PDHonline Course M196 (4 PDH)

HVAC Made Easy: A Guide to Heating

& Cooling Load Estimation

Instructor: A. Bhatia, B.E.

2020

PDH Online | PDH Center

5272 Meadow Estates Drive
Fairfax, VA 22030-6658
Phone: 703-988-0088
www.PDHonline.com

An Approved Continuing Education Provider

www.PDHcenter.com PDH Course M196 www.PDHonline.org

HVAC Made Easy: A Guide to Heating & Cooling Load Estimation

Course Content

AIR CONDITIONING SYSTEM OVERVIEW

Cooling & heating load calculations are normally made to size HVAC (heating, ventilating, and air-conditioning) systems
and their components. In principle, the loads are calculated to maintain the indoor design conditions. The first step in
any load calculation is to establish the design criteria for the project that involves consideration of the building concept,
construction materials, occupancy patterns, density, office equipment, lighting levels, comfort ranges, ventilations and
space specific needs. Architects and other design engineers converse at early stages of the project to produce design
basis & preliminary architectural drawings. The design basis typically includes information on:
1) Geographical site conditions (latitude, longitude, wind velocity, precipitation etc.)
2) Outdoor design conditions (temperature, humidity etc)
3) Indoor design conditions
4) Building characteristics (materials, size, and shape)
5) Configuration (location, orientation and shading)
6) Operating schedules (lighting, occupancy, and equipment)
7) Additional considerations (type of air-conditioning system, fan energy, fan location, duct heat loss and gain, duct
leakage, type and position of air return system…)

Climate data requirements

One of the most important things in building HVAC design is the climate you are designing. Let’s make first distinction in
terms “weather” and “climate”.
"Weather" is the set of atmospheric conditions prevailing at a given place and time. "Climate" can be defined as the
integration in time of weather conditions, characteristics of a certain geographical location. At the global level climates
are formed by the differential solar heat input and the uniform heat emission over the earth's surface.
Climate has a major effect on building performance, HVAC design and energy consumption. It is also pertinent to the
assessment of thermal comfort of the occupants. The key objectives of climatic design include:
1) To reduce energy cost of a building
2) To use "natural energy" as far as possible instead of mechanical system and power
3) To provide comfortable and healthy environment for people

Classification of climates
Many different systems of climate classification are in use for different purposes. Climatic zones such as tropical, arid,
temperature and cool are commonly found for representing climatic conditions. For the purposes of building design a
simple system based on the nature of the thermal problem in the particular location is often used.
1) Cold climates, where the main problem is the lack of heat (under heating), or excessive heat dissipation for all or
most parts of the year.
2) Temperate climates, where there is a seasonal variation between under heating and overheating, but neither is
very severe.
3) Hot-dry (arid) climates, where the main problem is overheating, but the air is dry, so the evaporative cooling
mechanism of the body is not restricted. There is usually a large diurnal (day - night) temperature variation.

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4) Warm-humid climates, where the overheating is not as great as in hot-dry areas, but it is aggravated by very high
humidity’s, restricting the evaporation potential. The diurnal temperature variation is small.

Six categories of climates:

1) Warm-humid - 15°N and South of the equator, e.g. Lagos, Mombassa, Colombo, Jakarta etc.
2) Warm-humid Island - equatorial and trade wind zones, e.g. Caribbean, Philippines and Pacific Islands etc.
3) Hot-dry desert - 15° to 30° North and South, e.g. Baghdad, Alice Springs, Phoenix etc.
4) Hot-dry maritime desert - latitudes as (3), coastal large landmass, Kuwait, Karachi etc.
5) Composite Monsoon - Tropic Cancer/Capricorn, Lahore, Mandalay, New Delhi etc.
6) Tropical uplands - Tropic Cancer/Capricorn, 900 to 1200 meters above sea level (plateau and mountains), Addis
Ababa, Mexico City, Nairobi etc.

