Evaluation of Ventilation

Strategies in New Construction 

Multifamily Buildings

S. Maxwell, D. Berger, and M. Zuluaga

Consortium for Advanced Residential Buildings

July 2014
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iii

Evaluation of Ventilation Strategies in

New Construction Multifamily Buildings

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

Sean Maxwell, David Berger, and Marc Zuluaga

Steven Winter Associates, Inc. 

Consortium for Advanced Residential Buildings (CARB)

61 Washington Street

Norwalk, CT 06854

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40342-04

July 2014
iv

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v

Contents

List of Figures ........................................................................................................................................... vii

List of Tables ............................................................................................................................................ viii

Definitions................................................................................................................................................... ix

Executive Summary.................................................................................................................................... x

1 Problem Statement............................................................................................................................... 1

1.1 Previous Research................................................................................................................2

1.2 Research Questions..............................................................................................................3

2 Technical Approach ............................................................................................................................. 4

2.1 Make-Up Air Ventilation Strategies....................................................................................4

2.1.1 Type 1: No Direct Make-Up Air Supplied to Apartments.......................................4

2.1.2 Type 2: Make-Up Air Through Ducted Supply Air.................................................5

2.1.3 Type 3: Make-Up Air Through HVAC Supply .......................................................5

2.1.4 Type 4: Make-Up Air Through Passive Vents ........................................................6

2.2 Factors Affecting Ventilation System Performance and Choice of Measurements ............6

2.3 Building Descriptions and Information ...............................................................................7

2.3.1 Type 1 Building: No Direct Make-Up Air Supplied to Apartments........................7

2.3.2 Type 2 Building: Make-Up Air Through Ducted Supply Air..................................8

2.3.3 Type 3 Building : Make-Up Air Through HVAC Supply .......................................9

2.3.4 Type 4 Building: Make-Up Air Through Passive Vents .......................................10

2.4 Measurements and Tests....................................................................................................11

2.4.1 Apartment Blower Door Testing ...........................................................................12

2.4.2 Supply and Exhaust Flow Testing .........................................................................12

2.4.3 Door Leakage Airflows..........................................................................................12

2.4.4 Pressure Monitoring...............................................................................................13

2.4.5 Supply and Exhaust Fan Power Consumption.......................................................14

2.4.6 Factors Affecting the Measurement.......................................................................14

3 Results and Findings ......................................................................................................................... 16

3.1 Type 1: No Direct Make-Up Air Supplied to Apartments.................................................16

3.1.1 Apartment Blower Door Testing ...........................................................................16

3.1.2 Supply and Exhaust Flow Testing .........................................................................16

3.1.3 Door Leakage Airflows..........................................................................................17

3.1.4 Pressure Monitoring: Cooling Season ...................................................................22

3.1.5 Pressure Monitoring: Heating Season....................................................................25

3.1.6 Fan Power Consumption........................................................................................28

3.2 Type 2: Make-Up Air Through Ducted Supply Air...........................................................28

3.2.1 Apartment Blower Door Testing ...........................................................................28

3.2.2 Supply and Exhaust Flow Testing .........................................................................29

3.2.3 Supply and Exhaust Fan Power Consumption.......................................................32

3.3 Type 3: Make-Up Air Through HVAC Supply .................................................................32

3.3.1 Apartment Blower Door Testing ...........................................................................32

3.3.2 Supply and Exhaust Flow Testing .........................................................................32

3.3.3 Door Leakage Airflows..........................................................................................33

3.3.4 Packaged Terminal Air Conditioning Make-Up Air .............................................34

3.3.5 Pressure Monitoring—Cooling Season..................................................................38

3.3.6 Pressure Monitoring—Heating Season..................................................................42

3.3.7 Supply and Exhaust Fan Power Consumption.......................................................45
vi

3.4 Type 4: Make-Up Air Through Passive Vents ..................................................................45

3.4.1 Apartment Blower Door Testing ...........................................................................45

3.4.2 Supply and Exhaust Flow Testing .........................................................................46

3.4.3 Passive Vent Testing..............................................................................................46

3.4.4 Door Leakage Airflows..........................................................................................49

3.4.5 Pressure Monitoring...............................................................................................51

3.4.6 Exhaust Fan Power Consumption..........................................................................55

4 Discussion........................................................................................................................................... 56

4.1 Type 1: No Direct Make-Up Air Supplied to Apartments.................................................56

4.2 Type 2: Make-Up Air Through Ducted Supply Air...........................................................56

4.3 Type 3: Make-Up Air Through HVAC Supply .................................................................57

4.4 Type 4: Make-Up Air Through Passive Vents ..................................................................57

4.5 The Relationship Between Apartment Airtightness and Make-Up Air.............................58

4.6 Comparison of Different Strategies...................................................................................63

5 Preliminary Conclusions ................................................................................................................... 68

References................................................................................................................................................. 74

Resources.................................................................................................................................................. 75
vii

List of Figures

Figure 1. Example of a building with no direct MUA to apartments ........................................................................4

Figure 2. Example building with central ducted supply (left) and a central supply unit (right) ............................5

