PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009

Hybrid Ventilation Systems and High Performance Buildings

D. MICHAEL UTZINGER1
1
Associate Professor, School of Architecture & Urban Planning, University of Wisconsin, Milwaukee, USA

ABSTRACT: Hybrid ventilation systems combined occupant controlled natural ventilation with mechanical ventilation
systems. This paper describes the performance of three buildings designed with hybrid ventilation systems. The design
and performance of each are detailed. Natural ventilation is shown to provide adequate ventilation when appropriately
designed. Proper control integration of natural ventilation into hybrid systems is shown to reduce energy consumption in
high performance buildings. Lessons learned from the three buildings are described.
Keywords: hybrid ventilation, natural ventilation, high performance buildings, occupant control, design process

INTRODUCTION development of radiant slabs and chilled beams to

Until the second half of the nineteenth century buildings provide sensible cooling. Ventilation rates were reduced
were ventilated through windows and shafts with winds to what was required by occupants for health and de-
and temperature differences providing the driving forces. humidification. Fan sizes went down and energy required
Concerns over air quality both inside and outside to ventilate and cool buildings was reduced considerably.
buildings led to the development of complex ventilation North American HVAC engineers, accustomed to the
systems driven by either fan or temperature differences systems they know, have generally resisted using the
and designed to filter incoming air and exhaust polluted natural ventilation and radiant cooling concepts
air from the building. The advent of air-conditioning in developed in Europe. Presumed inappropriate behavior
North America early in the twentieth century led to an by occupants opening windows at the wrong time and
integration of ventilation and air-conditioning systems. higher summer dew point temperatures in the eastern
With mechanical ventilation and air-conditioning, North USA have been given as reasons to stay with sealed
American architects began designing buildings with deep buildings and standard HVAC systems.
floor plates containing large interior zones that could
only be ventilated and air-conditioned mechanically. To This paper discusses hybrid ventilation design
keep floor to ceiling heights as high as possible, strategies and their impact on three high performance
architects typically required smaller supply and return buildings located in southern Wisconsin, USA. All three
duct designs leading to larger power requirements for the buildings were designed by the Kubala Washatko
fans. Reyner Banham provides an excellent history of Architects. The author served as energy consultant on all
HVAC systems and their impact on architectural form up three projects. His responsibilities included designing
to 1980[1]. the natural ventilation systems and integrating natural
ventilation into the HVAC control strategy. He was also
As high-rise offices began to appear in Northern responsible for post occupancy evaluation of building
Europe late in the twentieth century, air-conditioning and energy performance and has used all three buildings as a
ventilation systems were re-examined. Hybrid systems learning laboratory for architecture students. The Schlitz
integrating natural ventilation and mechanical ventilation Audubon Nature Center (SANC), completed in 2003, is
were developed as a means of conserving energy and an environmental education center open seven days per
meeting code requirements for operable windows in week. The Urban Ecology Center (UEC), completed in
occupied spaces. A North American office building 2004, is an ecological education center open seven days
might require 1.25 l/s/m2 for outdoor ventilation air, but per week. The Aldo Leopold Legacy Center (ALLC),
the air-conditioning system might require 5 to 8 l/s/m2. completed in 2007, houses a non-profit foundation. It is
A well designed HVAC system using air as a heat open five days per week during winter and six days per
transfer medium can transfer roughly 4 kJ/hr/°C per watt week during the rest of the year. The size, measured
of fan power. Using water as a heat transfer medium the energy utilization intensity and percentage of energy
system can transfer more than 70 kJ/hr/°C per watt of supplied by renewable solar power and bio-fuels on site
pump power. Recognition that water is significantly for each building is given below (Table 1). The hybrid
more efficient transferring heat than air led to the ventilation system of each building is considered in order
 PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009

of construction. Lessons learned from earlier buildings design, the HVAC system can and should be shut down
provide feedback to an evolving approach to hybrid when natural ventilation is employed.
ventilation.