Load Calculations Methods

Before one can design an efficient and effective air conditioning system, the load must first be calculated using
established techniques. There are various methods in use. The most basic of these methods is a rule-of-thumb value --
for example, square feet of floor area per ton of cooling. The "square-foot-per-ton" sizing method avoids calculating the
cooling load of the building and proceeds directly from the square footage of the building. While this approach is rapid
and simple, it does not account for orientation of the walls and windows, the difference in surface area between a one-
story and a two-story home of the same floor area, the differences in insulation and air leakage between different
buildings, the number of occupants, and many other factors. Such rules-of-thumb are useful in schematic design as a
means of getting an approximate handle on equipment size and cost.
The more refined methods available in the HVAC handbooks are:
1) Total Equivalent Temperature Difference/Time Average (TETD/TA)
2) Cooling Load Temperature Difference/Cooling Load Factor (CLTD/CLF)
3) Transfer Function Method (TFM)
4) Heat Balance (HB) & Radiant Time Series (RTS)
5) Manual J Method for Residential Applications & Manual N for Commercial Buildings: These methods are simplified
versions, jointly developed by Air conditioning contractors of America (ACCA) and the Air conditioning and
Refrigeration Institute (ARI).
These different methods may yield different results for the same input data. This is primarily due to the way; each
method handles the solar effect and building dynamics. But in true sense all the above approaches attempt to consider
the fundamental principle that heat flow rates are not instantaneously converted to loads and heat addition or extraction
incident upon the building do not immediately result in a change in temperature. Thermally heavy buildings can
effectively delay the cooling or heating load for several hours.
Most designers use the TETD and CLTD methods because these methods are simple to use, give component loads
and tend to predict load on conservative side. The most recent versions of the ASHRAE Fundamentals Handbook
(2001) provide more detailed discussion on the Radiant Time Series (RTS) and Heat Balance (HB) methods. The Heat
Balance method is the most accurate but is very laborious and cumbersome and is more suitable with the use of
computer programs. The RTS is a simplified method derived from heat balance (HB) method and effectively replaces
all other simplified (non-heat balanced) methods.
For strictly manual cooling loads calculation method, the most practical to use is the CLTD/CLF method. This course
discusses CLTD/CLF method in detail in succeeding sections.
A number of handbooks provide a good source of design information and criteria to use for CLTD/CLF method;
however engineering judgment is required in the interpretation of various custom tables and applying appropriate
correction factors. It is not the intent of this course to duplicate information but rather to provide a direction regarding the
proper use or application of the available data so that the engineers and designers can make an appropriate decision.

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PART 1 SUMMER COOLING LOAD

Preface
The term summer cooling load means much more than merely cooling the air in a building. In addition to cooling the air,
it also implies controlling:
1) The relative humidity
2) Providing proper ventilation
3) Filtering out contaminants (air cleaning) and
4) Distributing the conditioned air to the lived-in spaces in proper amounts, without appreciable drafts or objectionable
noise
This section deals with the design aspects and the equations used for summer cooling load calculations.

Design Conditions
The amount of cooling that has to be accomplished to keep buildings comfortable in hot summer depends on the
desired condition indoors and on the outdoor conditions on a given day. These conditions are, respectively, termed the
“indoor design condition” and the “outdoor design condition”.

Indoor Design Conditions

The indoor design conditions are directly related to human comfort. Current comfort standards, ASHRAE Standard 55-
1992 [4] and ISO Standard 7730 [5], specify a “comfort zone,” representing the optimal range and combinations of
thermal factors (air temperature, radiant temperature, air velocity, humidity) and personal factors (clothing and activity
level) with which at least 80% of the building occupants are expected to express satisfaction. As a general guideline for
summer air-conditioning design, the thermal comfort chapter of the ASHRAE fundamentals handbook (Chapter 8, 2001)
provides a snapshot of the psychrometric chart for the summer and winter comfort zones.
For most of the comfort systems, the recommended indoor temperature and relative humidity are:
1) Summer: 73 to 79°F; The load calculations are usually based at 75ºF dry bulb temperatures & 50% relative
humidity
2) Winter: 70 to 72°F dry bulb temperatures, 20 - 30 % relative humidity
The standards were developed for mechanically conditioned buildings typically having overhead air distribution systems
designed to maintain uniform temperature and ventilation conditions throughout the occupied space. The Psychrometric
chapter of the Fundamentals Handbook (Chapter 6, 2001) provides more details on this aspect.

Outdoor Design Conditions

Outdoor design conditions are determined from published data for the specific location, based on weather bureau or
airport records. Basic climatic and HVAC “design condition” data can be obtained from ASHRAE handbook, which
provides climatic conditions for 1459 locations in the United States, Canada and around the world. The information
includes values of dry-bulb, wet-bulb and dew-point temperature and wind speed with direction on percentage
occurrence basis.
Design conditions for the United States appear in Table 1a and 1b, for Canada in Tables 2a and 2b, and the
international locations in Tables 3a and 3b of 1997, ASHRAE fundamentals handbook chapter 26.
The information provided in table 1a, 2a and 3a are for heating design conditions that include:
1) Dry bulb temperatures corresponding to 99.6% and 99% annual cumulative frequency of occurrence,
2) Wind speeds corresponding to 1%, 2.5% and 5% annual cumulative frequency of occurrence,
3) Wind direction most frequently occurring with 99.6% and 0.4% dry-bulb temperatures and
4) Average of annual extreme maximum and minimum dry-bulb temperatures and standard deviations.