Figure 3. Example of a building with MUA from PTACs ...........................................................................................6

Figure 4. Apartment layout of test building “Type 1” ...............................................................................................8

Figure 5. Typical floor plan of test building #1 of “Type 2”......................................................................................9

Figure 6. Typical floor plan of test building #2 of “Type 2”......................................................................................9

Figure 7. Typical floor plan of test building “Type 3” .............................................................................................10

Figure 8. Typical floor plan of test building “Type 4” .............................................................................................11

Figure 9. Powered flow hood ....................................................................................................................................12

Figure 10. Setup for testing air leakage from a door frame....................................................................................13

Figure 11. Typical pressure patterns in a multistory building in winter................................................................14

Figure 12. Typical door weather-stripping coverage ..............................................................................................18

Figure 13. Concept for testing air leakage from a door frame (left) and actual test in progress (right).............18

Figure 14. Results of air leakage test from a single door.......................................................................................19

Figure 15. Results of air leakage test from all doors tested...................................................................................19

Figure 16. Results of air leakage test from all doors tested and average door performance .............................21

Figure 17. Incremental door airflow under various scenarios ...............................................................................22

Figure 18. Apartment pressure with respect to corridor—cooling season...........................................................23

Figure 19. Airflow across apartment door. Positive values indicate flow from apartment to corridor...............24

Figure 20. Apartment pressure with respect to outdoors—cooling season.........................................................25

Figure 21. Apartment pressure with respect to corridor—heating season...........................................................26

Figure 22. Apartment pressure with respect to outdoors—heating season.........................................................26

Figure 23. Apartment 1220 pressure with respect to outdoors and corridor—heating season..........................27

Figure 24. Apartment 1265 pressure with respect to outdoors and corridor—heating season..........................27

Figure 25. Apartment door—gap between door and threshold..............................................................................33

Figure 26. Results of air leakage test from all doors tested and average door performance .............................34

Figure 27. Testing apparatus for measuring airflow through PTAC fresh air kit (cover removed).....................35

Figure 28. Diagram of apparatus for measuring airflow through PTAC fresh air kit............................................35

Figure 29. Results from 3-in.2 orifice (L) and 4-in.2 orifice (R)................................................................................36

Figure 30. Combined results from both tests..........................................................................................................36

Figure 31. Setup for measuring pressures inside the PTAC blower cabinet........................................................37

Figure 32. Pressure at outlet of PTAC fresh air fan with respect to outdoors......................................................38

Figure 33. Pressure at outlet of PTAC fresh air fan with respect to outdoors, single day ..................................39

Figure 34. Apartment 4R—pressure, apartment temperature, and PTAC temperature .......................................40

Figure 35. Apartment pressures with respect to outdoors and corridor ..............................................................41

Figure 36. Apartment pressures with respect to outdoors and corridor—single day .........................................41

Figure 37. Pressure at outlet of PTAC fresh air fan with respect to outdoors—heating season ........................43

Figure 38. Apartment 3P pressures with respect to outdoors and corridor—heating season............................44

Figure 39. Apartment 3R pressures with respect to outdoors and corridor—heating season ...........................44

Figure 40. Common trickle vent (left) and installation in a window frame (right).................................................47

Figure 41. Setup for testing airflow through passive vents ...................................................................................47

Figure 42. Trickle vent flow at varying pressures ...................................................................................................48

Figure 43. Cut for trickle vent in window frame (left), trickle vent removed. Secondary holes drilled through 

frame to allow air passage (right)......................................................................................................................49

Figure 44. Setup for testing airflow through door gap with a plastic shroud and orifice plate...........................50

Figure 45. Pressure in several building spaces over time .....................................................................................51

Figure 46. Pressure data logged in building spaces over a 24-hour period .........................................................52

Figure 47. Pressures in apartments with respect to hallway .................................................................................53

Figure 48. Pressures in apartment with respect to hallway and outdoors ...........................................................54

Figure 49. All sides of an apartment contribute to leakage in a blower door test................................................59

Figure 50. Pressure differences on select surfaces in normal building operation ..............................................59

Unless otherwise noted, all figures were created by CARB.
viii

List of Tables

Table 1. Factors Affecting Ventilation Performance .................................................................................................7

Table 2. Measurements and Sampling Rates at Each Building Type ....................................................................12

Table 3. Type 1 Building Apartment Airtightness Metrics......................................................................................16

Table 4. Bathroom Exhaust Flows............................................................................................................................16

Table 5. Measured Corridor Supply Flows (CFM) by Floor.....................................................................................17

Table 6. Storage Room Supply Flows (CFM) by Floor............................................................................................17

Table 7. Garbage Room Exhaust Flows (CFM) by Floor.........................................................................................17

Table 8. Tabulated Results of Door Air Leakage Tests (CFM50)............................................................................20

Table 9. Average Apartment Supply and Exhaust Flows .......................................................................................22

Table 10. Exhaust Fan Performance—Power and Flow..........................................................................................28

Table 11. Supply Fan Performance—Power and Flow............................................................................................28

