installations, vertical termination through the roof SIZE TERMINATION
PER TABLE 9
is preferred. Figures 31 through 41 show typical termina-
tions.
D
1 − Exhaust and intake exits must be in same pressure
zone. Do not exit one through the roof and one on the
side. Also, do not exit the intake on one side and the
B
exhaust on another side of the house or structure. STRAIGHT
C APPPLICATION
2 − Intake and exhaust pipes should be placed as close A
together as possible at termination end (refer to il-
lustrations). Maximum separation is 3" (76mm) on roof
terminations and 6" (152mm) on side wall termina-
tions. 1/2" (13mm) ARMAFLEX INSULATION
IN UNCONDITIONED SPACE
3 − On roof terminations, the intake piping should termi-
nate straight down using two 90° elbows (See figure
31). D
* WALL E
4 − Exhaust piping must terminate straight out or up as
SUPPORT
shown. A reducer may be required on the exhaust pip-
ing at the point where it exits the structure to improve B
the velocity of exhaust away from the intake piping.
See table 9. A
C EXTENDED
NOTE − Care must be taken to avoid recirculation of
APPLICATION
exhaust back into intake pipe.

See venting table 8 for maximum venting lengths with this

arrangement.
* Use wall support every 24" (610 mm). Use two wall supports if
3"(76mm) MAX. extension is greater than 24" (610 mm) but less than 48" (1219 mm).
Inches(mm) SIZE TERMINATION
PIPE PER TABLE 9. NOTE − One wall support must be 6" (152 mm) from top of each pipe
(intake and exhaust)
UNCONDITIONED
ATTIC SPACE
8" (203mm) MIN 2" (51mm) 3" (76mm)
1/2" (13mm) FOAM Vent Pipe Vent Pipe
12" (305mm) ABOVE INSULATION IN A−Minimum clearance
AVERAGE SNOW UNCONDITIONED
SPACE above grade or average 12" (508MM) 12" (508MM)
ACCUMULATION
snow accumulation
B−Maximum horizontal
separation between 6" (152MM) 6" (152MM)
intake and exhaust
3" (76mm) OR
2" (51mm) PVC C−Minimum from
end of exhaust to 8" (203MM) 8" (203MM)
PROVIDE SUPPORT inlet of intake
FOR INTAKE AND D−Maximum exhaust
EXHAUST LINES 12" (305MM) 20" (508MM)
pipe length
DIRECT VENT ROOF TERMINATION KIT
(15F75 or 44J41) E−Maximum wall support
distance from top of each 6" (152MM) 6" (152MM)
FIGURE 31 pipe (intake/exhaust)

FIGURE 32

Page 27
 FIELD SUPPLIED WALL TERMINATION OR TABLE 9
(15F74) WALL RING TERMINATION KIT EXHAUST PIPE TERMINATION SIZE REDUCTION
With INTAKE ELBOW G51MP Termination
MODEL Exhaust Pipe Size Pipe Size
NOTE − FIELD PROVIDED 045 and 070 1−1/2" (38mm)
REDUCER MAY BE 2" (51mm), 2−1/2" (64mm),
1/2" (13mm) ARMAFLEX 090 3" (76mm) or 4" (102mm) 2" (51mm)
REQUIRED TO ADAPT
INSULATION IN UN- 110 2" (51mm)
LARGER VENT PIPE SIZE
CONDITIONED SPACE 135 3" (76mm) or 4" (102mm) 2" (51mm)
TO TERMINATION
5 − On field supplied terminations for side wall exit, ex-
SIZE TERMINATION
PER TABLE 9 haust piping may extend a maximum of 12 inches
(305mm) for 2" PVC and 20 inches (508mm) for 3"
D (76mm) PVC beyond the outside wall. Intake piping
should be as short as possible. See figures 32 and 33.
6 − On field supplied terminations, a minimum distance
B between the end of the exhaust pipe and the end of
STRAIGHT the intake pipe without a termination elbow is 8" and a
C APPPLICATION minimum distance of 6" with a termination elbow. See
A
figures 32 and 33.
7 − If intake and exhaust piping must be run up a side wall
1/2" (13mm) ARMAFLEX INSULATION to position above snow accumulation or other ob-
IN UNCONDITIONED SPACE structions, piping must be supported every 24"
(610mm) as shown in figures 32 and 33. In addition,
D close coupled wall termination kits must be extended
* WALL E for use in this application. See figures 39 and 40.
SUPPORT When exhaust and intake piping must be run up an
B outside wall, the exhaust piping must be terminated
with pipe sized per table 9.The intake piping may be
A equipped with a 90° elbow turndown. Using turndown
EXTENDED will add 5 feet (1.5m) to the equivalent length of the
C
APPLICATION pipe.
8 − Based on the recommendation of the manufacturer, a
multiple furnace installation may use a group of up to
See venting table 8 for maximum venting lengths with this four terminations assembled together horizontally, as
arrangement.
* Use wall support every 24" (610 mm). Use two wall supports if
shown in figure 36.
extension is greater than 24" (610 mm) but less than 48" (1219 mm).
NOTE − One wall support must be 6" (152 mm) from top of each pipe
(intake and exhaust)

2" (51mm) 3" (76mm)

Vent Pipe Vent Pipe
A−Minimum clearance
above grade or average 12" (508MM) 12" (508MM)
snow accumulation
B−Maximum horizontal
separation between 6" (152MM) 6" (152MM)
intake and exhaust
C−Minimum from
end of exhaust to 6" (152MM) 6" (152MM)
inlet of intake
D−Maximum exhaust
pipe length 12" (305MM) 20" (508MM)

E−Maximum wall support

distance from top of each 6" (152MM) 6" (152MM)
pipe (intake/exhaust)

FIGURE 33

Page 28
 1 1/2" (38mm) accelerator
provided on 71M80 & Front View EXHAUST VENT
44W92 kits for SIZE
G51MP−24B−045, TERMINATION
36B−045 & 36B−070 PIPE PER
FLASHING INTAKE TABLE 9.
12” (305mm) (Not Furnished) AIR
Minimum INTAKE
Above Average
Snow
Accumulation
Top View
1/2" (13mm) Foam Insulation
in Unconditioned Space
SHEET METAL STRAP
(Clamp and sheet metal strap FIELD−PROVIDED
must be field installed to support REDUCER MAY EXHAUST VENT
BE REQUIRED TO
the weight of the termination kit.)
ADAPT LARGER
CLAMP VENT PIPE SIZE INTAKE AIR
FIELD−PROVIDED TO TERMINATION
REDUCER MAY BE REQUIRED
TO ADAPT LARGER VENT 6 (152mm) MIN.
PIPE SIZE TO TERMINATION

DIRECT VENT CONCENTRIC ROOFTOP TERMINATION OUTSIDE WALL

71M80, 69M29 or 60L46 (US) DIRECT VENT WALL TERMINATION KIT
41W92 or 41W93 (Canada) (30G28 or 81J20)
FIGURE 34
FIGURE 37
1 1/2" (38mm) accelerator
FIELD−PROVIDED provided on 71M80 &
Front View EXHAUST VENT
REDUCER MAY BE 44W92 kits for
REQUIRED TO ADAPT G51MP−24B−045, SIZE
LARGER VENT PIPE 36B−045 & 36B−070 TERMINATION
SIZE TO TERMINATION OUTSIDE PIPE PER
WALL INTAKE INTAKE
AIR TABLE 9.
EXHAUST AIR
EXHAUST
AIR AIR

Top View
INTAKE 12" (305mm) Min.
INTAKE CLAMP AIR above grade or
1/2" (13mm) Foam Insulation
AIR (Not Furnished) in Unconditioned Space
average snow ac-
cumulation. FIELD−PROVIDED
GRADE REDUCER MAY EXHAUST VENT
BE REQUIRED TO
DIRECT VENT CONCENTRIC WALL TERMINATION ADAPT LARGER
71M80, 69M29 or 60L46 (US) VENT PIPE SIZE INTAKE AIR
41W92 or 41W93 (Canada) TO TERMINATION

FIGURE 35
8" (206mm) MIN.

OUTSIDE WALL
12" DIRECT VENT WALL TERMINATION KIT
EXHAUST (305mm)
VENT 5" 5−1/2" (22G44 or 44J40)
(127mm) (140mm)
INTAKE FIGURE 38
AIR
18" MAX.
(457mm) Front View
EXHAUST VENT
12" (305mm) Min.
above grade or
average snow ac-
Inches (mm) INTAKE cumulation.
AIR

optional intake elbow Side View

OPTIONAL VENT TERMINATION FOR MULTIPLE UNIT
INSTALLATION OF DIRECT VENT WALL TERMINATION KIT
(22G44, 44J40, 30G28 or 81J20)

FIGURE 36

Page 29
 WALL TERMINATION KITS (CLOSE−COUPLE)
EXTENDED VENT FOR GRADE CLEARANCE
2 inch (51 mm) 22G44 (US)
3 inch (76 mm) 44J40 (US)
If Intake and Exhaust Pipe is less than 12 in. (305 mm)
above snow accumulation or other obstructions, FIELD−PROVIDED
field fabricated piping must be installed. REDUCER MAY BE REQUIRED TO ADAPT
LARGER VENT PIPE SIZE TO TERMINATION
8” (203 mm) Min. for 2” (51 mm) & 3” (76 mm) DIA. pipe
between the end of the exhaust pipe and intake pipe
WALL SUPPORT* 12” (305 mm) Max. for 2” (51 mm) Dia. Exhaust
20” (508 mm) Max. for 3” (76 mm) Dia. Exhaust 12”
(305 mm)

6” (152 mm) EXHAUST 1/2” (13 mm)

Maximum AIR FOAM INSULATION
(Field Furnished)
INTAKE
INTAKE AIR EXHAUST
AIR AIR
5” (127 mm)

8” (203 mm) Min. 12” (305 mm) Minimum 12” (305 mm) Minimum
Above Grade or Average

ÉÉÉÉÉÉÉÉÉÉÉÉ ÉÉÉÉÉÉÉÉÉÉÉ
5−1/2” Above Grade or Average
Snow Accumulation (140 mm) Snow Accumulation

ÉÉÉÉÉÉÉÉÉÉÉÉ ÉÉÉÉÉÉÉÉÉÉÉ
GRADE GRADE
12” (305 mm) Max. for 2” (51 mm) Dia. Exhaust *Use wall support every 24" (610). Use two supports if
20” (508 mm) Max. for 3” (76 mm) Dia. Exhaust extension is greater than 24" but less than 48".

FIGURE 39

WALL TERMINATION KITS (CLOSE−COUPLE)

EXTENDED VENT FOR GRADE CLEARANCE
2 inch (51 mm) 30G28 (WTK Canada)
3 inch (76 mm) 81J20 (WTK Canada)
See Installation Instructions for additional information.