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The information provided in table 1b, 2b and 3b are for cooling and humidity control conditions that include:
1) Dry bulb temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and the
mean coincident wet-bulb temperature (warm). These conditions appear in sets of dry bulb (DB) temperature and
the mean coincident wet bulb (MWB) temperature since both values are needed to determine the sensible and
latent (dehumidification) loads in the cooling mode.
2) Wet-bulb temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and the
mean coincident dry-bulb temperature
3) Dew-point temperature corresponding to 0.4%, 1.0% and 2.0% annual cumulative frequency of occurrence and the
mean coincident dry-bulb temperature and humidity ratio (calculated for the dew-point temperature at the standard
atmospheric pressure at the elevation of the station).
4) Mean daily range (DR) of the dry bulb temperature, which is the mean of the temperature difference between daily
maximum and minimum temperatures for the warmest month (highest average dry-bulb temperature). These are
used to correct CLTD values.
In choosing the HVAC outdoor design conditions, it is neither economical nor practical to design equipment either for
the annual hottest temperature or annual minimum temperature, since the peak or the lowest temperatures might occur
only for a few hours over the span of several years. Economically speaking short duration peaks above the system
capacity might be tolerated at significant reductions in first cost; this is a simple risk - benefit decision for each building
design. Therefore, as a practice, the ‘design temperature and humidity’ conditions are based on frequency of
occurrence. The summer design conditions have been presented for annual percentile values of 0.4, 1 and 2% and
winter month conditions are based on annual percentiles of 99.6 and 99%.
The term “design condition” refers to the %age of time in a year (8760 hours), the values of dry-bulb, dew-point and
wet-bulb temperature exceed by the indicated percentage. The 0.4%, 1.0%, 2.0% and 5.0% values are exceeded on
average by 35, 88, 175 and 438 hours.
The 99% and 99.6% cold values are defined in the same way but are viewed as the values for which the corresponding
weather element are less than the design condition 88 and 35 hours, respectively. 99.6% value suggests that the
outdoor temperature is equal to or lower than design data 0.4% of the time.
Design condition is used to calculate maximum heat gain and maximum heat loss of the building. For comfort cooling,
use of the 2.5% occurrence and for heating use of 99% values is recommended.
The 2.5% design condition means that the outside summer temperature and coincident air moisture content will be
exceeded only 2.5% of hours from June to September or 73 out of 2928 hours (of these summer months) or 2.5% of
the time in a year, the outdoor air temperature will be above the design condition.

Cooling Loads Classified by Source

Cooling loads fall into the following categories, based on their sources:
1) Heat transfer (gain) through the building skin by conduction, as a result of the outdoor-indoor temperature
difference.
2) Solar heat gains (radiation) through glass or other transparent materials.
3) Heat gains from ventilation air and/or infiltration of outside air.
4) Internal heat gains generated by occupants, lights, appliances, and machinery.

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In cooling load calculation, there are four related heat flow terms; 1) space heat gain, 2) space cooling load, 3) space
heat extraction rate and 4) cooling coil load.
What does these terms mean?
1) The heat gain for a building is a simultaneous summation of all external heat flows plus the heat flows generated
inside the building. The heat gain varies throughout the 24 hours of the day, as the solar intensity, occupancy;
lights, appliances etc keep varying with time.
2) The cooling load is an hourly rate at which heat must be removed from a building in order to hold the indoor air
temperature at the design value. In other words, cooling load is the capacity of equipment required to account for
such a load. Theoretically, it may seem logical to address that the space heat gain is equivalent to space cooling
load but in practice “Heat gain ≠ cooling load.”
The primary explanation for this difference is the time lag or thermal storage affects of the building elements. Heat
gains that enter a building are absorbed/stored by surfaces enclosing the space (walls, floors and other interior
elements) as well as objects within the space (furniture, curtains etc.) These elements radiates into the space even
after the heat gain sources are no longer present. Therefore the time at which the space may realize the heat gain
as a cooling load is considerably offset from the time the heat started to flow. This thermal storage effect is critical
in determining the instantaneous heat gain and the cooling load of a space at a particular time. Calculating the
nature and magnitude of these re-radiated loads to estimate a more realistic cooling load is described in the
subsequent sections.