Table 12. Type 2 Building #1 Apartment Airtightness Metrics...............................................................................29

Table 13. Type 2 Building #2 Apartment Airtightness Metrics...............................................................................29

Table 14. Building #1—Actual and Design Supply and Exhaust Flow ..................................................................30

Table 15. Building #2—Actual and Design Supply and Exhaust Flow ..................................................................31

Table 16. Supply and Exhaust Fan Power ...............................................................................................................32

Table 17. Type 3 Building Apartment Airtightness Metrics....................................................................................32

Table 18. Apartment Exhaust Flow Test Results.....................................................................................................33

Table 19. Tabulated Results of Door Air Leakage Tests (CFM50)..........................................................................34

Table 20. Effect of Apartment Operating Conditions on Pressure at Outlet of PTAC Fresh Air Fan..................37

Table 21. Average Apartment Supply Flows as Percentage...................................................................................42

Table 22. Actual and Design Supply Flows From PTAC Fresh Air Fan.................................................................42

Table 23. Average Apartment Exhaust and Supply Flows (CFM) ..........................................................................45

Table 24. Supply and Exhaust Fan Power ...............................................................................................................45

Table 25. Type 4 Building Apartment Airtightness Metrics....................................................................................45

Table 26. Type 4 Exhaust Flow Measurements .......................................................................................................46

Table 27. Test Results Showing Calculated Leakage From a Plastic Shroud ......................................................50

Table 28. Summary Flow Data From Apartment/Corridor Doors ...........................................................................54

Table 29. Summary Flow Data From Trickle Vents .................................................................................................55

Table 30. Identified Sources of MUA in Apartments (CFM) ....................................................................................55

Table 31. Area of Trickle Vents Required at Various Pressure Differences To Deliver 30 CFM..........................58

Table 32. Leakage Sources of Tested Apartment....................................................................................................60

Table 33. Current and Proposed Building Characteristics for Properly Functioning Trickle Vents ...................61

Table 34. Proposed Leakage Levels for Functioning Trickle Vent ........................................................................61

Table 35. Two Scenarios of Apartment Tightness and MUA Sources...................................................................62

Table 36. Comparison of Ventilation Strategy Fan Power Applied to Two “Sample” Buildings.........................63

Table 37. Comparison of Supply Air Delivered by Each Ventilation Strategy ......................................................65

Table 38. Comparison of Ventilation Strategies......................................................................................................67

Table 39. Optimal Conditions for Each Ventilation System Type ..........................................................................73

Unless otherwise noted, all tables were created by CARB.
ix

Definitions

CARB Consortium of Advanced Residential Buildings

CFM Cubic Feet per Minute

CFM50 Cubic Feet per Minute Flow at A Test Pressure of 50 Pascals

CFM50/ft2 CFM at a Test Pressure of 50 Pascals per Square Foot of Enclosure Area

ERV Energy Recovery Ventilator

HVAC Heating, Ventilation, and Air Conditioning

HRV Heat Recovery Ventilator

kWh Kilowatt-hour

LEED Leadership in Energy & Environmental Design

MUA Make-Up Air

OA Outdoor Air

Pa Pascal

PTAC Packaged Terminal Air Conditioner

SWA Steven Winter Associates

TAB Testing and Balancing

W Watt
x

Executive Summary

In multifamily buildings, particularly in the Northeast, exhaust ventilation strategies are the norm 

as a means of meeting both local exhaust and whole-unit mechanical ventilation rates required by 

ASHRAE 62.2 (ASHRAE 2010b). Through this research and their role in multifamily program 

implementation work in the region, Consortium for Advanced Residential Buildings (CARB)

researchers believe that the majority of high performance, new construction, multifamily housing 

in the Northeast uses one of four general strategies for ventilation: 

1. Continuous exhaust only with no designated supply or make-up air (MUA) source 

2. Continuous exhaust with ducted MUA to apartments

3. Continuous exhaust with supply through a MUA device integral to the unit 

heating, ventilation, and air conditioning system

4. Continuous exhaust with supply through a passive inlet device, such as a trickle 

vent.

CARB researchers have also seen a subset of the first category, where the doors between interior 

corridors and apartments are undercut to allow MUA to flow from the corridor into the 

apartment. The general strategy is to pressurize the corridor using supply air and depressurize the 

apartments through continuous local exhaust. Though in many building codes this practice is 

essentially prohibited for reasons of fire safety, it is still a relatively common practice. 

Insufficient information is available to designers on how these various systems are best applied.

Product performance data are based on laboratory tests, and the assumption is that products 

perform similarly in the field. Since all ventilation systems are designed to work within a specific 

operating environment, proper application involves matching expected performance at expected 

building pressures. Still, in the case of passive ventilation systems that are dependent on 

environmental conditions, there is no guarantee that those conditions exist consistently in the 

finished building. This research effort sought to provide field validation of system performance. 

The performance of four substantially different strategies for providing MUA to apartments was 

evaluated.