If Intake and Exhaust Pipe is less than 12 in. (305 mm)

above snow accumulation or other obstructions, FIELD−PROVIDED
field fabricated piping must be installed. REDUCER MAY BE REQUIRED TO ADAPT
LARGER VENT PIPE SIZE TO TERMINATION

WALL SUPPORT* 12” (305 mm) Max. for 2” (51 mm) Dia. Exhaust
20” (508 mm) Max. for 3” (76 mm) Dia. Exhaust
12”
(305 mm)
EXHAUST 1/2” (13 mm)
6” (152 mm) AIR FOAM INSULATION
Maximum (Field Furnished)

6” (152 mm) INTAKE

Minimum AIR EXHAUST
INTAKE AIR
AIR
5” (127 mm)

6” (152 mm) 12” (305 mm) Minimum 12” (305 mm) Minimum
Minimum Above Grade or Average 5−1/2” Above Grade or Average

ÉÉÉÉÉÉÉÉÉÉÉÉ ÉÉÉÉÉÉÉÉÉÉÉ
Snow Accumulation (140 mm) Snow Accumulation
GRADE GRADE

12” (305 mm) Max. for 2” (51 mm) Dia. Exhaust *Use wall support every 24" (610). Use two supports if
20” (508 mm) Max. for 3” (76 mm) Dia. Exhaust extension is greater than 24" but less than 48".

FIGURE 40

Page 30
 G51MP DIRECT VENT APPLICATION
STRAIGHT−CUT OR SIZE TERMINATION
USING EXISTING CHIMNEY ANGLE−CUT IN DIRECTION PIPE PER TABLE 9.
OF ROOF SLOPE *
12" (305mm)
ABOVE AVE.
3" − 8"
SNOW 1/2" (13mm) FOAM
EXHAUST VENT
8" − 12" (76mm− 1/2" (13mm) ACCUMULATION INSULATION
(203mm − 305mm) 203mm)
WEATHERPROOF
INSULATION

Minimum 12" (305MM) INTAKE PIPE SHOULDER OF FITTINGS

INSULATION (optional) PROVIDE SUPPORT 3" (76mm) OR
above chimney top OF PIPE ON TOP PLATE UNCONDITIONED
plate or average snow SHEET 2" (51mm) PVC
METAL TOP ALTERNATE ATTIC SPACE
accumulation PLATE INTAKE PIPE
PROVIDE SUPPORT
FOR EXHAUST LINES
3" − 8"
(76mm−
203mm)
INSULATE EXTERIOR
TO FORM PORTION OF
SEAL CHIMNEY

*SIZE TERMINATION
NON−DIRECT VENT ROOF TERMINATION KIT
PIPE PER TABLE 9. (15F75 or 44J41)
FIGURE 42
NOTE − Do not discharge exhaust gases directly into any chimney or vent stack. If ver-
tical discharge through an existing unused chimney or stack is required, insert piping
12" (305mm) MAX. for 2" (51mm)
inside chimney until the pipe open end is above top of chimney and terminate as illus-
trated. In any exterior portion of chimney, the exhaust vent must be insulated. 20" (508mm) MAX. for 3" (76mm)
1/2" (13mm) ARMAFLEX
FIGURE 41 INSULATION IN
UNCONDITIONED SPACE 1/2" (13mm)
ARMAFLEX
FIELD−PROVIDED INSULATION
REDUCER MAY
Details of Exhaust Piping Terminations for Non-Direct BE REQUIRED TO
ADAPT LARGER
Vent Applications VENT PIPE SIZE PVC REDUCER
TO TERMINATION 12" MIN.
Exhaust pipes may be routed either horizontally through an SIZE TERMINATION (305mm)
Above Grade or
outside wall or vertically through the roof. In attic or closet PIPE PER TABLE 9. average snow
accumulation
installations, vertical termination through the roof is pre-
ferred. Figures 42 through 45 show typical terminations.
1 − Exhaust piping must terminate straight out or up as NON−DIRECT VENT FIELD SUPPLIED WALL TERMINATION OR
(15F74) WALL TERMINATION KIT
shown. The termination pipe must be sized as listed in
table 9.The specified pipe size ensures proper veloc-
ity required to move the exhaust gases away from the FIGURE 43
building. 3 − If exhaust piping must be run up a side wall to position
above snow accumulation or other obstructions, pip-
2 − On field supplied terminations for side wall exit, ex- ing must be supported every 24 inches (610mm) as
haust piping may extend a maximum of 12 inches shown in figure 44. When exhaust piping must be run
(305mm) for 2" PVC and 20 inches (508mm) for 3" up an outside wall, any reduction in exhaust pipe size
(76mm) PVC beyond the outside wall. See figure 43. must be done after the final elbow.

Page 31
 12" (305mm) MAX. for 2" (51mm) Condensate Piping
20" (508mm) MAX. for 3" (76mm) This unit is designed for either right- or left-side exit of con-
UNCONDITIONED
SPACE densate piping in either upflow or downflow applications;
6" (152mm)
however, it must be installed on the same side of the unit as
Max SIZE TER- the exhaust piping. In horizontal applications, the conden-
MINATION
*WALL SUPPORT PIPE PER sate trap should extend below the unit. A 5−1/2" service
OUTSIDE WALL
TABLE 9. clearance is required for the condensate trap. Refer to fig-
FIELD−PROVIDED
ure for condensate trap locations.
REDUCER MAY BE 12" (305mm)
REQUIRED TO ABOVE GRADE OR
AVERAGE SNOW
CONDENSATE TRAP LOCATIONS
ADAPT LARGER
ACCUMULATION (Unit shown in upflow position)
VENT PIPE SIZE TO
TERMINATION
Horizontal Horizontal
left and right and
1/2" (13mm) FOAM optional optional
INSULATION IN downflow downflow
UNCONDITIONED
SPACE 1/2" (13mm) FOAM
INSULATION
NON−DIRECT VENT FIELD SUPPLIED WALL TERMINATION
EXTENDED OR (15F74) WALL TERMINATION VENT PIPE
EXTENDED
*Use wall support every 24" (610). Use two supports if extension is
greater than 24" but less than 48".
FIGURE 44 Optional Optional
upflow upflow

G51MP NON−DIRECT VENT APPLICATION NOTE − In upflow applications where side return
USING EXISTING CHIMNEY air filter is installed on same side as the conden-
SIZE TERMINATION STRAIGHT−CUT OR
ANGLE−CUT IN DIRECTION sate trap, filter rack must be installed beyond
PIPE PER TABLE 9. OF ROOF SLOPE
condensate trap to avoid interference.
3" − 8"
(76mm−
EXHAUST VENT FIGURE 46
203mm)
1/2" (13mm)
Minimum 12" (305MM)
WEATHERPROOF
INSULATION
1 − Determine which side condensate piping will exit the
above chimney top
plate or average snow
unit. Remove plugs from the condensate collar at the
SHOULDER OF FITTINGS
accumulation PROVIDE SUPPORT
OF PIPE ON TOP PLATE
appropriate location on the side of the unit.
SHEET
METAL TOP
PLATE
NOTE − The condensate trap is factory−shipped with
two rubber O−rings and two rubber clean−out caps
3" − 8" installed. Check to make sure that these items are in
(76mm−
INSULATE
TO FORM
203mm)
EXTERIOR
place before installing the trap assembly.
SEAL PORTION OF
CHIMNEY 2 − Install condensate trap onto the condensate collar.
Use provided HI/LO screws to secure two upper
flanges of the trap to the collar. Use provided sheet
metal screw to secure bottom trap flange to side of
NOTE − Do not discharge exhaust gases directly into any chimney or vent stack. If ver-
tical discharge through an existing unused chimney or stack is required, insert piping
unit. DO NOT apply glue to secure condensate trap
inside chimney until the pipe open end is above top of chimney and terminate as illus- to cabinet. All other joints must be glued. See figure
trated. In any exterior portion of chimney, the exhaust vent must be insulated.
47.
FIGURE 45 NOTE − In upflow and downflow applications, con-
densate trap must be installed on the same side as
exhaust piping.

Page 32
 CONDENSATE ASSEMBLY
CAUTION HI/LO SCREWS
(DO NOT use power
DO NOT use a power driver to tighten screws which O−RINGS VENT
driver. Hand−tighten
secure condensate trap to cabinet. Screws should using screw driver.)
NIPPLE
be hand−tightened using a screw driver to avoid the
possibility of damage to the trap assembly.

3 − The condensate trap provided with the unit is

manufactured using ABS material. Use ABS to PVC TEE
transition solvent cement to glue a field−provided PVC CAP NIPPLE
coupling or PVC pipe to the trap. Install a tee and vent
COUPLING
pipe near the trap.
CLEAN−OUT ACCESS
NOTE − The condensate trap drain stubs (both sides) (both sides)
have an outer diameter which will accept a standard CONDENSATE
3/4" PVC coupling. The inner diameter of each stub SCREW
TRAP
will accept standard 1/2" diameter PVC pipe.
NOTE − Use screws to secure condensate trap to cabinet. DO
NOTE − Vinyl tubing may be used for condensate NOT apply glue to this joint. All other joints must be glued.
drain. Tubing must be 1−1/4" OD X 1" ID and should be FIGURE 47
attached to the drain stubs on the trap using a hose
clamp.
Gas Piping
4 − Glue the field−provided drain line to the tee. Route the
drain line to an open drain. As an alternate, clear vinyl CAUTION
tubing may be used to drain condensate away from If a flexible gas connector is required or allowed by
the trap. Secure the vinyl tubing to the drain stubs on the authority that has jurisdiction, black iron pipe
the trap using a hose clamp. Do not overtighten the shall be installed at the gas valve and extend outside
hose clamp. the furnace cabinet.
Condensate line must be sloped downward away from
condensate trap to drain. If drain level is above con-
densate trap, condensate pump must be used. Con-
WARNING
densate drain line should be routed within the condi- Do not exceed 600 in−lbs (50 ft−lbs) torque when at-
tioned space to avoid freezing of condensate and taching the gas piping to the gas valve.
blockage of drain line. If this is not possible, a heat 1 − Gas piping may be routed into the unit through either
cable kit may be used on the condensate trap and line. the left- or right-hand side. Supply piping enters into
Heating cable kit is available from Lennox in various the gas valve from the side of the valve as shown in
lengths; 6 ft. (1.8m) − kit no. 26K68; 24 ft. (7.3m) − kit figure 50 or 50.
no. 26K69; and 50 ft. (15.2m) − kit no. 26K70.
IMPORTANT
The Honeywell VR8205 gas valve requires a low inlet
CAUTION pressure switch in LP/propane applications. A 4" BIP
Do not use copper tubing or existing copper nipple must be installed in the gas valve inlet when
condensate lines for drain line. right−side gas entry is used in LP/propane applica-
tions. See figure 49 or 50.
5 − If unit will be started immediately upon completion of 2 − When connecting gas supply, factors such as length of
installation, prime trap per procedure outlined in Unit run, number of fittings and furnace rating must be con-
Start−Up section. sidered to avoid excessive pressure drop. Table 10
lists recommended pipe sizes for typical applications.
6 − Glue the provided cap onto the unused condensate NOTE − Use two wrenches when connecting gas pip-
drain line stub. ing to avoid transferring torque to the manifold.