Convective Component Heat Extraction by

Instantaneous Instantaneous
Heat Gain Equipment
Cooling Load

Radiative Component Convection (with time delay)

Furnishings, structure,
variable heat storage

Schematic Relation of Heat Gain to Cooling Load

The convective heat flows are converted to space cooling load instantaneously whereas radiant loads tend to be
partially stored in a building. The cooling load for the space is equal to the summation of all instantaneous heat

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gain plus the radiant energy that has been absorbed by surfaces enclosing the space as well as objects within the
space. Thus heat gain is often not equal to cooling load.
In heating load calculations however, the instantaneous heat loss from the space can be equated to the space-
heating load and it can be use directly to size the heating equipment.
3) The space heat extraction rate is usually the same as the space-cooling load but with an assumption that the
space temperature remains constant.
4) The cooling coil load is the summation of all the cooling loads of the various spaces served by the equipment plus
any loads external to the spaces such as duct heat gain, duct leakage, fan heat, and outdoor makeup air.

Cooling Loads Classified by Kinds of Heat

There are two distinct components of the air conditioning load; (1) the sensible load (heat gain) and (2) the latent load
(water vapor gain).

Sensible Loads
Sensible heat gain is the direct addition of heat to a space,which shall result in increase in space temperatures. The
factors influencing sensible cooling load:
1) Solar heat gain through building envelope (exterior walls, glazing, skylights, roof, floors over crawl space)
2) Partitions (that separate spaces of different temperatures)
3) Ventilation air and air infiltration through cracks in the building, doors, and windows
4) People in the building
5) Equipment and appliances operated in the summer
6) Lights

Latent Loads
A latent heat gain is the heat contained in water vapor. Latent heat does not cause a temperature rise, but it constitutes
a load on the cooling equipment. Latent load is the heat that must be removed to condense the moisture out of the air.
The sources of latent heat gain are:
1) People (breathing)
2) Cooking equipment
3) Housekeeping, floor washing etc.
4) Appliances or machinery that evaporates water
5) Ventilation air and air infiltration through cracks in the building, doors, and windows
The total cooling load is the summation of sensible and latent loads.

Cooling Loads Classified by Inside-Outside Environment

Buildings can be classified as envelope-load-dominated and interior-load-dominated. The envelope heat flows are
termed “external loads”, in that they originate with the external environment. The other loads are termed “internal loads”,
in that they are generated from within the building itself. The percentage of external versus internal load varies with
building type, site climate, and building design decisions. It is useful to identify whether internal or external loads will
dominate a building, as this information should substantially change the focus of design efforts related to control and
energy efficiency.

External Loads
External cooling loads consist of the following:

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1) Sensible loads through opaque envelope assemblies (roofs, walls, floors)

2) Sensible loads through transparent or translucent envelope assemblies (skylights, windows, glazed openings)
3) Sensible loads through ventilation and infiltration (air leakage)
4) Latent loads through ventilation and infiltration.
Because of the inherent differences in these types of heat flows, they are calculated (estimated) using four different
equations:
1) Roofs, External Walls & Conduction through Glass
The equation used for sensible loads from the opaque elements such as walls, roof, partitions and the conduction
through glass is:
Q = U * A * (CLTD)
U = Thermal Transmittance for roof or wall or glass. See 1997 ASHRAE Fundamentals, Chapter 24 or 2001
ASHRAE Fundamentals, chapter 25.
A = area of roof, wall or glass calculated from building plans
CLTD = Cooling Load Temperature Difference for roof, wall or glass. Refer 1997 ASHRAE Fundamentals, Chapter
28, tables 30, 31, 32, 33 and 34.
2) Solar Load through Glass
The equation used for radiant sensible loads from the transparent/translucent elements such as window glass,
skylights and plastic sheets is:

Q = A * (SHGC) * (CLF)
A = area of roof, wall or glass calculated from building plans
SHGC = Solar Heat Gain Coefficient. See 1997 ASHRAE Fundamentals, Chapter 28, table 35
CLF = Solar Cooling Load Factor. See 1997 ASHRAE Fundamentals, Chapter 28, and Table 36.
3) Partitions, Ceilings & Floors
The equation used for sensible loads from the partitions, ceilings and floors:

Q = U * A * (Ta - Trc)
U = Thermal Transmittance for roof or wall or glass. See 1997 ASHRAE Fundamentals, Chapter 24 or 2001
ASHRAE Fundamentals, and Chapter 25.
A = area of partition, ceiling or floor calculated from building plans
Ta = Temperature of adjacent space (Note: If adjacent space is not conditioned and temperature is not available,
use outdoor air temperature less 5°F)
Trc = Inside design temperature of conditioned space (assumed constant)
4) Ventilation & Infiltration Air
Ventilation air is the amount of outdoor air required to maintain Indoor Air Quality for the occupants (sees ASHRAE
Standard 62 for minimum ventilation requirements) and makeup for air leaving the space due to equipment
exhaust, exfiltration and pressurization.
Q sensible = 1.08 * CFM * (To – Tc)
Q latent = 4840 * CFM * (W o – Wc)
Q total = 4.5 * CFM * (ho – hc)
CFM = Ventilation airflow rate.
To = Outside dry bulb temperature, °F
Tc = Dry bulb temperature of air leaving the cooling coil, °F