Testing indicated that there is considerable variation in the amount of MUA provided by passive 

devices (System 4). Since operation of these devices is based on a pressure difference, any 

factors that affect the pressure across the device directly affect the flow. The role of wind is 

clearly visible by the short-term fluctuation of the pressure monitoring results. The airtightness 

of the apartment plays a major role in the performance of these devices. A well-sealed and 

compartmentalized apartment is needed in order to establish an adequately depressurized space.

In the case of the PTAC MUA fan (System 3), it was found that this strategy provided supply 

flows that were approximately 15% of the MUA. The balance of MUA was split roughly in half 

between leakage around the apartment door and other sources of infiltration. Despite the poor 

performance, this strategy still offers promise. Several changes in the system design could 

significantly improve the performance. In addition, a tight, well-compartmentalized building 
xi

would result in a greater pressure differential, which the MUA fan experiences, and thus a 

greater flow through the fan.

Active MUA strategies (System 2), such as dedicated supply air from a central rooftop unit, can 

be highly effective at delivering specified quantities of MUA to apartments; however, a thorough 

testing and balancing process is required. In the units tested under this study, the performance 

varied from 12% to 132% of the design. This wide range indicates the need for clear, welldefined specifications and careful construction oversight with commissioning and verification of 

system performance.

Testing of local exhaust-only (System 1) indicated that approximately 25% of the MUA for 

apartment exhaust was being drawn from the corridor through leakage around the door, while the 

remainder came from other sources of infiltration. This is most likely a combination of 

infiltration from the exterior, from other apartments, and from interstitial spaces. With so many 

potential sources of MUA, it is not accurate to assume that one source predominates. While some 

factors that influence air movement can be controlled, most cannot. As a result, this strategy does 

not consistently deliver adequate MUA. 

In order for a MUA strategy to function properly, it is necessary that the exhaust system operate 

as designed. On several occasions, it was observed that exhaust fans were not performing at their

design levels. CARB also found situations where the same model fan was installed in each 

apartment, yet the performance was drastically different. These issues are the result of poor 

design, poor installation, or a combination of the two. Again, thorough testing and 

commissioning help to prevent these issues and ensure proper function of both exhaust and MUA

systems.

The airtightness of an apartment plays a major role in the performance of MUA systems, 

especially when passive strategies are employed. Further study should be conducted to determine 

the minimum level of air sealing necessary to ensure performance. This is particularly important 

in maintaining pressure differentials and minimizing fluctuations due to environmental 

conditions.
1

1 Problem Statement

In multifamily buildings, particularly in the Northeast, exhaust ventilation strategies are the norm 

as a means of meeting both local exhaust and whole-unit mechanical ventilation rates required by 

ASHRAE 62.2. According to this Standard, an exhaust system is “one or more fans that remove 

air from the building, causing outdoor air to enter by ventilation inlets or normal leakage paths 

through the building envelope.” If participating in a high performance building program, like 

ENERGY STAR® Certified Homes, exhaust ventilation system flow rates must be verified in the 

field to demonstrate compliance with ASHRAE 62.2 ventilation rates, but the source and 

quantity of make-up air (MUA) is rarely addressed. Although ENERGY STAR Certified Homes, 

Version 3, Rev. 07, requires that “ventilation air comes directly from outdoors, not from adjacent 

dwelling units, garages, crawlspaces, or attics,” dedicated MUA is not explicitly required, and 

the source and quantity are not verified (EPA 2010). The issue of where the “fresh” air is coming 

from is gaining significance as airtightness standards for enclosures become more stringent, and 

the “normal leakage paths through the building envelope” disappear.

Even with few data available to support it, some high performance building programs, such as 

the U.S. Green Building Council’s Leadership in Energy & Environmental Design (LEED) for 

Homes and the New York State Energy Research and Development Authority’s Multifamily 

Performance Program, have recognized the importance of MUA. These programs have already 

included additional prescriptive requirements for MUA strategies, beyond what is required by 

ASHRAE 62.2, but still without any commissioning requirements to verify that the MUA system 

actually works. For example, LEED for Homes Mid-Rise requires that “outdoor air must be 

provided to each unit directly from the outdoors” (LEED 2010). While ASHRAE 62.2-2010, 

Section 6.1.1 accepts a ventilation system design that “explicitly requires transfer air from 

corridors into units,” LEED for Homes Mid-Rise explicitly prohibits “systems that rely on 

transfer air from pressurized hallways or corridors, adjacent dwelling units, attics, etc.” Other 

programs, like ENERGY STAR’s Multifamily High Rise Program, are waiting for building 

research to inform their ventilation requirements.

As the industry waits for research results, exhaust ventilation approaches and corresponding 

MUA strategies (if any) are being implemented in high performance new construction 

multifamily buildings to meet program or code requirements, but a lack of clear guidance is 

resulting in poor implementation of ventilation systems despite the best intentions of the 

programs or standards cited above. Provisions for MUA, without depending on infiltration, come 

in many forms, ranging from fully ducted systems that deliver MUA directly to apartments, to 

designed holes in the envelope such as trickle vents, or outdoor air (OA) dampers tied to the 

space conditioning system. Passive systems are some of the least expensive options for providing 

MUA directly to apartments, but not enough is known about how effective they actually are. 