Page 33
3 − Gas piping must not run in or through air ducts, clothes Leak Check
chutes, chimneys or gas vents, dumb waiters or eleva- After gas piping is completed, carefully check all piping
tor shafts. Center gas line through piping hole. Gas connections (factory− and field−installed) for gas leaks. Use
line should not touch side of unit. See figure 50 or 50. a leak detecting solution or other preferred means.
4 − Piping should be sloped 1/4 inch per 15 feet (6mm per The furnace must be isolated from the gas supply system
5.6m) upward toward the gas meter from the furnace. by closing its individual manual shut-off valve during any
The piping must be supported at proper intervals, ev- pressure testing of the gas supply system at pressures less
ery 8 to 10 feet (2.44 to 3.05m), using suitable hangers than or equal to 1/2 psig (3.48 kPa, 14 inches w.c.).
or straps. Install a drip leg in vertical pipe runs to serve as
a trap for sediment or condensate.
5 − A 1/8" N.P.T. plugged tap or pressure post is located IMPORTANT
on the gas valve to facilitate test gauge connection. When testing pressure of gas lines, gas valve must
See figures 57 and 58. be disconnected and isolated. See figure 48. Gas
6 − In some localities, codes may require installation of a valves can be damaged if subjected to pressures
manual main shut-off valve and union (furnished by in- greater than 1/2 psig (3.48 kPa).
staller) external to the unit. Union must be of the
ground joint type.
WARNING
IMPORTANT FIRE OR EXPLOSION HAZARD
Failure to follow the safety warnings exactly could
Compounds used on threaded joints of gas piping result in serious injury, death, or property damage.
must be resistant to the actions of liquified petro- Never use an open flame to test for gas leaks. Check
leum gases. all connections using a commercially available soap
solution made specifically for leak detection.Some
MANUAL MAIN SHUT−OFF
FURNACE soaps used for leak detection are corrosive to certain
VALVE WILL NOT HOLD metals. Carefully rinse piping thoroughly after leak
NORMAL TEST PRESSURE
test has been completed.
1/8 NPT PLUG ISOLATE
GAS VALVE

CAP

FIGURE 48

Upflow Application AUTOMATIC Upflow Application

GAS VALVE
Left Side Piping (with manual Right Side Piping
(Standard) shut−off valve) (Alternate)
MANUAL
MAIN SHUT−OFF
VALVE MANUAL
MAIN SHUT−OFF
VALVE

GROUND GROUND
JOINT JOINT
UNION UNION

DRIP LEG
FIELD
PROVIDED
DRIP LEG AND INSTALLED In LP/propane applications using a Honeywell
VR8205 gas valve, a 4" BIP nipple must be installed
to accommodate the low inlet pressure switch.

FIGURE 49

Page 34
 Horizontal Applications
Possible Gas Piping Configurations

MANUAL
MAIN SHUT−OFF
VALVE Horizontal Application
MANUAL Left−Side Air Discharge
MAIN SHUT−OFF
VALVE
GROUND
JOINT
UNION

GROUND
JOINT DRIP LEG
UNION

DRIP LEG

MANUAL
MAIN SHUT−OFF
Horizontal Application VALVE
Right−Side Air Discharge (With 1/8 in. NPT
Plugged Tap
Shown)
GROUND
JOINT
In LP/propane applications using a UNION
Honeywell VR8205 gas valve, a 4" BIP
DRIP LEG
nipple must be installed to accommodate
the low inlet pressure switch.

FIGURE 50
TABLE 10
GAS PIPE CAPACITY − FT3/HR (kL/HR)
Nominal Internal Length of Pipe−Feet(m)
Iron Pipe Size Diameter 10 20 30 40 50 60 70 80 90 100
−Inches(mm) −Inches(mm) (3.048) (6.096) (9.144) (12.192) (15.240) (18.288) (21.336) (24.384) (27.432) (30.480)
1/4 .364 43 29 24 20 18 16 15 14 13 12
(6.35) (9.246) (1.13) (.82) (.68) (.57) (.51) (.45) (.42) (.40) (.37) (.34)
3/8 .493 95 65 52 45 40 36 33 31 29 27
(9.53) (12.522) (2.69) (1.84) (1.47) (1.27) (1.13) (1.02) (.73) (.88) (.82) (.76)
1/2 .622 175 120 97 82 73 66 61 57 53 50
(12.7) (17.799) (4.96) (3.40) (2.75) (2.32) (2.07) (1.87) (1.73) (1.61) (1.50) (1.42)
3/4 .824 360 250 200 170 151 138 125 118 110 103
(19.05) (20.930) (10.19) (7.08) (5.66) (4.81) (4.28) (3.91) (3.54) (3.34) (3.11) (2.92)
1 1.049 680 465 375 320 285 260 240 220 205 195
(25.4) (26.645) (19.25) (13.17) (10.62) (9.06) (8.07) (7.36) (6.80) (6.23) (5.80) (5.52)
1−1/4 1.380 1400 950 770 660 580 530 490 460 430 400
(31.75) (35.052) (39.64) (26.90) (21.80) (18.69) (16.42) (15.01) (13.87) (13.03) (12.18) (11.33)
1−1/2 1.610 2100 460 1180 990 900 810 750 690 650 620
(38.1) (40.894) (59.46) (41.34) (33.41) (28.03) (25.48) (22.94) (21.24) (19.54) (18.41) (17.56)
2 2.067 3950 2750 2200 1900 1680 1520 1400 1300 1220 1150
(50.8) (52.502) (111.85) (77.87) (62.30) (53.80) (47.57) (43.04) (39.64) (36.81) (34.55) (32.56)
2−1/2 2.469 6300 4350 3520 3000 2650 2400 2250 2050 1950 1850
(63.5) (67.713) (178.39) (123.17) (99.67) (84.95) (75.04) (67.96) (63.71) (58.05) (55.22) (52.38)
3 3.068 11000 7700 6250 5300 4750 4300 3900 3700 3450 3250
(76.2) (77.927) (311.48) (218.03) (176.98) (150.07) (134.50) (121.76) (110.43) (104.77) (97.69) (92.03)
4 4.026 23000 15800 12800 10900 9700 8800 8100 7500 7200 6700
(101.6) (102.260) (651.27) (447.39) (362.44) (308.64) (274.67) (249.18) (229.36) (212.37) (203.88) (189.72)
NOTE − Capacity given in cubic feet of gas per hour (kilo liters of gas per hour) and based on 0.60 specific gravity gas.

Page 35
Electrical The unit is equipped with a field make−up box. The make−
up box may be moved to the right side of the furnace to fa-
ELECTROSTATIC DISCHARGE (ESD)
cilitate installation. If the make−up box is moved to the right
Precautions and Procedures side, the excess wire must be pulled into the blower
compartment. Secure the excess wire to the existing harn-
CAUTION ess to protect it from damage.
Electrostatic discharge can affect electronic com- Refer to figure 54 for field wiring and figure 56 for schematic
ponents. Take precautions during furnace installa- wiring diagram and troubleshooting.
tion and service to protect the furnace’s electronic
controls. Precautions will help to avoid control ex- Select circuit protection and wire size according to the
posure to electrostatic discharge by putting the fur- unit nameplate. The power supply wiring must meet
nace, the control and the technician at the same Class I restrictions.
electrostatic potential. Neutralize electrostatic
charge by touching hand and all tools on an un- Holes are on both sides of the furnace cabinet to facili-
painted unit surface, such as the gas valve or blower tate wiring.
deck, before performing any service procedure. Install a separate disconnect switch (protected by ei-
ther fuse or circuit breaker) near the furnace so that
INTERIOR MAKE−UP BOX INSTALLATION power can be turned off for servicing.
MAKE−UP Before connecting the thermostat or the power wiring,
BOX
check to make sure the wires will be long enough for
servicing at a later date. Remove the blower access
panel to check the length of the wire.
Complete the wiring connections to the equipment.
Use the provided unit wiring diagram and the field wir-
ing diagram shown in figure 54. Use 18−gauge wire or
Right Side
larger that is suitable for Class II rating for thermostat
connections.
Electrically ground the unit according to local codes or,
in the absence of local codes, according to the current
National Electric Code (ANSI/NFPA No. 70) for the
FIGURE 51 USA and current Canadian Electric Code part 1 (CSA
standard C22.1) for Canada. A green ground wire is
INTERIOR MAKE−UP BOX INSTALLATION provided in the field make−up box.
NOTE − The G51MP furnace contains electronic com-
MAKE−UP
Left side BOX ponents that are polarity sensitive. Make sure that the
furnace is wired correctly and is properly grounded.
One line voltage EAC" accessory terminal is provided
on the furnace control board. Any electronic air cleaner
rated up to one amp can be connected to this terminal
with the neutral leg of the circuit being connected to the
any of the NEUTRAL" terminals. See figure 55 for con-
trol board configuration. This terminal is energized
whenever the blower is operating.
One line voltage HUM" accessory terminal is provided
on the furnace control board. Any humidifier rated up
to one amp can be connected to this terminal with the
neutral leg of the circuit being connected to any of the
NEUTRAL" terminals. See figure 55 for control board
configuration. This terminal is energized in the heating
mode whenever the combustion air inducer is operat-
FIGURE 52 ing.

Page 36
 G51MP and CONDENSING UNIT
mon leg of the circuit being connected to the C" ter-
THERMOSTAT DESIGNATIONS minal of the thermostat terminal block, which is lo-
(Refer to specific thermostat and outdoor unit.) cated on the control board.
Install the room thermostat according to the instruc-
Thermostat G51MP Condensing
Furnace
tions provided with the thermostat. See figure 53 for
Unit
thermostat designations. If the furnace is being
POWER
R R matched with a heat pump, refer to the FM21 installa-
W1
HEAT
W
tion instruction.
CONDENSING
Y COOLING Y
UNIT Indoor Blower Speeds

G
INDOOR BLOWER
G
When the thermostat is set to FAN ON," the indoor
COMMON
CONDENSING
UNIT COMMON
blower will run continuously on the continuous low
C C speed (FAN) when there is no cooling or heating de-
mand.
FIGURE 53 When the G51MP is running in the heating mode, the
One 24 volt 24V HUM" terminal is provided on the indoor blower will run on the heating speed.
furnace control board. Any humidifier rated up to 0.5 When there is a cooling demand, the indoor blower will
amp can be connected to this terminal with the com- run on the cooling speed.