SCHLITZ AUDUBON NATURE CENTER (SANC)

Table 1: Gross floor areas, energy utilization intensity (EUI) All occupied spaces should have lights off during sunny
and site renewable energy percent of buildings studied. days. During mild weather windows should be open and
HVAC systems off. These were two design goals
Building Gross Floor Annual Site Renewable established during programming of the Schlitz Audubon
Area Measured EUI Energy Percent
Nature Center. To achieve these goals all occupied
SANC 2,785 m2 97.9 kWh/m2 3.8%
UEC 1,880 m2 157.1 kWh/m2 18.1% spaces needed to be perimeter zones with access to light
ALLC 1,244 m2 55.1 kWh/m2 70.6% and air. With the exception of six classrooms and two
offices along a double-loaded corridor on the ground
floor, all occupied spaces on the ground and upper floor
CLIMATE AND NATURAL VENTILATION have operable windows on at least two different
Southern Wisconsin is located between 42.5° and exposures (Fig. 1). The classrooms are lined with
44.0° North latitude. The Schlitz Audubon Nature windows facing the corridor providing natural light from
Center and the Urban Ecology Center are located in north and south. The windows are capped with operable
Milwaukee, Wisconsin near the shore of Lake Michigan transom lights to allow cross-flow ventilation. Occupied
in the upper reaches of the St Lawrence River watershed. basement spaces were added to the program after design
The Aldo Leopold Legacy Center is located in a forested development. Some were neither daylit nor naturally
area near the Wisconsin River in the upper reaches of the ventilated.
Mississippi River watershed. Heating season in southern
Wisconsin runs from early November to late April or mid
May. The average January dry bulb temperature is
roughly -10°C with design dry bulb temperature of -
26°C. Occasional arctic air masses reach Southern
Wisconsin and temperatures drop to -30°C. Air-
conditioning is required when dew point temperatures
become excessive. This occurs intermittently from mid
June to mid September with an accumulated duration of
four to eight weeks. Building zones with large
occupancies or high plug load densities will require
longer periods of air-conditioning. Poorly designed
glazing systems will increase the length of the air-
conditioning season due to solar gains. However, there is
no excuse for poorly designed glazing systems. The
average July dry bulb temperature in southern Wisconsin Figure 1: Ground Floor Plan, Schlitz Audubon Nature Center.
is roughly 23°C and the average dew point temperature is
roughly 19°C. The design dry bulb temperature is 32°C
with coincident 23°C dew point. Hot and humid air from Ground source water-to-air heat pumps (one for each
the Gulf of Mexico can bring dry bulb temperatures up to of the 22 occupied spaces) provide heating and cooling.
40°C. 90 vertical wells, 38 meters deep, connect the earth
source/sink to the heat pumps. Outdoor air is ducted
Hybrid ventilation systems employ natural ventilation directly to the three upper floor zones and the auditorium
during mild weather and controlled mechanical zone. Outdoor air for all remaining occupied spaces is
ventilation during heating or air-conditioning operations. provided via a heat recovery ventilator and water-to-air
Based on the experience of the three buildings studied in heat pump. The heat recovery ventilator served occupied
this paper, natural ventilation can meet outdoor air and spaces in the basement that did not have windows.
thermal comfort requirements when the dry bulb Therefore it was always on during occupancy whether
temperature is above 10°C and below 25°C with the windows were open or not. Outdoor air flow rates met
coincident dew point temperature below 17°C. This the requirements of ASHRAE Standard 62.1-1999. As
range assumes solar gains are minimized and internal described below, the natural ventilation system was also
heat generation is not excessive. Given the southern designed to meet the required ASHRAE ventilation rates.
Wisconsin climate, natural ventilation can be used in lieu
of mechanical systems for a total of three to four months Volumetric flow rates for wind driven and stack
per year. If natural ventilation is well integrated into the natural ventilation systems were estimated using
 PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009