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Wo = Outside humidity ratio, lb (water) per lb (dry air)

Wc = Humidity ratio of air leaving the cooling coil, lb (water) per lb (dry air)
ho = Outside/Inside air enthalpy, Btu per lb (dry air)
hc = Enthalpy of air leaving the cooling coil Btu per lb (dry air)
Refer to 1997 ASHRAE Fundamentals, Chapter 25, for determining infiltration

Internal Loads
Internal cooling loads consist of the following:
1) Sensible & latent loads due to people
2) Sensible loads due to lighting
3) Sensible loads due to power loads and motors (elevators, pumps, fans & other machinery)
4) Sensible & latent loads due to appliances
An internal load calculation is “the area of engineering judgment.” The internal loads are sometimes about 60% of the
load; however, these data are generally the least amount of information available to you at the design stage and
therefore the generic rules are most often employed to fix the variables. The equations used in estimating internal loads
are:
1) People
Q sensible = N * (QS) * (CLF)
Q latent = N * (QL)
N = number of people in space.
QS, QL = Sensible and Latent heat gain from occupancy is given in 1997 ASHRAE Fundamentals Chapter 28,
Table 3
CLF = Cooling Load Factor, by hour of occupancy. See 1997 ASHRAE Fundamentals, Chapter 28, table 37.Note:
CLF = 1.0, if operation is 24 hours or of cooling is off at night or during weekends.
2) Lights
The lights result in sensible heat gain.
Q = 3.41 * W * FUT * FBF * (CLF)
W = Installed lamp watts input from electrical lighting plan or lighting load data
FUT = Lighting use factor, as appropriate
FBF = Blast factor allowance, as appropriate
CLF = Cooling Load Factor, by hour of occupancy. See 1997 ASHRAE Fundamentals, Chapter 28, Table 38.
Note: CLF = 1.0, if operation is 24 hours or if cooling is off at night or during weekends.

3) Power Loads & Motors

Three different equations are used under different scenarios:
a. Heat gain of power driven equipment and motor when both are located inside the space to be conditioned
Q = 2545 * (P / Eff) * FUM * FLM
P = Horsepower rating from electrical power plans or manufacturer’s data
Eff = Equipment motor efficiency, as decimal fraction
FUM = Motor use factor (normally = 1.0)
FLM = Motor load factor (normally = 1.0)

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Note: FUM = 1.0, if operation is 24 hours

b. Heat gain of when driven equipment is located inside the space to be conditioned space and the motor is
outside the space or air stream
Q = 2545 * P * FUM * FLM
P = Horsepower rating from electrical power plans or manufacturer’s data
Eff = Equipment motor efficiency, as decimal fraction
FUM = Motor use factor
FLM = Motor load factor
Note: FUM = 1.0, if operation is 24 hours
c. Heat gain of when driven equipment is located outside the space to be conditioned space and the motor is
inside the space or air stream
Q = 2545 * P * [(1.0-Eff)/Eff] * FUM * FLM
P = Horsepower rating from electrical power plans or manufacturer’s data
Eff = Equipment motor efficiency, as decimal fraction
FUM = Motor use factor
FLM = Motor load factor
Note: FUM = 1.0, if operation is 24 hours
4) Appliances
Q = 3.41 * W * Fu * Fr * (CLF)
W = Installed rating of appliances in watts. See 1997 ASHRAE Fundamentals, Chapter 28; Table 5 thru 9 or use
manufacturer’s data. For computers, monitors, printers and miscellaneous office equipment, see 2001 ASHRAE
Fundamentals, Chapter 29, Tables 8, 9, & 10.
Fu = Usage factor. See 1997 ASHRAE Fundamentals, Chapter 28, Table 6 and 7
Fr = Radiation factor. See 1997 ASHRAE Fundamentals, Chapter 28, Table 6 and 7
CLF = Cooling Load Factor, by hour of occupancy. See 1997 ASHRAE Fundamentals, Chapter 28, Table 37 and
39. Note: CLF = 1.0, if operation is 24 hours or of cooling is off at night or during weekends.
Heat Gain from HVAC System
a. Supply Fan Heat Load
Supply and/or return fans that circulate or supply air to the space add heat to the space or system depending
on the location relative to the conditioned space. The heat added may take one or all of the following forms:
Instantaneous temperature rise in the air stream due to fan drive inefficiency.
Temperature rise in the air stream when the air is brought to static equilibrium and the static and kinetic energy
is transformed into heat energy.
The location of the fan and motor relative to the cooling coil and space being conditioned determines how the
heat is added to the system. If the fan is downstream of the cooling coil (draw-thru) then the fan heat load is
added to the space-cooling load. If the fan is upstream of the cooling coil, then the fan heat load is added to
the system cooling coil load.
The heat energy is calculated as follows:
Q = 2545 * [P / (Eff1 * Eff2)]
P = Horsepower rating from electrical power plans or manufacturer’s data
2545 = conversion factor for converting horsepower to Btu per hour
Eff1 = Full load motor and drive efficiency
Eff2 = Fan static efficiency