TYPICAL G51MP FIELD WIRING DIAGRAM

FIGURE 54

Page 37
SURELIGHT® INTEGRATED CONTROL BOARD
TERMINAL DESIGNATIONS
COOL Blower − Cooling Speed (120VAC)
HEAT Blower − Heating Speed (120VAC)
PARK Unused Blower (Not energized)
FAN Continuous Low Blower Speed
EAC Accessory Terminal (120VAC)
XFMR Transformer (120VAC)
LINE Input (120VAC)
HUM Heat Only Accessory (120VAC)
5 Terminals 120 Volt Neutral
FS Flame Sensor
24V HUM Heat Only Accessory (24VAC)

1/4" FEMALE
QUICK
CONNECT
*

*Factory setting

FIGURE 55

Page 38
TYPICAL G51MP WIRING DIAGRAM

FIGURE 56

Page 39
Unit Start−Up BEFORE LIGHTING the unit, smell all around the fur-
nace area for gas. Be sure to smell next to the floor be-
FOR YOUR SAFETY READ BEFORE OPERATING cause some gas is heavier than air and will settle on the
floor.
WARNING The gas valve on the G51MP is equipped with a gas con-
Do not use this furnace if any part has been under- trol switch (lever). Use only your hand to move the switch.
water. A flood−damaged furnace is extremely dan- Never use tools. If the the switch will not move by hand,
gerous. Attempts to use the furnace can result in do not try to repair it. Force or attempted repair may result
fire or explosion. Immediately call a qualified ser- in a fire or explosion.
vice technician to inspect the furnace and to replace
all gas controls, control system parts, and electrical Placing the furnace into operation:
parts that have been wet or to replace the furnace, G51MP units are equipped with a SureLight® ignition
if deemed necessary.
system. Do not attempt to manually light burners on this
furnace. Each time the thermostat calls for heat, the
WARNING burners will automatically light. The ignitor does not get
hot when there is no call for heat on units with SureLight®
Danger of explosion. Can cause injury ignition system.
or product or property damage. Should
the gas supply fail to shut off or if
overheating occurs, shut off the gas
valve to the furnace before shutting off WARNING
the electrical supply. If you do not follow these instructions exactly, a fire
or explosion may result causing property damage,
personal injury or death.
CAUTION
Before attempting to perform any service or mainte- Gas Valve Operation (Figures 57 and 58)
nance, turn the electrical power to unit OFF at dis-
connect switch. 1 − STOP! Read the safety information at the beginning of
Priming Condensate Trap this section.
The condensate trap should be primed with water prior to 2 − Set the thermostat to the lowest setting.
start−up to ensure proper condensate drainage. Either
pour 10 fl. oz. (300 ml) of water into the trap, or follow 3 − Turn off all electrical power to the unit.
these steps to prime the trap: 4 − This furnace is equipped with an ignition device which
1 − Follow the lighting instructions to place the unit into op- automatically lights the burners. Do not try to light the
eration. burners by hand.
2 − Set the thermostat to initiate a heating demand.
5 − Remove the upper access panel.
3 − Allow the burners to fire for approximately 3 minutes.
4 − Adjust the thermostat to deactivate the heating de- 6 − White Rodgers 36G Gas Valve − Move gas valve
mand. switch to OFF. See figure 57 for the White Rodgers
36G valve.
5 − Wait for the combustion air inducer to stop. Set the
Honeywell VR8205 Gas Valve − Move gas valve
thermostat to initiate a heating demand and again al-
switch to OFF. See figure 58 for Honeywell VR8205
low the burners to fire for approximately 3 minutes.
valve.
6 − Adjust the thermostat to deactivate the heating de-
mand and again wait for the combustion air inducer to 7 − Wait five minutes to clear out any gas. If you then smell
stop. At this point, the trap should be primed with suffi- gas, STOP! Immediately call your gas supplier from a
cient water to ensure proper condensate drain opera- neighbor’s phone. Follow the gas supplier’s instruc-
tion. tions. If you do not smell gas go to next step.

Page 40
 4 − White Rodgers 36G Gas Valve − Move gas valve
White Rodgers 36G Series Gas Valve
switch to OFF.
OUTLET Honeywell VR8205 Gas Valve − Move gas valve
PRESSURE
POST
switch to OFF.
5 − Replace the upper access panel.
Heating Sequence Of Operation
INLET MANIFOLD When thermostat calls for heat, combustion air inducer
PRESSURE PRESSURE starts.
POST ADJUSTMENT
SCREW Combustion air pressure switch proves blower opera-
GAS VALVE SHOWN IN OFF POSITION tion. Switch is factory set and requires no adjustment.
FIGURE 57 After a 15−second prepurge, the hot surface ignitor en-
ergizes.
Honeywell VR8205 Gas Valve with ON/OFF Switch
After a 20−second ignitor warm−up period, the gas
MANIFOLD
PRESSURE MANIFOLD valve solenoid opens.
PRESSURE
ADJUSTMENT
OUTLET
Gas is ignited, flame sensor proves the flame, and the
SCREW
(under
combustion process continues.
barbed If flame is not detected after first ignition trial, the igni-
fitting)
tion control will repeat steps 3 and 4 four more times
INLET before locking out the gas valve (WATCHGUARD"
PRESSURE
PORT flame failure mode). The ignition control will then auto-
GAS VALVE SHOWN IN ON POSITION matically repeat steps 1 through 6 after 60 minutes.
FIGURE 58 To interrupt the 60−minute WATCHGUARD" period,
8 − White Rodgers 36G Gas Valve − Move gas valve move thermostat from Heat" to OFF" then back to
switch to ON. See figure 57 for the White Rodgers Heat". Heating sequence then restarts at step 1.
36G valve
Honeywell VR8205 Gas Valve − Move gas valve
Gas Pressure Adjustment
switch to ON. See figure 58 for Honeywell VR8205 Gas Flow (Approximate)
valve. 1 − Operate unit at least 15 minutes before checking gas
9 − Replace the upper access panel. flow. Determine the time in seconds for one revolu-
10− Turn on all electrical power to to the unit. tions of gas through the meter. A portable LP gas me-
11− Set the thermostat to desired setting. ter (17Y44) is available for LP applications.
NOTE − When unit is initially started, steps 1 through 11 2 − Compare the number of seconds and the gas meter
may need to be repeated to purge air from gas line. size in table 11 to determine the gas flow rate. Multiply
12− If the appliance will not operate, follow the instructions the gas flow rate by the heating value to determine the
Turning Off Gas to Unit" and call the gas supplier. unit input rate. If manifold pressure is correct and the
Turning Off Gas to Unit unit input rate is incorrect, check gas orifices for proper
1 − Set the thermostat to the lowest setting. size and restriction.
2 − Turn off all electrical power to the unit if service is to be 3 − Remove temporary gas meter if installed.
performed. NOTE − To obtain accurate reading, shut off all other gas
3 − Remove the upper access panel. appliances connected to meter.

Page 41
 TABLE 11 1 − Remove the threaded plug from the outlet side of the
Gas Flow Rate (Ft.3/Hr.) gas valve and install a field−provided barbed fitting.
Seconds for 1 Gas Meter Size Connect test gauge +" connection to barbed fitting to
Revolution 1/2 cu ft Dial 1 cu ft Dial measure manifold pressure.
10 180 360 2 − Tee into the gas valve regulator vent hose and connect
12 150 300 test gauge −" connection.
14 129 257 3 − Start unit and allow 5 minutes for unit to reach steady
16 113 225 state.
18 100 200
4 − While waiting for the unit to stabilize, notice the flame.
20 90 180
Flame should be stable and should not lift from burner.
22 82 164
24 75 150 Natural gas should burn blue.
26 69 138 5 − After allowing unit to stabilize for 5 minutes, record
28 64 129 manifold pressure and compare to value given in table
30 60 120 12.
32 56 113 NOTE − Shut unit off and remove manometer as soon as an
34 53 106 accurate reading has been obtained. Take care to remove
36 50 100 barbed fitting and replace threaded plug.
38 47 95
40 45 90
42
44
43
41
86
82
IMPORTANT
46 39 78 The White Rodgers 36G gas valve (figure 57) is
48 38 75 equipped with pressure posts for measuring supply
50 36 72 and manifold pressures. The posts provide built−in
52 35 69 hose connections and have an integral 3/32" Allen−
54 33 67 head screw. Rotate the screw counterclockwise one
56 32 64 full turn to permit pressure measurement. Reseat the
58 31 62 screw (rotate one full turn clockwise) after measure-
60 30 60 ments have been taken to prevent gas leakage.
Gas Supply Pressure Measurement
A threaded plug on the inlet side of the gas valve provides
access to the supply pressure tap. Remove the threaded
plug, install a field−provided barbed fitting and connect a
manometer to measure supply pressure. Replace the
threaded plug after measurements have been taken.
Manifold Pressure Measurement
To correctly measure manifold pressure, the differential
pressure between the positive gas manifold and the nega-
tive burner box must be considered. Use pressure test
adapter kit (available as Lennox part 10L34) to assist in
measurement.

Page 42
 High Altitude Information TABLE 12
Manifold Pressure (Outlet) inches w.c.
NOTE − In Canada, certification for installations at eleva-
Model Altitude (feet)
tions over 4500 feet (1372 m) is the jurisdiction of local au-
Fuel Input 0− 4501− 5501− 6501− 7501−
thorities. Sizes 4500 5500 6500 7500 10,000
The manifold pressure may require adjustment to ensure Nat. All
proper operation at higher altitudes. Refer to table 12 for 3.5 3.5 3.5 3.5 3.5*
Gas sizes
proper manifold pressure settings at varying altitudes. L.P. All
10.0** 10.0** 10.0** 10.0** 10.0**
Table 13 lists required pressure switch changes and con- Gas sizes
version kits at varying altitudes. *Conversion kit required for applications at altitudes above 7501
ft. above sea level.
The combustion air pressure switches are factory−set and **Conversion kit required for applications at all altitudes.
require no adjustment. NOTE − A natural to L.P. propane gas changeover kit is nec-
essary to convert this unit. Refer to the changeover kit
installation instruction for the conversion procedure.
TABLE 13
Conversion Kit and Pressure Switch Requirements at Varying Altitudes
High Altitude High Altitude Pressure LPG/Propane Kit
Orifice Kit Switch Kit
Input Natural Gas Only
7501−10,000 ft. 4501−7500 ft. 7501−10,000 ft. 0−7500 ft. 7501−10,000 ft.
−045 44W51 −−− 95M22 83M74 83M75
−070 44W51 56M05 56M06 83M74 83M75
−090 44W51 75M20 56M07 83M74 83M75
−110 44W51 75M20 56M07 83M74 83M75
−135 44W51 56M04 60M35 83M74 83M75
Pressure switch is factory set. No adjustment necessary. All models use the factory installed pressure switch from 0−4500 feet (0−1370 m).