procedures given by the American Society of Heating building manager could check the window status screen
and Refrigeration and Air-Conditioning Engineers [2]. at the end of the day to determine which of the 115
The volume flow rate equations given by ASHRAE were operable windows might be open.
modified to output zone air change rates. The air change
rate for wind driven ventilation is given by While the operation of the window control system
was very successful, installation was problematic. At the
ACH = 3600 Cvent Aopening vwind /Vzone (1) time of construction wireless controls were not available.
Each contact had to be wired through the window frame
Where ACH is the number of zone air changes per to the operable sash. At commissioning 20% of the
hour; Aopening is the smaller of the inlet or outlet opening contacts did not function properly and needed to be re-
areas given in square meters; vwind is the wind velocity in wired. In addition, the contacts and installation added
meters per second; Vzone is the occupied zone air volume $7,800 to the controls budget (the installer would charge
in cubic meters; and Cvent is the effectiveness of the a higher price in the future after their experience). The
opening path to wind driven ventilation. The value of design team felt that use of contacts as a part of the
Cvent is between 0.25 and 0.65 depending on whether building control strategy would require wireless contacts
winds are quartering or perpendicular. For each zone the and a less expensive controls package.
air change rate was estimated as a function of the percent
of window opening area (inlet or outlet) actually open The Schlitz Audubon Nature Center is a LEED
and wind velocity ranging from 20% to 100% of the Gold certified building. The impact of natural ventilation
average summer wind velocity. Cvent was assumed to be on annual energy performance was not modelled in the
0.25. The expected natural ventilation rate was energy simulation as the control strategy was deemed to
compared with the ventilation rate required by ASHRAE. complex to model to insure the comfort performance
The environmental classrooms required an air change required for LEED compliance.
rate of 5.4 or higher. This ventilation could be met at
low wind velocities when windows were fully open
(Table 2). Only the grey areas did not meet the required URBAN ECOLOGY CENTER (UEC)
air change rate. The Urban Ecology Center is more compact than the
Schlitz Audubon Nature Center (Fig. 2 and 3). The site
was more restricted and ground source heat pumps were
Table 2: Environmental classroom wind driven air change not an option. Construction budget was an issue and the
rates (ACH) as a function of wind velocity and opening area. cost of the LEED submission determined prohibitive.
The team choose to emphasize daylight, high enclosure
Percent of Average Summer Wind Velocity
insulation levels (double code levels in the walls and
20% 40% 60% 80% 100%
20% 1.58 3.16 4.75 6.33 7.91
roof) and natural ventilation. The building was divided
40% 3.16 6.33 9.49 12.66 15.82 into three thermal zones: the staff offices, the public and
60% 4.75 9.49 14.24 18.99 23.73 classroom spaces and the basement occupied spaces.
80% 6.33 12.66 18.99 25.32 31.64 Each zone is served by a constant volume air handling
100% 7.91 15.82 23.73 31.64 39.56 unit. A high efficiency boiler delivers hot water to the
air handling units and four cabinet heaters at fire stairs
and vestibules. Only the office zone was provided with
Proper integration of window operation by occupants air-conditioning. The public spaces were naturally
with the HVAC system is critical for energy efficient ventilated in summer and the basement zone was
building performance. The control strategy was to shut mechanically ventilated.
down the zone heat pump any time a window in the zone
was opened. This was accomplished by placing a The window opening area and distribution were
window contact on each operable window in a zone. determined using the same procedures as on the Schlitz
When a window was opened, the magnet on the operable Audubon Nature Center. The control strategy was much
sash moved away from a sensor and the building simpler. From late May to mid November the public
automated control system sensed a window was opened spaces and office spaces are in natural ventilation mode.
in a thermal zone. The heat pump in that zone was shut If conditions in summer become extremely hot or humid,
down. Once the window was closed, the heat pump the office air handling unit is turned on in air-
would return to its normal mode of operation. The staff conditioning mode and office windows to the outside and
appreciated this control strategy. They choose to enter or the public spaces are closed. The building manager
leave natural ventilation mode by opening or closing switches the system on or off through a web interface to
windows. The audible sound of the heat pump shutting the building controls. After the second year of
down or starting up provides feedback that window occupancy, a cooling coil was added to the public space.
operation is integrated with the systems. In addition, the The staff felt guilty that they would retreat to their air-
 PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009

conditioned offices when visitors suffered in the heat and carbon dioxide concentrations rose. On cold, sunny
humidity. When weather is mild, windows are open in November days the office temperature would be 17°C at
the office and public zones and the air handling units are 8 A.M. and would rise to 21°C by mid afternoon. Staff
off. A staff member checks the entire bu building to close would dress in layers allowing adjustment between
and lock all windows at the end of the day. comfort conditions over the day. Over the entire period,
the heating system was on for 10 days; the cooling
system was on for 7 days and the building operated in
natural ventilation mode for 85 days (over 12 weeks).

S umme r Ve nt Fa ll Ve nt
Air Conditioning He a ting
800 ppm

600 ppm

400 ppm

200 ppm
-10 C 0C 10 C 20 C 30 C 40 C

Figure 4: Average daily UEC office CO2 concentration as a

Figure 2: Upper Floor Plan, Urban Ecology Center
Center. function of average daily outdoor air temperature for heating,
cooling and natural ventilation.