Pressurizing corridors by oversizing corridor space conditioning systems to provide an indirect 

source of MUA to apartments is a controversial but common practice that, thus far, has led to 

high energy use with no data to justify the perceived indoor air quality benefit to the apartment. 

As ventilation rates increase along with air-tightness requirements, the MUA issue is only going 

to be exacerbated. Joe Lstiburek’s article “Unintended Consequences Suck” in the June 2013 

ASHRAE Journal expounds upon this (Lstiburek 2013). However, regardless of the ventilation 
2

design rate being implemented, design support to improve the performance of multifamily 

mechanical ventilation systems is warranted. 

Insufficient information is available to designers on how these various systems are best applied.

Product performance data are based on laboratory tests, and the assumption is that products 

perform similarly in the field. Since all ventilation systems are designed to work within a specific 

operating environment, proper application involves matching expected performance at expected 

building pressures. Still, in the case of passive ventilation systems that are dependent on 

environmental conditions, there is no guarantee that those conditions exist consistently in the 

finished building. Evaluation is needed to validate system performance in the field.

1.1 Previous Research

Steven Winter Associates/the Consortium for Advanced Residential Buildings (SWA/CARB)

have been researching residential ventilation systems for more than a decade. In 2006, CARB

tested and compared ventilation systems in high performing single-family homes in south 

Chicago, to evaluate performance and cost effectiveness (Aldrich 2006). In 2008, SWA 

evaluated the performance of self-balancing air dampers in multifamily central exhaust 

ventilation systems. The importance of air sealing the exhaust duct work was evident and 

methods including the Aeroseal technology were examined. This research was funded by the 

Partnership for Advancing Technology in Housing Program and the report was published by the 

National Association of Home Builders Research Center. The work was frequently cited by 

Building Science Corporation in its ventilation research study performed as part of an energy 

retrofit of an existing Philadelphia multifamily building (BSC 2012). 

In 2009, SWA, Camroden Associates (Terry Brennan), and the National Center for Healthy 

Homes developed two best practice guides for exhaust-only systems in multifamily buildings 

applicable to both new construction and retrofits. These guides provide a blueprint for designing 

and implementing exhaust-only ventilation systems in multifamily buildings that meet ASHRAE 

62.2 local exhaust and whole-unit ventilation rate requirements (NCHH 2009a, 2009b).

In 2013, CARB again evaluated ventilation systems in single-family homes to determine 

performance and cost effectiveness; however, this time the focus was retrofits of existing homes 

(CARB 2013). Neither the Building Science Corporation research nor these SWA/CARB

research projects evaluated the source or quantity of the MUA.

In 2010, SWA performed testing in a multifamily building that had taken the extra step to pair 

exhaust ventilation with appropriately sized trickle vents, in order to provide a dedicated source 

of MUA. To support the development of the ENERGY STAR Multifamily High Rise program, 

EPA funded SWA to perform similar ventilation research in three additional midrise multifamily 

buildings. Through this work, a repeatable protocol for evaluating the performance of the 

compartmentalized apartment/unitized ventilation design has been developed and further refined 

for the current research.

The Canadian Mortgage and Housing Corporation has conducted considerable research on 

residential ventilation systems. A survey of corridor ventilation systems in midrise and high-rise 

multifamily buildings found that actual corridor and exhaust flows were considerably lower than 
3

the design levels. Environmental conditions, building airtightness, and compartmentalization all 

affected the flows. The study found that designers assumed MUA would be provided from the 

corridor and infiltration; however, little was done in the design or construction to ensure this.

Further research by the Canadian Mortgage Housing Association has shown that ventilation 

systems can be affected by a complex interaction of factors. Under- and overventilation are 

common occurrences. In corridor supply systems it has been found that much of the air bypasses 

the apartments and exits through stairwell, elevator shafts, and other points. In the study, four 

alternative ventilation strategies were evaluated (CMHC 2003), including passive vents and three 

variations of heat recovery ventilator (HRV) systems. It was found that passive vents were 

susceptible to wind and stack pressures and require compartmentalization of apartments to work 

properly. HRVs performed well, but were expensive and required greater maintenance.

Another study looked at the impacts of corridor ventilation on building energy consumption. The 

common notion this study examined was the belief that corridor supply systems can displace 

space heating by pressurizing the building. The results of the study disprove this concept. The 

corridor system did not displace significant amounts of infiltration or space heating load. The 

conclusion was heated air form the corridor system was finding direct routes to the outside and 

bypassing most areas.

Building Science Corporation has found that central ventilation systems in multifamily buildings 

often have poor overall performance. It recommend compartmentalizing apartments, sealing 

ductwork, and installing variable-speed, pressure-controlled fans with electronically commutated 

motors. The resulting system can improve performance and reduce heating energy and fan 

electric energy.

1.2 Research Questions

The following research questions were the focus of this project:

• What type of exhaust ventilation systems and corresponding MUA strategies (if any) are 

being implemented in high performance new construction multifamily buildings to meet 

whole-unit ventilation rates of ASHRAE 62.2?