Page 43
Testing for Proper Venting and Sufficient Combustion Air for Non−Direct Vent Applications

horizontal pitch. Determine there is no blockage or re-

WARNING striction, leakage, corrosion, or other deficiencies
which could cause an unsafe condition.
CARBON MONOXIDE POISONING HAZARD! 3− To the extent that it is practical, close all building doors
Failure to follow the steps outlined below for each and windows and all doors between the space in which
appliance connected to the venting system being the appliances connected to the venting system are lo-
placed into operation could result in carbon monox- cated and other spaces of the building.
ide poisoning or death. 4− Close fireplace dampers.
The following steps shall be followed for each ap-
5− Turn on clothes dryers and any appliances not con-
pliance connected to the venting system being
placed into operation, while all other appliances con- nected to the venting system. Turn on any exhaust
nected to the venting system are not in operation. fans, such as range hoods and bathroom exhausts, so
they will operate at maximum speed. Do not operate a
summer exhaust fan.
After the G51MP gas furnace has been started, the follow-
6− Follow the lighting instruction to place the appliance
ing test should be conducted to ensure proper venting and
being inspected into operation. Adjust thermostat so
sufficient combustion air has been provided to the G51MP, appliance will operate continuously.
as well as to other gas-fired appliances which are separate- 7− Use the flame of match or candle to test for spillage of
ly vented. flue gases at the draft hood relief opening after 5 min-
If a G51MP furnace replaces a Category I furnace which utes of main burner operation.
was commonly vented with another gas appliance, the size 8− If improper venting is observed during any of the
of the existing vent pipe for that gas appliance must be above tests, the venting system must be corrected or
sufficient combustion/make-up air must be provided.
checked. Without the heat of the original furnace flue prod- The venting system should be re-sized to approach
ucts, the existing vent pipe is probably oversized for the the minimum size as determined by using the ap-
single water heater or other appliance. The vent should be propriate tables in appendix G in the current standards
checked for proper draw with the remaining appliance. of the National Fuel Gas Code ANSI−Z223.1/NPFA 54
in the U.S.A., and the appropriate Natural Gas and
The test should be conducted while all appliances (both in Propane appliances venting sizing tables in the cur-
operation and those not in operation) are connected to the rent standard of the CSA−B149 Natural Gas and Pro-
venting system being tested. If the venting system has pane Installation Codes in Canada.
been installed improperly, or if provisions have not been 9− After determining that each appliance remaining
made for sufficient amounts of combustion air, corrections connected to the common venting system properly
must be made as outlined in the previous section. vents when tested as indicated in step 3, return
doors, windows, exhaust fans, fireplace dampers
1 − Seal any unused openings in the venting system. and any other gas-burning appliance to their previous
2 − Visually inspect the venting system for proper size and condition of use.

Page 44
Other Unit Adjustments Electrical
1 − Check all wiring for loose connections.
2 − Check for the correct voltage at the furnace (furnace
Primary and Secondary Limits operating).
The primary limit is located on the heating compartment 3 − Check amp-draw on the blower motor.
vestibule panel. The secondary limits are located in the Motor Nameplate__________Actual__________
blower compartment, attached to the back side of the blow- NOTE − Do not secure the electrical conduit directly to the
air ducts or structure.
er. These limits are factory set and require no adjustment.
Blower Speeds
Flame Rollout Switches (Two) NOTE − CFM readings are taken external to unit with a dry
These manually reset switches are located on the burner evaporator coil and without accessories.
box. If tripped, check for adequate combustion air before Turn off electrical power to furnace.
resetting. Remove blower access panel.
Disconnect existing speed tap at control board speed
Pressure Switches (Two) terminal.
The pressure switches are located in the heating compart- NOTE − Termination of any unused motor leads must be
ment on the combustion air inducer. These switches check insulated.
for proper combustion air inducer operation before allow- Refer to blower speed selection chart on unit wiring dia-
ing ignition trial. The switches are factory−set and require gram for desired heating or cooling speed. See blower
no adjustment. performance data beginning on Page 46.
Connect selected speed tap at control board speed
Temperature Rise
terminal.
After the furnace has been started and supply and return air Resecure blower access panel.
temperatures have been allowed to stabilize, check the Turn on electrical power to furnace.
temperature rise. If necessary, adjust the blower speed to Electronic Ignition
maintain the temperature rise within the range shown on The SureLight® integrated control has an added feature of
the unit nameplate. Increase the blower speed to decrease an internal Watchguard control. The feature serves as an
the temperature. Decrease the blower speed to increase automatic reset device for ignition control lockout caused by
the temperature rise. Failure to adjust the temperature rise ignition failure. This type of lockout is usually due to low gas
may cause erratic limit operation. line pressure. After one hour of continuous thermostat de-
mand for heat, the Watchguard will break and remake ther-
Fan Control mostat demand to the furnace and automatically reset the
Heating Mode −− The fan on delay of 45 seconds is not control to begin the ignition sequence.
adjustable. The fan off delay (amount of time that the Exhaust and Air Intake Pipe
blower operates after the heat demand has been satis- 1 − Check exhaust and air intake connections for tightness
fied) may be adjusted by setting S1 switches 1 and 2 lo- and to make sure there is no blockage.
cated on the SureLight® control. The unit is shipped with
a factory fan off setting of 90 seconds. The fan off delay 2 − Are pressure switches closed? Obstructed exhaust
affects comfort and is adjustable to satisfy individual ap- pipe will cause unit to shut off at pressure switches.
plications. Adjust the fan off delay to achieve a supply air Check termination for blockages.
temperature between 90° and 110°F at the exact mo- 3 − Reset manual flame rollout switches on burner box
ment that the blower is de−energized. Longer off delay cover.
settings provide lower return air temperatures; shorter Failure To Operate
settings provide higher return air temperatures. See fig- If the unit fails to operate, check the following:
ure 55. 1 − Is the thermostat calling for heat?
Cooling Mode −− The cooling mode fan off delay (amount 2 − Are access panels securely in place?
of time that the blower operates after the cooling demand
3 − Is the main disconnect switch closed?
has been satisfied) may be adjusted by setting S1 switch
4 − Is there a blown fuse?
3 located on the SureLight® control. In the off position,
5 − Is the filter dirty or plugged? Dirty or plugged filters will
the cooling fan off delay is 2 seconds. In the on position,
cause the limit control to shut the unit off.
the cooling fan off delay is 45 seconds. See figure 55.
6 − Is gas turned on at the meter?
Thermostat Heat Anticipation 7 − Is the manual main shut-off valve open?
Set the heat anticipator setting (if adjustable) according to 8 − Is the internal manual shut-off valve open?
the amp draw listed on the wiring diagram that is attached 9 − Is the unit ignition system in lock out? If the unit locks out
to the unit. again, inspect the unit for blockages.

Page 45
 Blower Motor Performance
G51MP−24B−045 PERFORMANCE (Less Filter) G51MP−36B−045 PERFORMANCE (Less Filter)
Air Volume / Watts at Different Blower Speeds Air Volume / Watts at Different Blower Speeds
External External
Static Medium− Medium− Static Medium− Medium−
High High Low Low High Low
Pressure Pressure High Low
in. w.g. in. w.g.
cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts
0.00 1125 425 900 360 745 295 690 265 0.00 1555 630 1410 585 1190 520 1030 435
0.10 1115 410 905 350 760 285 695 260 0.10 1515 605 1385 555 1190 485 1020 415
0.20 1090 395 895 335 755 280 685 255 0.20 1470 580 1345 520 1170 455 1010 400
0.30 1060 380 880 325 740 270 675 250 0.30 1410 555 1310 495 1155 440 1000 385
0.40 1020 365 860 315 720 260 665 240 0.40 1350 535 1250 465 1120 410 980 360
0.50 970 345 835 300 700 255 640 230 0.50 1290 505 1205 450 1080 390 950 345
0.60 920 335 795 290 665 240 600 220 0.60 1220 485 1145 420 1020 365 905 320
0.70 860 315 730 270 620 230 565 215 0.70 1145 460 1080 400 975 345 860 300
0.80 760 295 665 250 560 215 510 200 0.80 1050 425 985 365 870 320 785 285
0.90 675 275 585 230 495 200 465 175 0.90 945 410 900 345 825 305 730 270
NOTES − Air volume based on bottom return air. Actual air volume may vary on side return air applications.

G51MP−36B−070 PERFORMANCE (Less Filter) G51MP−36C−090 PERFORMANCE (Less Filter)

Air Volume / Watts at Different Blower Speeds Air Volume / Watts at Different Blower Speeds
External External
Static Medium− Medium− Static Medium− Medium−
High Low High Low
Pressure High Low Pressure High Low
in. w.g. cfm Watts cfm Watts cfm Watts cfm Watts in. w.g. cfm Watts cfm Watts cfm Watts cfm Watts
0.00 1640 660 1415 575 1160 485 1005 410 0.00 1630 745 1360 635 1125 540 975 440
0.10 1600 635 1395 550 1160 460 1000 385 0.10 1620 715 1365 610 1160 515 1000 430
0.20 1540 605 1370 525 1160 445 995 375 0.20 1590 680 1365 580 1160 495 990 405
0.30 1495 580 1345 505 1145 425 990 365 0.30 1550 655 1355 565 1170 475 985 395
0.40 1420 545 1275 480 1125 395 965 345 0.40 1520 630 1330 545 1160 460 980 380
0.50 1360 525 1245 450 1080 375 945 325 0.50 1465 605 1300 515 1140 440 960 360
0.60 1275 490 1165 410 1025 350 900 305 0.60 1415 570 1250 490 1095 420 940 350
0.70 1170 465 1085 385 965 335 860 295 0.70 1350 545 1215 470 1065 400 905 335
0.80 1080 440 1010 360 865 310 775 270 0.80 1260 510 1140 440 1005 375 850 310
0.90 945 400 840 320 765 275 710 245 0.90 1165 475 1035 395 900 335 730 285
NOTES − Air volume based on bottom return air. Actual air volume may vary on side return air applications.

G51MP−48C−090 PERFORMANCE (Less Filter) G51MP−48C−110 PERFORMANCE (Less Filter)

External Air Volume / Watts at Different Blower Speeds Air Volume / Watts at Different Blower Speeds
Static External
Pressure Medium− Medium− Static Medium− Medium−
High Low High Low
in. w.g. High Low Pressure High Low
cfm Watts cfm Watts cfm Watts cfm Watts in. w.g. cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2180 930 1835 790 1520 630 1280 510 0.00 2160 880 1880 755 1490 602 1235 485
0.10 2135 885 1825 750 1510 610 1275 495 0.10 2100 850 1855 730 1480 585 1230 475
0.20 2085 840 1810 720 1505 580 1270 475 0.20 2035 805 1815 690 1475 560 1225 460
0.30 2030 800 1775 685 1500 565 1265 460 0.30 1965 750 1755 650 1475 545 1220 445
0.40 1940 760 1735 650 1480 535 1250 440 0.40 1885 725 1715 625 1465 510 1215 430
0.50 1865 725 1660 600 1430 505 1215 425 0.50 1780 680 1630 580 1420 490 1150 400
0.60 1740 670 1590 575 1380 475 1175 410 0.60 1690 660 1550 550 1360 460 1110 380
0.70 1645 640 1475 520 1290 450 1105 375 0.70 1575 620 1410 505 1210 405 1035 350
0.80 1540 600 1340 465 1175 405 1020 355 0.80 1375 550 1230 450 1125 380 970 325
0.90 1335 540 1170 440 1070 375 950 330 0.90 1225 520 1120 415 1050 365 885 310