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Note: See 1997 ASHRAE Fundamentals, Chapter 28; Table 4 for motor heat gain.
b. Duct Heat Gain
Unless the return ductwork system is extensive and uninsulated or passes over a non-conditioned space, only
the heat gained by the duct supply system is significant. This heat gain is normally estimated as a percentage
of the space sensible cooling load (usually 1% to 5%) and applied to the temperature of the air leaving the
cooling coil in the form of temperature increase.

TOTAL LOAD
The total load is the summation of external and internal load or both sensible and latent loads. Usually 10% safety
margin is added but it all depends on how accurate are the inputs. The final load is than used to size the HVAC
equipment. HVAC equipment is rated in Btuh, but is commonly expressed in tonnage. A Btu (British thermal unit) is the
amount of heat needed to raise one pound of water one degree Fahrenheit. A “Ton” of cooling load is actually 12,000
Btu per hour heat extraction equipment. The term ton comes from the amount of cooling provided by two thousand
pounds or one ton of ice.
Traditionally, cooling loads are calculated based on worst case scenarios. Cooling loads are calculated with all
equipment & lights operating at or near nameplate values, occupant loads are assumed to be at a maximum, and the
extreme outdoor conditions are assumed to prevail 24 hours per day. Real occupant loads are seldom as high as
design loads. In detailed designing, the internal and external loads are individually analyzed, since the relative
magnitude of these two loads have a bearing on equipment selection and controls. For example check the figure below:

Analysis of this breakup provides an idea of how much each component of the building envelope contributes to the
overall cooling load and what can be done to reduce this load. Reducing solar heat gain through windows is clearly one
of the key areas.
The architect must also be aware of the heat load equations and the calculation methodology as these influence the
architectural design decisions that will in turn influence the energy consumption and comfort potential of the facility. The
majority of these decisions are made--either explicitly or by default--during the architectural design process.
Information on each of the components of cooling load equations -- and the design decisions that lie behind these
equations -- is covered in the subsequent sections.

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PART 2 SOLAR HEAT GAIN THROUGH WALLS, ROOF, PARTITIONS

Preface
The equation used for sensible loads from the opaque elements such as walls, roof, partitions and the raised floors is:
Q = U * A * (CLTD)
U = Thermal transmittance for roof or wall or glass. See 1997 ASHRAE Fundamentals, Chapter 24 or 2001 ASHRAE
Fundamentals, chapter 25.
A = area of roof, wall or glass calculated from building plans
CLTD = Cooling Load Temperature Difference for roof, wall or glass. Refer 1997 ASHRAE Fundamentals, Chapter 28,
tables 30, 31, 32, 33 and 34.
This section deals with the interpretation of solar heat gain equations, tabulated information and the load reduction
strategies through building envelope opaque surfaces.

Heat Gain through Building Walls

The heat flow through the wall is calculated by the following equation:
Q = U x Ax (Ti - To)
OR

A * (T - T )
i o
Q=
R
Total
In the equation above:
1) Q describes ‘Sensible heat flow’ that affects HVAC equipment size and energy consumption. For overall economics
and efficiency the goal is to minimize this value. Higher Q value shall impose high first- and recurring operation
costs on HVAC system.
2) A is the area of wall, which is a function of building form. The area values are computed from building plans and
elevations drawings.
3) U-value describes the rate of heat flow through a building element. It is the reciprocal of the R-value; U = 1/R Total
where R Total is the total resistance of the materials used in the construction of wall. The higher the R-value, the
higher shall be the insulating value of the material or the lower the U-value the higher shall be the insulation value
of the material.
The units of the U-value are BTU/hr sq-ft °F. A maximum value of U-factor is often set by energy efficiency
standards and is calculated from the material information shown in building drawings. The first step in estimating
the heat transfer through wall is to determine the design heat transmission coefficient or overall thermal resistances
of the various components that make up the envelope of the space. ASHRAE Fundamentals edition 2001, Chapter
25, or 1997 ASHRAE Fundamentals Chapter 24 provides detailed procedures and the thermal values which may
be used to calculate the thermal resistances of building walls, floors, roofs and ceilings.
3) ΔT i.e. (Ti - To) is the temperature difference between the inside design and outdoor temperature.
The conductive heat gain equation Q = U x Ax (Ti - To) is OK for estimating heat loss in winter months but for
summer months, the combined effect of convection, conduction, radiation and thermal (time) lag (for opaque
surfaces) are to be considered. The time lag is the difference between the time of peak outside temperature and
the time of the resulting indoor temperature. All of the transmitted solar radiation does not immediately act to
increase the cooling load; some is absorbed by wall and is radiated back to indoor space even after the sunset.
Therefore the time at which the space may realize the heat gain as a cooling load is considerably offset from the
time the heat started to flow. This phenomenon is caller “thermal storage effect” and is dependent on the thermal
mass of the material (the concept is discussed later in this section).