Natural ventilation was not desired on the hottest day

observed (Fig. 4).
). The staff wanted air-conditioning,
air but
the three staff members with access to the control
systems password were out of the office. The control
system set the building in natural ventilation mode at the
start of each day. Staff would choose to enter air- air
conditioning by closing windows and entering the
controls web site to change the HVAC mode. This
control system strategy requires more staff with
knowledge and skill manipulating building control
systems.
Figure 3: Building Section, Urban Ecology Center
Center.

ALDO LEOPOLD LEGACY CENTER (ALCC)

To determine the effectiveness of natural ventilation a The Board of Directors of the Aldo Leopold Foundation
carbon dioxide sensor was located on a work surface requested that the design
n team provide a carbon neutral
between two UEC administrators. This location was design for the Aldo Leopold Legacy Center.
Center The design
chosen because the administrators were often at their team and board agreed that this meant designing a net-
desks and the location was not ideal for the natural zero building. To accurately model all energy flows in
ventilation flow paths.. Carbon dioxide concentrations the building, the design team integrated natural
and outdoor air temperatures
emperatures were recorded at 5 minute ventilation processes into the simulation model. An
intervals from June 21, 2005 to November 28, 2005. experiment comparing measured and modelled
Each day the office was open average
verage carbon dioxide ventilation rates had been conducted at the Schlitz
concentrations during office hours were plotted as a Audubon Nature Center er [3]. The results of that study
function of average outdoor air temperature during office convinced the design team that natural ventilation could
hours for (Fig. 4).
). With the exception of cold fall days, be reasonably simulated with a multi-node
multi bulk flow
natural ventilation performed as well or better than model. The CONTAMW bulk flow pollution and air
mechanical ventilation.. As temperatures during fall ventilation model [4] was modified to allow variation of
became cooler, stafff restricted window openings and the window
dow openings during simulation and integrated
 PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009

into the TRNSYS thermal simulation program [5]. The automated controls system issues an alarm that
simulation model of ventilation systems for the Aldo conditions are not appropriate for natural ventilation.
Leopold Legacy Center has been presented elsewhere When outdoor air temperatures are between 10°C and
[6]. Of note here is that the simulation model was used 25°C and the dew point is below 16°C, the controls
to study the effectiveness of different operable windows system issues a message indicating conditions are
on different orientations. The administrative offices appropriate for natural ventilation. The building systems
were designed with individual offices on the north, operator can switch the building between natural
common office activities in the center of the space and a ventilation mode and cooling (or heating) mode. When
circulation zone along the south wall (Fig. 5). Cross the building is switched into natural ventilation mode, the
flow ventilation moves through the three spaces. air handling unit shuts down and the ground source heat
Clerestory windows above the offices were found to be pumps, ground loop circulation pumps and floor loop
less effective for total building ventilation. As a result, pumps shut down.
the number of operable clerestory windows was reduced.

Figure 6: Floor Plan, Aldo Leopold Legacy Center

Figure 5: Building section through administrative offices, Aldo The simulation model indicates that natural
Leopold Legacy Center, looking east. ventilation saves 1,156 kWh per year (0.93 W/m2 per
year), nearly 10% of the cooling and associated fan and
pump loads and roughly 1.7% of the total annual energy
The Aldo Leopold Legacy Center has four building load. While this number may seem small, every bit of
components (Fig. 6). The Workshop and Seed Hall are savings is important. Comparing the energy utilization
not tempered spaces. From April to November the Seed intensity of the three buildings studied is illuminating
Hall is used for classes and meetings and is rented to (see Table 1). The Aldo Leopold Legacy Center requires
groups. The remainder of the year the space is not used. half the energy per unit floor area as the Schlitz Audubon
The Conference Wing is used intermittently. Those Nature Center and just over one third of the energy
spaces are maintained at 55 during the heating season demand of the Urban Ecology Center. For buildings
and a wood-burning stove brings the space up to comfort designed by the same architect with similar wall and roof
when it is occupied. The administrative offices and insulation levels, one would not expect so much
exhibit spaces are heated and cooled by a radiant slab. difference between energy requirements. The Urban
Slab temperatures are maintained by ground source Ecology Center is the most compact of the three
water-to-water heat pumps. When the building is not in buildings with the Aldo Leopold Legacy Center having
natural ventilation mode, a 100% outdoor air mechanical the most enclosure area per unit floor area. Energy use is
ventilation system provides required ventilation air. The the opposite. On the one hand, the decision to not pursue
air-handling unit cools and dehumidifies or heats the LEED for the Urban Ecology Center led to a decision to
ventilation air as required. Using a radiant slab for forgo commissioning and energy simulation. Both of
cooling creates difficulties for natural ventilation these activities provide a second look at all the pieces in
integration. Because the slab is maintained at 18°C, the design allowing questions to be raised concerning the
natural ventilation should be prohibited of the outdoor air appropriateness of decisions. After examining the
dew point temperature exceeds 17°C. Otherwise there is energy use of SANC and UEC, the design team decided
a risk that condensation will occur on the cooled slab. to always recommend energy simulation modelling and
Dew point temperature is monitored at three locations in commissioning to a client that did not wish to invest in
the administration and exhibit wing as well as the LEED certification. Simulation modelling of the Aldo
outdoor dew point. When the outdoor dew point is Leopold Legacy Center was well integrated into the
within 1°C of the slab temperature, the building design process informing decisions concerning operable
 PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009