Page 46
G51MP−60C−090 PERFORMANCE (Less Filter)
Air Volume / Watts at Different Blower Speeds
External Bottom Return Air, Side Return Air with Optional RAB Single Side Return Air − Air volumes in bold require field
Static Return Air Base, Return Air from Both Sides or Return fabricated transition to accommodate 20 x 25 x 1 in. air
Pressure − Air from Bottom and One Side. filter in order to maintain proper air velocity.
in. w.g. High Medium−High Medium−Low Low High Medium−High Medium−Low Low
cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2840 1450 2345 1105 1895 900 1515 700 2835 1495 2340 1155 1800 895 1440 695
0.10 2765 1415 2365 1080 1950 885 1580 700 2785 1475 2345 1135 1805 865 1515 690
0.20 2695 1385 2345 1050 1985 870 1620 695 2715 1435 2275 1080 1825 845 1560 685
0.30 2605 1335 2315 1030 1990 850 1645 690 2620 1380 2260 1035 1840 825 1600 680
0.40 2530 1300 2265 990 1990 825 1665 675 2550 1350 2230 1015 1845 815 1620 670
0.50 2420 1260 2210 955 1970 800 1675 665 2450 1305 2175 990 1850 790 1615 655
0.60 2330 1220 2145 925 1930 775 1665 650 2365 1270 2130 940 1830 775 1615 640
0.70 2250 1190 2050 885 1875 745 1645 630 2240 1205 2070 915 1815 760 1595 620
0.80 2135 1140 2000 865 1810 715 1620 615 2185 1190 1965 865 1775 745 1555 605
0.90 2030 1090 1885 830 1720 685 1560 590 2015 1150 1820 820 1690 715 1440 580

G51MP−60C−110 PERFORMANCE (Less Filter)

Air Volume / Watts at Different Blower Speeds
External Bottom Return Air, Side Return Air with Optional RAB Single Side Return Air − Air volumes in bold require field
Static Return Air Base, Return Air from Both Sides or Return fabricated transition to accommodate 20 x 25 x 1 in. air
Pressure Air from Bottom and One Side. filter in order to maintain proper air velocity.
in. w.g. High Medium−High Medium−Low Low High Medium−High Medium−Low Low
cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2720 1385 2410 1090 2055 935 1620 710 2625 1350 2310 1080 1885 885 1515 700
0.10 2665 1355 2385 1075 2025 885 1730 735 2570 1330 2325 1060 1910 865 1575 700
0.20 2585 1315 2350 1015 2030 865 1680 690 2410 1305 2285 1035 1930 845 1620 690
0.30 2505 1275 2290 990 2025 840 1695 675 2425 1265 2230 990 1925 825 1635 675
0.40 2435 1250 2235 940 2030 830 1695 660 2335 1220 2175 950 1910 810 1640 660
0.50 2350 1205 2170 930 1975 790 1735 665 2270 1195 2120 935 1895 785 1640 640
0.60 2255 1185 2100 895 1915 765 1720 650 2170 1155 2045 885 1860 765 1630 630
0.70 2160 1150 2005 840 1865 730 1680 635 2110 1130 1950 855 1795 730 1590 610
0.80 2020 1090 1905 825 1810 710 1625 610 2035 1090 1885 820 1745 705 1540 580
0.90 1910 1050 1820 795 1705 675 1540 590 1900 1055 1760 780 1665 680 1470 565

G51MP−60D−135 PERFORMANCE (Less Filter)

Air Volume / Watts at Different Blower Speeds
External Bottom Return Air, Side Return Air with Optional RAB Single Side Return Air − Air volumes in bold require field
Static Return Air Base, Return Air from Both Sides or Return fabricated transition to accommodate 20 x 25 x 1 in. air
Pressure Air from Bottom and One Side. filter in order to maintain proper air velocity.
in. w.g. High Medium−High Medium−Low Low High Medium−High Medium−Low Low
cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2730 1465 2425 1125 2055 915 1560 680 2665 1440 2325 1100 1865 890 1410 690
0.10 2670 1440 2400 1100 2065 890 1590 675 2615 1405 2310 1065 1915 865 1465 685
0.20 2600 1400 2365 1070 2045 865 1620 665 2530 1370 2280 1055 1925 850 1570 675
0.30 2525 1360 2315 1045 2035 845 1615 655 2470 1330 2235 1015 1920 825 1590 670
0.40 2445 1325 2260 1015 2020 820 1615 645 2380 1290 2175 985 1910 805 1590 655
0.50 2360 1280 2195 985 1960 790 1610 635 2310 1265 2120 965 1890 790 1595 645
0.60 2290 1255 2130 965 1900 755 1600 615 2200 1230 2055 935 1835 765 1580 630
0.70 2205 1220 2035 910 1825 730 1570 600 2120 1190 1970 900 1790 740 1545 605
0.80 2110 1195 1945 880 1765 710 1540 580 2025 1160 1890 875 1720 710 1515 590
0.90 1970 1120 1835 830 1680 690 1540 545 1930 1110 1800 835 1655 685 1440 570

Page 47
Service Winterizing and Condensate Trap Care
1 − Turn off power to the unit.
2 − Have a shallow pan ready to empty condensate water.
WARNING 3 − Remove the drain plug from the condensate trap and
empty water. Inspect the trap then reinstall the drain
ELECTRICAL SHOCK, FIRE, plug.
OR EXPLOSION HAZARD.
Cleaning Heat Exchanger
Failure to follow safety warnings exactly could result If cleaning the heat exchanger becomes necessary, follow
in dangerous operation, serious injury, death or
the below procedures and refer to figure 1 when disassem-
property damage.
Improper servicing could result in dangerous opera- bling unit. Use papers or protective covering in front of fur-
tion, serious injury, death, or property damage. nace while removing heat exchanger assembly.
Before servicing, disconnect all electrical power to 1 − Turn off electrical and gas supplies to the furnace.
furnace. 2 − Remove the upper and lower furnace access panels.
When servicing controls, label all wires prior to dis- 3 − Mark all gas valve wires and disconnect them from
connecting. Take care to reconnect wires correctly. valve.
Verify proper operation after servicing.
4 − Remove gas supply line connected to gas valve. Re-
move gas valve/manifold assembly.
At the beginning of each heating season, system should be 5 − Remove sensor wire from sensor. Disconnect 2-pin or
checked as follows by a qualified service technician: 3−pin plug from the ignitor.
Blower 6 − Disconnect wires from flame roll−out switches.
7 − Remove burner box cover and remove four burner box
Check the blower wheel for debris and clean if necessary. screws at the vestibule panel and remove burner box.
The blower motors are prelubricated for extended bearing Set burner box assembly aside.
life. No further lubrication is needed. NOTE − If necessary, clean burners at this time. Follow
procedures outlined in Burner Cleaning section.
8 − Loosen three clamps and remove flexible exhaust tee.
WARNING 9 − Remove 3/8 inch rubber cap from condensate drain
The blower access panel must be securely in place plug and drain. Replace cap after draining.
when the blower and burners are operating. Gas 10 − Disconnect condensate drain line from the conden-
fumes, which could contain carbon monoxide, can sate trap. Remove condensate trap (it may be neces-
be drawn into living space resulting in personal inju- sary to cut drain pipe). Remove screws that secure
ry or death. condensate collars to either side of the furnace and re-
move collars. Remove drain tubes from cold end
Filters header collector box.
11 − Disconnect condensate drain tubing from flue collar.
All G51MP filters are installed external to the unit. Filters Remove screws that secure both flue collars into
should be inspected monthly. Clean or replace the filters place. Remove flue collars. It may be necessary to cut
when necessary to ensure proper furnace operation. the exiting exhaust pipe for removal of the fittings.
Table 3 lists recommended filter sizes.
12 − Disconnect the 2-pin plug from the combustion air in-
Exhaust and air intake pipes ducer. Disconnect the two wires to the secondary limit,
if applicable. Remove four screws which secure com-
Check the exhaust and air intake pipes and all connections bustion air inducer to collector box Remove combus-
for tightness and to make sure there is no blockage. tion air inducer assembly. Remove ground wire from
NOTE − After any heavy snow, ice or frozen fog event the vest panel.
furnace vent pipes may become restricted. Always check 13 − Mark and disconnect all combustion air pressure tub-
the vent system and remove any snow or ice that may be ing from cold end header collector box.
obstructing the plastic intake or exhaust pipes. 14 − Mark and remove wires from pressure switches. Re-
move pressure switches. Keep tubing attached to
Electrical
pressure switches.
1 − Check all wiring for loose connections. 15 − Remove electrical junction box from the side of the fur-
nace.
2 − Check for the correct voltage at the furnace (furnace
operating). 16 − Mark and disconnect any remaining wiring to heating
compartment components. Disengage strain relief
3 − Check amp−draw on the blower motor. bushing and pull wiring and bushing through the hole in
Motor Nameplate__________Actual__________ the blower deck.

Page 48
17 − Remove the primary limit from the vestibule panel. 37 − Reconnect sensor wire and reconnect 2−pin plug from
18 − Remove two screws from the front cabinet flange at ignitor.
the blower deck. Spread cabinet sides slightly to allow 38 − Secure burner box assembly to vestibule panel using
clearance for removal of heat exchanger. four existing screws. Make sure burners line up in
19 − Remove screws along vestibule sides and bottom center of burner ports.
which secure vestibule panel and heat exchanger as- 39 − Reinstall gas valve manifold assembly. Reconnect
sembly to cabinet. Remove two screws from blower gas supply line to gas valve.
rail which secure bottom heat exchanger flange. Re- 40 − Reinstall burner box cover.
move heat exchanger from furnace cabinet.
41 − Reconnect wires to gas valve.
20 − Back wash heat exchanger with soapy water solution
42 − Replace the blower compartment access panel.
or steam. If steam is used it must be below 275°F
(135°C) . 43 − Refer to instruction on verifying gas and electrical con-
nections when re−establishing supplies.
21 − Thoroughly rinse and drain the heat exchanger. Soap
solutions can be corrosive. Take care to rinse entire 44 − Follow lighting instructions to light and operate fur-
assembly. nace for 5 minutes to ensure that heat exchanger is
clean and dry and that furnace is operating properly.
22 − Reinstall heat exchanger into cabinet making sure that
the clamshells of the heat exchanger assembly are 45 − Replace heating compartment access panel.
resting on the support located at the rear of the cabi- Cleaning the Burner Assembly
net. Remove the indoor blower to view this area 1 − Turn off electrical and gas power supplies to furnace.
through the blower opening. Remove upper and lower furnace access panels.
23 − Re-secure the supporting screws along the vestibule 2 − Mark all gas valve wires and disconnect them from the
sides and bottom to the cabinet. valve.
24 − Reinstall cabinet screws on front flange at blower 3 − Disconnect the gas supply line from the gas valve. Re-
deck. move gas valve/manifold assembly.
25 − Reinstall the primary limit on the vestibule panel. 4 − Mark and disconnect sensor wire from the sensor. Dis-
26 − Route heating component wiring through hole in blow- connect 2-pin plug from the ignitor at the burner box.
er deck and reinsert strain relief bushing. 5 − Remove burner box cover and remove four screws
27 − Reinstall pressure switches and reconnect pressure which secure burner box assembly to vest panel. Re-
switch wiring. move burner box from the unit.
28 − Carefully connect combustion air pressure switch 6 − Use the soft brush attachment on a vacuum cleaner to
hosing from pressure switches to proper stubs on gently clean the face of the burners. Visually inspect
cold end header collector box. the inside of the burners and crossovers for any block-
29 − Reinstall condensate collars on each side of the fur- age caused by foreign matter. Remove any blockage.
nace. Reconnect drain tubing to collector box. 7 − Reconnect the sensor wire and reconnect the 2−pin
30 − Reinstall condensate trap on same side as exhaust plug to the ignitor wiring harness.
pipe. Reconnect condensate drain line to the conden- 8 − Reinstall the burner box assembly using the existing
sate trap. four screws. Make sure that the burners line up in the
31 − Reinstall electrical junction box. center of the burner ports.
32 − Reinstall the combustion air inducer. Reconnect the 9 − Reinstall the gas valve manifold assembly. Reconnect
2−pin or 3−pin plug to the wire harness. Reconnect the the gas supply line to the gas valve. Reinstall the burn-
two wires to the secondary limit, if applicable. er box cover.
33 − Use securing screws to reinstall flue collars to either 10 − Reconnect the gas valve wires to the gas valve.
side of the furnace. Reconnect exhaust piping and ex- 11 − Replace the blower compartment access panel.
haust drain tubing. 12 − Refer to instruction on verifying gas and electrical con-
34 − Replace flexible exhaust tee on combustion air induc- nections when re−establishing supplies.
er and flue collars. Secure using three existing hose 13 − Follow lighting instructions to light and operate fur-
clamps. nace for 5 minutes to ensure that heat exchanger is
35 − Reinstall burner box assembly in vestibule area. clean and dry and that furnace is operating properly.
36 − Reconnect flame roll−out switch wires. 14 − Replace heating compartment access panel.