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The heat gain equation for cooling load is thus modified to include empirical value termed as ‘Cooling load
temperature difference (CLTD) that takes into account heat storage and time lag effects.
Q = U x Ax (CLTD)
Or
A * CLTD
Q=
R
Total

Where to get CLTD values?

The CLTD values can be found from tables listed in ASHRAE handbook of fundamentals.
The CLTD is determined by the type of wall (assembly construction) and is affected by thermal mass, indoor &
outdoor temperatures, daily temperature range, orientation, tilt, month, day, hour, latitude, solar absorbance, wall
facing direction and other variables. Corrections and adjustments are made if the conditions are different. Table- 1
& 2, below provide a snapshot of U-values, wall type & category (description & number), type of construction, mass
of wall and CLTD values for Type-D wall construction at various wall facing directions and solar time.
Table -1
U-values for Walls

Weight, U value, Code

Group
Wall description Description of construction numbers for
Number
layers
lb/ft2 Btu/h·ft2·°F
A0 A2 B1 A2
C Airspace + 4-in face brick 83 0.358
E0
A0 A2 C4 E1
D 4-in common brick 90 0.415
E0
1-in insulation or airspace + 4-in A0 A2 C4
C 90 0.174-0.301
4-in face brick + common brick B1/B2 E1 E0
brick A0 A2 B3 C4
B 2-in insulation + 4-in common brick 88 0.111
E1 E0
A0 A2 C9 E1
B 8-in common brick 130 0.302
E0
Insulation or airspace + 8-in common A0 A2 C9
A 130 0.154-0.243
brick B1/B2 E1 E0
A0 A2 C2 E1
E 4-in block 62 0.319
E0
A0 A2 C2
D Airspace or insulation + 4-in block 62 0.153-0.246
4-in face brick + B1/B2 E1 E0
(lightweight or
A0 A2 C7 A6
heavyweight D 8-in block 70 0.274
E0
concrete block)
Airspace or 1-in insulation + 6-in or 8- A0 A2 B1
C 73-89 0.221-0.275
in block C7/C8 E1 E0
A0 A2 B3
B 2-in Insulation + 8-in block 89 0.096-0.107
C7/C8 E1 E0
Heavyweight A0 A1 C5 E1
E 4-in concrete 63 0.585
concrete wall + E0
(finish) A0 A1 C5
D 4-in concrete + 1-in or 2-in insulation 63 0.119-0.200
B2/B3 E1 E0
A0 A1 B6 C5
C 2-in insulation + 4-in concrete 63 0.119
E1 E0
A0 A1 C10
C 8-in concrete 109 0.49
E1 E0
A0 A1 C10
B 8-in concrete + 1-in or 2-in insulation 110 0.115-0.187
B5/B6 E1 E0
A0 A1 B3
A 2-in insulation + 8-in concrete 110 0.115
C10 E1 E0
A0 A1 C11
B 12-in concrete 156 0.421
E1 E0

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Weight, U value, Code

Group
Wall description Description of construction numbers for
Number
layers
lb/ft2 Btu/h·ft2·°F
A0 C11 B6
A 12-in concrete + insulation 156 0.113
A6 E0
A0 A3
With/without airspace + 1-in/2-in/3-in
Metal curtain wall G 06-May 0.091-0.230 B5/B6/B12
insulation
A3 E0
A0 A1 B1
Frame wall G 1-in to 3-in insulation 16 0.081-0.178 B2/B3/B4 E1
E0