• How can the air delivery performance of passive MUA strategies such as trickle vents, 

OA dampers, or door undercuts be measured and quantified in actual installations? How 

variable is the performance and what are the key factors that affect it—apartment 

location, extent of compartmentalization, season, etc.? 

• Do the MUA strategies for apartments, both passive and active, function as designed 

when installed in actual buildings?

• Can specifications be developed to better ensure proper performance from all MUA

strategies?

• What is the energy cost associated with the fan power of mechanical supply systems?

• Considering the choices for various ventilation approaches and the factors impacting their 

cost-effectiveness—material cost, efficiency, and performance—under what conditions is 

each system optimal? 
4

2 Technical Approach

2.1 Make-Up Air Ventilation Strategies

This research project evaluated the performance of four substantially different strategies for 

providing MUA to apartments: 

• No direct MUA supplied to apartments

• MUA through ducted supply air

• MUA through HVAC supply (i.e. packaged terminal air conditioner [PTAC] OA damper)

• MUA through passive vents.

2.1.1 Type 1: No Direct Make-Up Air Supplied to Apartments

In multifamily buildings, both high-rise and low-rise, although local mechanical exhaust and 

whole-unit ventilation may be provided, there might not be a provision for direct MUA to the 

apartments (see Figure 1). In the case of buildings constructed to low-rise residential building 

codes with no common spaces (e.g., townhouses), MUA may be assumed to come from air leaks 

in the exterior envelope. Also, when a building is designed with a common hallway and interior 

apartment entries, OA may be delivered to corridors but none is supplied directly to apartments.

The intention, however controversial, may be that this air passes into apartments indirectly 

through door undercuts.

Figure 1. Example of a building with no direct MUA to apartments

Newer building codes for high-rise buildings essentially prohibit this latter approach, in part 

because it is not compatible with fire-resistant construction principles that call for complete 

separation of apartments from each other and from common spaces. ASHRAE Standard 62.1-

2010 calls for very little corridor ventilation (0.06 CFM/ft2 of corridor area), removing the 

rationale for pressurizing corridors with high amounts of OA. Still, a very common design for 

high-rise buildings calls for apartments with exhaust-only ventilation, no provision for direct 

supply air, and the expectation that at least some MUA will be supplied by the overventilated 
5

corridors. This research assesses whether corridor air makes its way into apartments through air

leakage around the door frame and door undercut, intentionally or not.

2.1.2 Type 2: Make-Up Air Through Ducted Supply Air

This system type generally has continuous in-unit or central exhaust from apartments and 

continuous central fresh MUA directly supplied to each apartment (see Figure 2). In some 

instances, the supply air is provided by an energy recovery ventilator (ERV) or HRV that 

captures energy from central apartment exhaust; in others, it can be provided by the same space 

conditioning system serving the corridor. Verifying the delivery of MUA to apartments is 

straightforward, involving measurement at supply registers within the apartment. Research goals 

for this system type included verifying supply air delivery rates, making measurements of power 

consumption of the unit’s exhaust and supply fans, and measuring the heat recovery efficiency 

(if present).

Figure 2. Example building with central ducted supply (left) and a central supply unit (right)

2.1.3 Type 3: Make-Up Air Through HVAC Supply

This system type uses the heating and/or cooling system of individual apartments as the source 

of MUA delivery through one of a variety of methods. In buildings with PTACs, MUA can be 

provided either through passive OA dampers that draw in air during normal blower operation or 

through a dedicated, powered OA supply fan in each PTAC unit (see Figure 3). The strategy is 

similar for buildings with fan coils or with ducted in-apartment air handlers using air cyclers or 

other integrated fresh air devices.
6

Figure 3. Example of a building with MUA from PTACs

2.1.4 Type 4: Make-Up Air Through Passive Vents

This system type uses engineered passive vents in the exterior envelope to provide MUA from 

outside. Exhaust in apartments is provided by either central or in-unit fans operating

continuously. The negative pressure that these fans create is meant to draw MUA into the 

apartment through these passive vents.

The research focus for this system type is the functioning of passive vents under various pressure 

conditions. Since the passive vents are meant to operate in a general environment of negative 

apartment pressure, the research assessed whether these negative pressures prevail through a 

variety of environmental conditions.

2.2 Factors Affecting Ventilation System Performance and Choice of 

Measurements

Many factors affect the performance of a ventilation system and its ability to deliver fresh air to 

apartments, including elements of the proposed building design, its as-built construction, and 

human and environmental interaction with the building during occupation (see Table 1). Because 

the tests chosen for evaluation of performance in the study buildings are only a sample of a large 

number of possible tests, this study cannot provide final conclusions for ventilation systems of 

every type.
7

Table 1. Factors Affecting Ventilation Performance

Design Factors

• Overall ventilation scheme

• Supply and exhaust design flow rates

• Choice of ventilation equipment

• Size and shape of building

• Choice of building materials and techniques

Construction 

Factors

• Quality of ventilation equipment and duct installation

• Airtightness of the building’s exterior envelope and interior 

partitions

Environmental 

Factors

• Indoor and outdoor temperature

• Wind pressures

Human Factors

• Operation of ventilation system’s “modes” (high, low, etc.)