Page 49
Planned Service Return air duct − Must be properly attached and provide
an air seal to the unit.
A service technician should check the following items dur-
ing an annual inspection. Power to the unit must be shut off Operating performance − Unit must be observed during
for safety. operation to monitor proper performance of the unit and the
vent system.
Fresh air grilles and louvers (on the unit and in the room
where the furnace is installed) − Must be open and unob- Combustion gases − Flue products must be analyzed and
structed to provide combustion air. compared to the unit specifications.

Burners − Must be inspected for rust, dirt, or signs of water. Problems detected during the inspection may make it nec-
essary to temporarily shut down the furnace until the items
Vent pipe − Must be inspected for signs of water, damaged,
can be repaired or replaced.
cracked or sagging pipe, or disconnected joints.
Instruct the homeowners to pay attention to their
Unit appearance − Must be inspected for rust, dirt, signs
furnace. Situations can arise between annual furnace in-
of water, burnt or damaged wires, or components.
spections that may result in unsafe operation. For instance,
Blower access door − Must be properly in place and pro- items innocently stored next to the furnace may obstruct
vide a seal between the return air and the room where the the combustion air supply. This could cause incomplete
furnace is installed. combustion and the production of carbon monoxide gas.

Repair Parts List

The following repair parts are available through Lennox dealers. When ordering parts, include the complete furnace model
number listed on the CSA nameplate −− Example: G51MP−24B−045−1. All service must be performed by a licensed pro-
fessional installer (or equivalent), service agency, or gas supplier.
Cabinet Parts Heating Parts
Upper access panel Flame Sensor
Blower access panel Heat exchanger assembly
Top cap Gas manifold
Control Panel Parts Combustion air inducer
Transformer Gas valve
SureLight® integrated control board Main burner cluster
Door interlock switch Main burner orifices
Circuit breaker Pressure switches
Blower Parts Ignitor
Primary limit control
Blower wheel
Secondary limit control
Motor
Flame rollout switches
Motor mounting frame
Combustion air inducer auxiliary limit
Motor capacitor
Blower housing cutoff plate

Page 50
SureLight® Troubleshooting: Heating Sequence of Operation

SURELIGHT CONTROL
HEATING SEQUENCE OF OPERATION

NORMAL HEATING MODE ABNORMAL HEATING MODE

POWER ON
GAS VALVE OFF. COMBUSTION AIR INDUCER ON.
INDOOR BLOWER ON.
CONTROL SELF−CHECK OKAY? DS1 ALTERNATING FAST FLASH
NO DS2 ALTERNATING FAST FLASH
CHECK FOR BROKEN IGNITOR OR
OPEN IGNITOR CIRCUIT

GAS VALVE OFF. COMBUSTION AIR INDUCER OFF.

YES INDOOR BLOWER DELAY OFF.
IS POLARITY REVERSED? DS1 ON
DS2 ON
NO (RESET CONTROL BY TURNING MAIN POWER OFF.)

POLARITY REVERSED.
DS1 −− FAST FLASH
IS VOLTAGE NO DS2 −− SLOW FLASH
ABOVE 75 VOLTS?
YES DS1 AND DS2 ALTERNATING FAST
NO FLASH. SIGNAL HOLDS UNTIL VOLTAGE
ROLLOUT SWITCHES CLOSED? RISES ABOVE 75 VOLTS.
YES
GAS VALVE OFF. COMBUSTION AIR INDUCER OFF.
INDOOR BLOWER OFF WITH DELAY.
NO DS1 −− ON. DS2 −− SLOW FLASH.
BURNER OFF?
(CONTINUOUS FLAME CHECK) SEQUENCE HOLDS UNTIL ROLLOUT SWITCH CLOSES.

YES
NORMAL OPERATION:
DS1 −− SLOW FLASH GAS VALVE OFF. COMBUSTION AIR INDUCER ON.
DS2 −− SLOW FLASH INDOOR BLOWER ON HEATING SPEED.
DS1 −− SLOW FLASH
DS2 −− OFF
THERMOSTAT CALLS FOR HEAT:
DS1 −− FAST FLASH
DS2 −− FAST FLASH
GAS VALVE OFF.
YES COMBUSTION AIR INDUCER OFF.
NO INDOOR BLOWER OFF WITH DELAY.
ARE COMBUSTION AIR
PRESSURE SWITCHES OPEN? DS1 OFF
DS2 SLOW FLASH
YES (Sequence holds until pressure switch
IS COMBUSTION AIR INDUCER ENERGIZED? closes or thermostat resets control.)
(HUM TERMINALS ARE ENERGIZED WITH CAI)
YES
PRESSURE SWITCHES IN WATCHGUARD MODE.
HAVE COMBUSTION AIR PRESSURE NO GAS VALVE OFF. COMBUSTION AIR INDUCER OFF.
SWITCHES CLOSED IN 2.5 MINUTES? INDOOR BLOWER OFF WITH DELAY.
DS1 −− OFF. DS2 −− SLOW FLASH.
YES IS 5-MINUTE RESET PERIOD COMPLETE?
CONTINUED NEXT PAGE

Page 51
SureLight® Troubleshooting: Heating Sequence of Operation (Continued)

NORMAL HEATING MODE ABNORMAL HEATING MODE

15-SECOND COMBUSTION AIR INDUCER PREPURGE
INITIATED BY CLOSED PRESSURE SWITCH.
IS VOLTAGE ABOVE 75 VOLTS? NO
YES
NO YES DS1 & DS2
NO
IGNITOR WARM-UP −− 20 SECONDS. IS THERE A PROPER GROUND? SIGNAL
ALTERNATING
YES YES YES NO FAST FLASH.
IS IGNITOR INTACT AND CONNECTED?
4-SECOND TRIAL FOR IGNITION.
GAS VALVE OPENS. IGNITOR ENERGIZED FOR
UP TO 4 SECONDS.
GAS VALVE OFF. COMBUSTION AIR INDUCER ON.
INDOOR BLOWER OFF.
YES NO HAS CONTROL FAILED TO SENSE FLAME FOR NO

FLAME STABILIZATION PERIOD. FIVE CONSECUTIVE TRIES DURING A SINGLE

HEAT DEMAND?
4 SECONDS
YES
FLAME RECTIFICATION CURRENT IGNITION WATCHGUARD MODE. GAS VALVE OFF.
CHECK. CAN FLAME BE PROVEN WITHIN COMBUSTION AIR INDUCER OFF.
4 SECONDS AFTER GAS VALVE OPENS? INDOOR BLOWER OFF WITH DELAY
(u0.24 microamps) DS1 ALT. SLOW FLASH DS2 ALT. SLOW FLASH
IS 60-MINUTE RESET PERIOD COMPLETE?
YES
YES
NO NO
FLAME PRESENT? HAS CONTROL RESET IGNITION
SEQUENCE FOUR TIMES?

YES
NO LOW FLAME SIGNAL
(Does not affect operation of control) YES
FLAME SIGNAL ABOVE 0.34 MICROAMPS? DS1 −− SLOW FLASH
YES DS2 −− FAST FLASH LIMIT SWITCH WATCH-
INDOOR BLOWER ON DELAY BEGINS GUARD MODE. GAS
(45 seconds.) GAS VALVE DE−ENERGIZED. VALVE OFF. COMB. AIR
COMBUSTION AIR INDUCER INDUCER OFF. INDOOR
YES DE−ENERGIZED.INDOOR BLOWER BLOWER OFF WITH
PRIMARY AND SECONDARY LIMIT NO ON UNTIL SWITCHES CLOSE. DS1 DELAY. DS1 − SLOW
SWITCHES CLOSED? −− SLOW FLASH. DS2 −− ON. FLASH DS2 − ON. IS
COMBUSTION AIR INDUCER IS LIMIT SWITCH CLOSED? 60MINUTE RESET
AUXILIARY LIMIT SWITCH CLOSED? PERIOD COMPLETE?
YES
HAVE PRIMARY/SECONDARY/CAI NO
AUX. LIMITS RESET (CLOSED)
YES WITHIN 3 MINUTES? YES

GAS VALVE POWER OFF.

NO COMBUSTION AIR INDUCER POWER ON.
ROLLOUT SWITCHES CLOSED? INDOOR BLOWER ON WITH DELAY
DS1 −− ON. DS2 −− SLOW FLASH.
YES SEQUENCE HOLDS UNTIL ROLLOUT SWITCH AND
THERMOSTAT IS RESET.
COMBUSTION AIR PRESSURE NO
SWITCH CLOSED?
YES
YES GAS VALVE DE−ENERGIZED.
COMBUSTION AIR INDUCER ON. YES
THERMOSTAT DEMAND SATISFIED. INDOOR BLOWER OFF WITH DELAY
YES DS1 −− OFF. DS2 −− SLOW FLASH.
HAS CAB SWITCH CLOSED IN 2.5 MINUTES?
DS1 & DS2 SIMULTANEOUS SLOW FLASHES.
NO
YES
COMB. AIR INDUCER CONTINUES 5-SECOND 5-MINUTE PRESSURE SWITCH YES
POST PURGE AFTER T’STAT DEMAND IS SATISFIED. WATCHGUARD MODE.
INDOOR AIR BLOWER COMPLETES SELECTED OFF"
DELAY BEFORE SHUTTING OFF.

Page 52
SureLight® Troubleshooting: Cooling Sequence of Operation

SURELIGHT CONTROL
COOLING SEQUENCE OF OPERATION
NORMAL COOLING MODE ABNORMAL COOLING MODE

POWER ON.

IGNITION CONTROL MAIN POWER ON.

GAS VALVE OFF. COMBUSTION AIR INDUCER OFF.

CONTROL SELF DIAGNOSTIC CHECK. NO INDOOR BLOWER OFF WITH NORMAL DELAY.
IS CONTROL OPERATING NORMALLY? SIGNAL CIRCUIT BOARD FAILURE AT DS1 & DS2.
INTERRUPT MAIN POWER TO RESET CONTROL.
YES

YES
IS POLARITY REVERSED? DS1 FAST FLASH DS2 SLOW FLASH.