window areas, insulation levels, the effectiveness of the buildings and giving their time for my numerous
circulation hall along the south wall as a thermal buffer questions. The Kubala Washatko Architects provided
and the appropriateness of earth tubes to pre-treat building plan and section images.
ventilation air. The commissioning agent pressed the
design team to increase the efficiency of fans and pumps.
The installed fan power at SANC is 5.7 Watts per square REFERENCES
meter. It is 1.9 Watts per square meter at ALLC. 1. Banham, Reyner (1984). The Architecture of the Well-
tempered Environment, 2ND Edition, The University of Chicago
Press, Chicago, Illinois, USA.
2. 1997 ASHRAE Handbook of Fundamentals. Atlanta:
CONCLUSION
American Society of Heating, Refrigerating and Air-
To achieve energy savings with natural ventilation, the Conditioning Engineers, Inc.
HVAC system must be integrated with the architectural 3. Bradley, D. E. and Utzinger, D. M., (2006). Natural
design and building program to allow the HVAC system Ventilation Measurements and Simulation at Two Milwaukee
to be shut down if windows are open. This is not easy. Nature Centers, In SimBuild 2006 Conference Proceedings,
Two of the three buildings studied had the program Cambridge, MA, USA, August 2-6.
expand into the basement during the design development 4. Dols, W.S. and G.N. Walton, (2002). CONTAMW 2.0 User
process. For the Schlitz Audubon Nature Center, this Manual. National Institute of Standards and Technology.
meant a fresh air ventilation system that could not be Gaithersburg, MD, USA.
shut down when ground floor spaces were in natural 5. Klein, S.A. et. al. (2005) TRNSYS: A Transient System
Simulation Program. Madison, WI: Solar Energy Laboratory.
ventilation mode. If building zones are not compatible University of Wisconsin – Madison
with natural ventilation because of occupancy use (e.g. 6. Bradley, D. and Utzinger, D. M., (2007). The Enhancement
large auditoriums or sensitive laboratories), those zones and Use of Combined Simulation Tools in the Assessment of
should have a separate ventilation system. The Urban Hybrid Natural/Mechanical Ventilation Systems, ASHRAE
Ecology Center has a separate HVAC system for the Transactions, Vol. 113 (2).
basement zones.

Simulation modelling, when it can accurately include

natural ventilation and control strategies, is indispensible
to evaluation of high performance building design.
However, the integration of natural ventilation systems
and building automatic control systems requires careful
discussions with the client/owner that will be operating
the building. Contacts on windows that automatically
shut down the HVAC system for the space when a
window is open seem like the best solution. It is if each
space is a separate zone (eg. a school building
classroom). With multi-zone HVAC systems, it is more
appropriate to shift the control of going into or out of
natural ventilation mode to a building system operator
allowing communication to all staff that windows can be
opened or should be closed.

When well designed, a natural ventilation system can

result in significant reduction of cooling and ventilation
energy. More important, occupants often have a greater
sense of control over their workspace when they can
control windows near their desk. In all three buildings,
no staff member wanted to return to a sealed building.
All spoke of enjoying the feeling of fresh air moving
through the building on the first warm spring day. The
emotional attachment to an outdoor connection may be
as important as the energy savings.

ACKNOWLEDGEMENTS. The author wishes to

acknowledge the assistance of the staff of the Schlitz
Audubon Nature Center, Urban Ecology Center and Aldo
Leopold Foundation providing performance data for their