• Resident operation of windows and other air pathways

• Temperatures of building spaces

The tests below were chosen to evaluate construction and environmental factors in this study 

where possible. No attempt was made to quantify design factors because they are too varied to 

consider in this study. The influence of human factors was not adequately prepared for in this 

study and led to difficulty in evaluating some of the other factors. Further studies should more 

fully anticipate their influence.

2.3 Building Descriptions and Information 

Test buildings were selected that represented each of the four ventilation strategies. Selection 

was based on construction schedules and owner enthusiasm for participating in this study. The 

aim was to choose buildings that had just completed construction, but were not yet fully 

occupied. This was to facilitate initial testing in the apartments. All of the test buildings in this 

study completed construction between late 2012 and 2013.

2.3.1 Type 1 Building: No Direct Make-Up Air Supplied to Apartments

The “No Direct Make-Up Air” test building is part of a senior housing complex in the greater 

Albany area of New York. The complex consists of a central building and numerous residential 

buildings. The structures are all connected; however, each building is separated by fire doors. 

The portion of the complex that was used for testing is circled in Figure 4. The apartments where 

pressure monitoring was conducted are highlighted in yellow. The three numbers in each box 

indicate the apartment number on each floor, lowest being floor 1, highest being floor 3. 
8

Figure 4. Apartment layout of test building “Type 1”

The test building is three stories tall. There is a corridor (~3,800 ft2

) that runs the length of the 

building, one elevator bank with two elevators, and four stairwells at the ends and midpoints of 

the building. Each floor has 26 apartments, two storage rooms, two lobbies, three garbage rooms, 

and two janitor closets. 

Ducted OA is supplied to the corridors by nine vents on each floor, with a design supply of 850 

CFM, or 0.22 CFM/ft2. Although ASHRAE 62.1 requires a minimum of only 0.06 CFM/ft2 for 

corridors, the system was intentionally designed to provide an additional 20–30 CFM per 

apartment, to indirectly provide MUA to the units (ASHRAE 2010a). In addition to the corridor 

ventilation, there is supply to each storage room (designed at 250 CFM/floor), exhaust from the 

three garbage rooms (designed at 420 CFM/floor), and exhaust from the two janitor closets 

(designed at 100 CFM/floor).

To meet the local exhaust and whole-unit ventilation rates required by ASHRAE 62.2, each 

apartment has a bathroom exhaust fan with occupancy-sensor boost that exhausts directly 

through the exterior wall of the apartment. When motion is detected in the bathroom, the fans 

ramp up from a low continuous ventilation rate of ~30 CFM to a high boost. In addition, kitchens 

in apartments are equipped with range hoods that exhaust directly to the exterior. The range 

hoods have three settings: high, low, and off; and are manually operated.

In theory, the design supplies 5,250 CFM of OA to the corridors and common spaces, and 

exhausts only 1,560 CFM from the garbage rooms and janitor closets, which should leave the 

corridor positively pressurized, with more than 40 CFM of OA per apartment.

2.3.2 Type 2 Building: Make-Up Air Through Ducted Supply Air

Two buildings were selected that represent the “Type 2” strategy. Both buildings are located in 

eastern Massachusetts. Both were existing former industrial buildings that were completely 

rehabilitated. Each building participated in utility conservation programs and the LEED for 
9

Homes certification process during its construction. Participation included testing of apartment 

airtightness and testing and balancing (TAB) of heating, ventilation, and air conditioning 

(HVAC) system flows. Building #1 is four to five stories with 62 apartments, while Building #2 

is five stories with 64 apartments. Each building has a central ERV on the roof that continuously 

exhausts air from apartment bathrooms and kitchens and delivers fresh MUA back to each

apartment and corridor.

To meet the local exhaust ventilation rates required by ASHRAE 62.2, each apartment bathroom 

was designed to exhaust 25–30 CFM continuously and each apartment kitchen was designed to 

exhaust 50–60 CFM continuously, with the exception of the few units on the fifth floor in 

Building #1, where the kitchens are intermittently vented through the roof and not attached to the 

ERV. To meet the whole-unit ventilation rates of ASHRAE 62.2 and the prescriptive OA

requirements for LEED for Homes certification, the design called for 50–90 CFM of MUA, 

ducted directly into the apartment. By design, the supply to the apartments was not intended to 

provide the same CFM as exhausted, but rather to meet the whole-unit ventilation rates of 

ASHRAE 62.2, which are lower than the continuous local exhaust requirements. Typical floor 

plans of both buildings are shown in Figure 5 and Figure 6. 