Table - 2
CLTD for Group D Walls

Wall Solar time, h

facing 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
N 15 13 12 10 9 7 6 6 6 6 6 7 8 10 12 13 15 17 18 19 19 19 18 16
NE 17 15 13 11 10 8 7 8 10 14 17 20 22 23 23 24 24 25 25 24 23 22 20 18
E 19 17 15 13 11 9 8 9 12 17 22 27 30 32 33 33 32 32 31 30 28 26 24 22
SE 20 17 15 13 11 10 8 8 10 13 17 22 26 29 31 32 32 32 31 30 28 26 24 22
S 19 17 15 13 11 9 8 7 6 6 7 9 12 16 20 24 27 29 29 29 27 26 24 22
SW 28 25 22 19 16 14 12 10 9 8 8 8 10 12 16 21 27 32 36 38 38 37 34 31
W 31 27 24 21 18 15 13 11 10 9 9 9 10 11 14 18 24 30 36 40 41 40 38 34
NW 25 22 19 17 14 12 10 9 8 7 7 8 9 10 12 14 18 22 27 31 32 32 30 27

*The CLTD values for all other wall groups can be referred in the 1997 ASHRAE Fundamentals Handbook.
Note that the values shown in the tables assume a mean of 85ºF (tm), a room temperature (tr) of 78ºF, a daily range
(DR) of 21ºF, dark surface, and a clear sky on the July 21. When conditions are different, CLTD values from the table
must be corrected before being used in heat transfer equation. CLTDCor can be found using equation as follows:
CLTDCor = CLTD Table + (78 – t r) + (tm – 85), where tm = (to + t r) / 2 = to - (DR / 2)
The revised equation Q = U x Ax CLTDcor shall than be used for your specific project site application.

Heat Gain through Building Roof

The heat flow through the roof is calculated by the following equation:
Q = U x Ax (CLTD)
Or
A * CLTD
Q=
R
Total

Just like the case with the walls, the materials used in the construction of roof shall be used to calculate the resistance
(R) and the U-value of the roof. The area of the roof is calculated from building plans. The materials parameters and the
solar time shall than be used to determine the maximum CLTD value for the roof. Below is the reference Table-3 on
typical CLTD values for a flat roof without suspended ceiling.
Table - 3
CLTD for Flat Roofs without Suspended Ceilings

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U
Description
Wt. value,
of Solar time
lb/ft2 Btu/hr·
construction
ft2·°F

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Steel sheet 7 0.213

with 1" (or 2") 1 -2 -3 -3 -5 -3 6 19 34 49 61 71 78 79 77 70 59 45
(8) (0.12)
insulation
1" wood with 8 0.17 6 3 0 -1 -3 -3 -2 4 14 27 39 52 62 70 74 74 70 62
1" insulation
4" lightweight 18 0.213 9 5 2 0 -2 -3 -3 1 9 20 32 44 55 64 70 73 71 66
concrete

2"
heavyweight 0.206
29 12 8 5 3 0 -1 -1 3 11 20 30 41 51 59 65 66 66 62
concrete with (0.12)
1" (or 2")
insulation
1" wood with 9 0.109 3 0 -3 -4 -5 -7 -6 -3 5 16 27 39 49 57 63 64 62 57
2 " insulation
6" lightweight 24 0.158 22 17 13 9 6 3 1 1 3 7 15 23 33 43 51 58 62 64
concrete
2.5" wood
with 1" 13 0.13 29 24 20 16 13 10 7 6 6 9 13 20 27 34 42 48 53 55
insulation
8" lightweight 21 0.126 35 30 26 22 18 14 11 9 7 7 9 13 19 25 33 39 46 50
concrete

4"
heavyweight 52 0.200
25 22 18 15 12 9 8 8 10 14 20 26 33 40 46 50 53 53
concrete with (52) (0.12)
1" (or 2")
insulation
2.5" wood
with 2" 13 0.093 30 26 23 19 16 13 10 9 8 9 13 17 23 29 36 41 46 49
insulation
Roof terrace 75 0.106 34 31 28 25 22 19 16 14 13 13 15 18 22 26 31 36 40 44
system

6"
heavyweight 75 0.192
31 28 25 22 20 17 15 14 14 16 18 22 26 31 36 40 43 45
concrete with (75) (0.12)
1" (or 2")
insulation
4" wood with 17 0.106
1" (or 2") 38 36 33 30 28 25 22 20 18 17 16 17 18 21 24 28 32 36
(18) (0.07)
insulation

*The CLTD values for flat roof with suspended ceiling and other details can be referred in Table 5, p. 26.8, Chap 26,
ASHRAE Fundamental Handbook, 1985 along with notes, limitations and adjustments)

Heat Gain through Exposed Floors & Slabs

The exposed floor is that portion of the building that has a vacant space below for example 2nd floor apartment building
with open air parking garage below. Heat loss from floors to open spaces is treated as a wall or roof (Q = U * A * ΔT).

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