NO
DS1 AND DS2 ALTERNATING FAST
IS VOLTAGE NO
FLASH. SIGNAL HOLDS UNTIL VOLTAGE
ABOVE 75 VOLTS? RISES ABOVE 75 VOLTS.

YES
NO GAS VALVE OFF. COMBUSTION AIR INDUCER OFF.
ROLLOUT SWITCHES MONITORED CONTINUOUSLY. INDOOR BLOWER OFF WITH NORMAL DELAY.
ARE ROLLOUT SWITCHES CLOSED? SIGNAL CIRCUIT BOARD FAILURE AT DS1 & DS2.
SEQUENCE HOLDS UNTIL ROLLOUT SWITCH CLOSES.
YES

DS1 & DS2: SLOW FLASH RATE REMAINS

UNCHANGED THROUGHOUT COOLING CYCLE.

THERMOSTAT CALLS FOR COOLING.

COMPRESSOR CONTACTOR AND SYSTEM FAN

ENERGIZED WITH 2-SECOND DELAY. COOL
SPEED TERMINAL IS ENERGIZED. EAC
TERMINAL IS ENERGIZED.

THERMOSTAT OPENS.

COMPRESSOR OFF.

SYSTEM FAN AND EAC TERM. OFF

WITH 2− OR 45-SECOND DELAY.

Page 53
 SureLight® Troubleshooting: Continuous Fan Sequence of Operation
SURELIGHT CONTROL
CONTINUOUS FAN SEQUENCE OF OPERATION
DS1 & DS2: SLOW FLASH RATE REMAINS
UNCHANGED THROUGHOUT SEQUENCE.

MANUAL FAN SELECTION MADE AT THERMOSTAT.

CONTROL (G) ENERGIZES INDOOR BLOWER AT
FAN SPEED. EAC TERM. IS ENERGIZED.
HUM (120 VAC & 24VAC)
THERMOSTAT CALLS FOR HEAT (W). TERMINALS ENERGIZED
WITH COMB. AIR INDUCER.
NO YES
NO
THERMOSTAT CALLS FOR COOLING.
INDOOR BLOWER SWITCHES TO HEAT SPEED
YES AFTER NORMAL DELAY.

INDOOR BLOWER SWITCHES TO COOL SPEED.

THERMOSTAT OPENS. COMBUSTION AIR INDUCER
AND HUM TERMINALS ARE DE−ENERGIZED
FOLLOWING 5−SECOND POST−PURGE.
THERMOSTAT OPENS.

INDOOR BLOWER SELECTED TIME OFF DELAY BEGINS.

AFTER DELAY, HEAT SPEED DE−ENERGIZED.
(Blower remains energized during continuous fan mode).

INDOOR BLOWER SWITCHES TO SELECTED

BLOWER SPEED. EAC TERM. REMAINS
ENERGIZED.

Ignition Control Board Diagnostic Codes

DIAGNOSTIC CODES
Make sure to Identify LED’S Correctly.
Refer to Installation Instructions for control board layout.
DS1 DS2
DESCRIPTION
(Red or Green) (Green)
SIMULTANEOUS SIMULTANEOUS Power on −− Normal operation.
SLOW FLASH SLOW FLASH Also signaled during cooling and continuous fan.
SIMULTANEOUS SIMULTANEOUS
Normal operation −− Signaled when heating demand initiated at thermostat.
FAST FLASH FAST FLASH
Primary or secondary limit switch open. Limit must close within 3 minutes or unit goes into
SLOW FLASH ON
1−hour Watchguard.
Pressure switch open;
OR: Blocked inlet/exhaust vent;
OFF SLOW FLASH
OR: Condensate line blocked;
OR: Pressure switch closed prior to activation of combustion air inducer.
ALTERNATING ALTERNATING Watchguard −− Burners failed to ignite, limit open longer than 3 minutes, or flame sense lost 5
SLOW FLASH SLOW FLASH times in one heating cycle.
SLOW FLASH OFF Flame sensed without gas valve energized.
ON SLOW FLASH Rollout switch open.
ON ON
Circuit board failure or control wired incorrectly. Check 24−volt and 115−volt connections to
ON OFF
board.
OFF ON
FAST FLASH SLOW FLASH Main power polarity reversed. Switch line and neutral. Improper main ground.
SLOW FLASH FAST FLASH Low flame signal. Check flame sensor.
The following conditions are sensed during the ignitor warm−up period only:
ALTERNATING ALTERNATING 1) Improper main ground;
FAST FLASH FAST FLASH 2) Broken ignitor; OR: Open ignitor circuit;
3) Line voltage below 75 volts.
NOTE − Slow flash rate equals 1 Hz (one flash per second). Fast flash rate equals 3 Hz (three flashes per second).
Drop out flame sense current = 0.15 − 0.24 microAmps.

Page 54
Vent Pipe Sizing Worksheet

Step 1 Proposed vent pipe size : ______ Equivalent Feet

Step 2 Termination kit catalog number : _____________
Vent pipe equivalency value from table 5 : ______
Step 3 Total number of 90° elbows required (indoors and outdoors)
______ X 5 = ______ equivalent feet of pipe
Step 4 Total number of 45° elbows required (indoors and outdoors)
______ X 2.5 = ______ equivalent feet of pipe
Step 5 Linear feet of straight pipe required : ______
Add equivalent feet of vent pipe listed in TOTAL
Step 6
steps 2 through 5.

If the total is equal to, or less than, the allowable maximum given in table 8, the proposed pipe size is acceptable. If the
total exceeds the maximum allowed vent pipe length, repeat the process above using the next larger diameter pipe until
an acceptable total is achieved.
NOTE − In Direct Vent systems, total the equivalent length of either the exhaust OR intake piping run, depending upon which
will be LONGER. Intake and exhaust pipe diameter must be the same size and must be terminated in the same pressure
zone. Intake and exhaust pipe should be roughly the same length.

G51MP Start−Up & Performance Check List

Job Name Job No. Date

Job Location City State
Installer City State
Unit Model No. Technician
Serial No.
Heating Section
Electrical Connections Tight?
Supply Voltage Blower Motor H.P.
Blower Motor Amps Gas Piping Connections
Tight & Leak−Tested?
Fuel Type: Natural Gas? LP/Propane Gas?
Furnace Btu Input
Line Pressure
Regulator Pressure w.c. − Nat.: w.c. − LP/Propane
Flue Connections Tight? Proper Draft?
Condensate Connections Tight?
Combustion Gas Tested? CO2 CO
Fan Control Setting
(45 Seconds Fixed On)
Fan Control Off Setting Temperature Rise
Filter Clean & Secure? Vent Clear?
Thermostat
Calibrated? Heat Anticipator Properly Set? Level?

Page 55
Requirements for Commonwealth of Massachusetts
Modifications to NFPA−54, Chapter 10 4 − INSPECTION. The state or local gas inspector of the
Revise NFPA−54 section 10.8.3 to add the following re- side wall, horizontally vented, gas−fueled equipment
quirements: shall not approve the installation unless, upon inspec-
tion, the inspector observes carbon monoxide detec-
For all side wall, horizontally vented, gas−fueled equipment
tors and signage installed in accordance with the pro-
installed in every dwelling, building or structure used in
visions of 248 CMR 5.08(2)(a) 1 through 4.
whole or in part for residential purposes, including those
EXEMPTIONS: The following equipment is exempt
owned or operated by the Commonwealth and where the
from 24 CMR 5.08(2)(a) 1 through 4:
side wall exhaust vent termination is less than seven (7)
1 − The equipment listed in Chapter 10 entitled Equip-
feet above the finished grade in the area of the venting, in-
ment Not Required to Be Vented" in the most current
cluding but not limited to decks and porches, the following
edition of NFPA 54 as adopted by the Board; and
requirements shall be satisfied:
2 − Product Approved side wall, horizontally vented, gas−
1 − INSTALLATION OF CARBON MONOXIDE DETEC- fueled equipment installed in a room or structure sepa-
TORS. At the time of installation of the side wall, hori- rate from the dwelling, building or structure used in
zontally vented, gas−fueled equipment, the installing whole or in part for residential purposes.
plumber or gasfitter shall observe that a hard−wired
MANUFACTURER REQUIREMENTS −
carbon monoxide detector with an alarm and battery
GAS EQUIPMENT VENTING SYSTEM PROVIDED.
backup is installed on the floor level where the gas
equipment is to be installed. In addition, the installing When the manufacturer of Product Approved side wall,
plumber or gasfitter shall observe that a battery−oper- horizontally vented, gas−fueled equipment provides a vent-
ated or hard−wired carbon monoxide detector with an ing system design or venting system components with the
alarm is installed on each additional level of the dwell- equipment, the instructions provided by the manufacturer
ing, building or structure served by the side wall, hori- for installation of the equipment and the venting system
zontally vented, gas−fueled equipment. It shall be the shall include:
responsibility of the property owner to secure the ser-
vices of qualified licensed professionals for the instal- 1 − Detailed instructions for the installation of the venting
lation of hard−wired carbon monoxide detectors. system design or the venting system components:
and
a − In the event that the side wall, horizontally vented,
gas−fueled equipment is installed in a crawl space 2 − A complete parts list for the venting system design or
or an attic, the hard−wired carbon monoxide detec- venting system.
tor with alarm and battery back−up may be installed
MANUFACTURER REQUIREMENTS −
on the next adjacent floor level.
GAS EQUIPMENT VENTING SYSTEM NOT PROVIDED.
b − In the event that the requirements of this subdivi-
sion cannot be met at the time of completion of When the manufacturer of Product Approved side wall,
installation, the owner shall have a period of thirty
horizontally vented, gas−fueled equipment does not pro-
(30) days to comply with the above requirements;
provided, however, that during said thirty (30) day vide the parts for venting the flue gases, but identifies spe-
period, a battery−operated carbon monoxide detec- cial venting systems," the following requirements shall be
tor with an alarm shall be installed. satisfied by the manufacturer:
2 − APPROVED CARBON MONOXIDE DETECTORS.
Each carbon monoxide detector as required in accor- 1 − The referenced special venting system" instructions
dance with the above provisions shall comply with shall be included with the appliance or equipment
NFPA 720 and be ANSI/UL 2034 listed and IAS certi- installation instructions; and
fied. 2 The special venting systems" shall be Product Ap-
3 − SIGNAGE. A metal or plastic identification plate shall proved by the Board, and the instructions for that sys-
be permanently mounted to the exterior of the building tem shall include a parts list and detailed installation
at a minimum height of eight (8) feet above grade di- instructions.
rectly in line with the exhaust vent terminal for the hori- A copy of all installation instructions for all Product
zontally vented, gas−fueled heating appliance or Approved side wall, horizontally vented, gas−fueled
equipment. The sign shall read, in print size no less equipment, all venting instructions, all parts lists for
than one−half (1/2) inch in size, GAS VENT DIRECT- venting instructions, and/or all venting design instruc-
LY BELOW. KEEP CLEAR OF ALL OBSTRUC- tions shall remain with the appliance or equipment at
TIONS." the completion of the installation.

Page 56