Figure 5. Typical floor plan of test building #1 of “Type 2” 

Figure 6. Typical floor plan of test building #2 of “Type 2” 

2.3.3 Type 3 Building : Make-Up Air Through HVAC Supply

The “Type 3” test building is part of a senior housing complex in Westchester County, New 

York (Figure 7). It is a four-story, L-shaped building. There are 17 apartments per floor. There 
10

are a laundry room, a two-elevator bank, and three stairwells per floor. A typical floor plan is 

shown in the figure below. Pressure monitoring was conducted in apartments P and R on floors 

2, 3, and 4. The locations of these apartments are indicated by the shaded blue boxes.

Figure 7. Typical floor plan of test building “Type 3” 

The building participated in LEED for Homes Certification and the New York State Energy 

Research and Development Authority’s Multifamily Performance Program during its 

construction, which required a provision for OA and included testing of apartment airtightness 

and TAB of HVAC system flows. The apartments are heated and cooled by PTACs with hot 

water coils. To meet the local exhaust required by ASHRAE 62.2, each apartment has an inline 

exhaust fan that draws air from the kitchen and bathroom. These fans operate continuously and 

vent directly to the exterior. The design calls for 30 CFM from the kitchen and 25 CFM from the 

bathroom. To provide MUA to meet the whole-unit ventilation rates, a PTAC with an OA 

damper and powered-fan was installed, which was intended to provide 40 CFM continuously, 

whether or not the main PTAC blower fan was running. Studio apartments have one PTAC, 

while one-bedroom units have two. All PTACS are equipped with fresh air kits.

2.3.4 Type 4 Building: Make-Up Air Through Passive Vents

The “Type 4” test building is located in the Bronx, New York. It is a five-story, T-shaped 

building. There are 13 apartments per floor. There are a laundry room, a refuse room, a storage 

room, an elevator bank, and two stairwells per floor. A typical floor plan is shown in Figure 8. 

Pressure monitoring was conducted in apartments 3F, 4K, 4L, 4M, 5A, and 5H.

Apt 

P

Apt 

R
11

Figure 8. Typical floor plan of test building “Type 4”

The building participated in LEED for Homes Certification and the New York State Energy 

Research and Development Authority’s Multifamily Performance Program during its 

construction, which required a provision for OA and included testing of apartment airtightness 

and TAB of HVAC system flows. The apartments are heated by hydronic baseboard convectors.

To meet the local exhaust and whole-unit ventilation rates required by ASHRAE 62.2, the 

apartments are ventilated by inline fans drawing from kitchens and bathrooms and operating 

continuously. MUA is intended to come from trickle vents installed in window frames. There is 

one trickle vent in each living room and one per bedroom. It is not clear how the trickle vents 

were “sized” or selected for this building. They are all the same make and model.

2.4 Measurements and Tests

The tests listed in Table 2 were used in the analyses of the different ventilation strategies. For 

each building type they can be grouped into three main stages: detailed tests on building 

components, pressure monitoring in the winter, and pressure monitoring in the summer. For each 

building type, at least one building was tested in detail. Minimum sampling rates for each test are

also shown below. These rates were determined based on the availability of previous test data 

and access to units for additional testing. Pressure monitoring was conducted for 2 weeks during 

the winter and 2 weeks during the summer. The length of the sampling period was based on the 

availability of equipment, availability of occupied units, and need to collect data that are 

representative of the season. The aim was to make the sampling period as long as possible, while 

still being able to monitor all of the buildings.
12

Figure 9. Powered flow hood

Table 2. Measurements and Sampling Rates at Each Building Type

Test

Number of Samples per Building Type (Percentage)

No Direct 

MUA

Supplied to 

Apartments

MUA

Through 

Ducted 

Supply Air

MUA

Through 

PTAC

MUA

Through 

Passive Vents

Apartment Blower Door Testing 27 (35%) 29 (23%) 9 (15%) 9 (14%)

Ducted MUA Supply Flows to 

Apartment 45 (36%)

Corridor Supply Flows 31 (94%)

Door Leakage Airflow 8 (10%) 7 (11%) 5 (8%)

PTAC OA Damper Airflows 5 (8%)

Passive Vent Testing 5 (3%)

Apartment Exhaust Flows 11 (14%) 45 (36%) 6 (10%) 14 (22%)

Pressure Monitoring 5 (6%) 5 (8%) 5 (8%)

Details for the tests designed to provide data on various aspects of building and ventilation 

performance are given in Section 2.4.1 through Section 2.4.6. For improved context, details on 

the tests specific to a particular building type are provided in Section 3.

2.4.1 Apartment Blower Door Testing

Apartment airtightness (compartmentalization) is an important aspect of high performance 

construction, impacting the function of ventilation systems and other airflow patterns in an 

operating building. Evaluating apartment blower door data alongside ventilation performance 

measurements reveals correlations between the two. Buildings selected for evaluation have 

recently participated in high-performance building programs, where single-point, single-unit 

unguarded depressurization tests were conducted at 50 Pa, for more than 10% of the units.

2.4.2 Supply and Exhaust Flow Testing

The proposed testing involves using an orifice/pressure box or 

powered flow hood to measure airflow at apartment ventilation 

supply and exhaust registers, and comparing this to design values

(see Figure 9). Airflow at corridor supply registers and common 

area exhaust registers will also be tested by this method.

2.4.3 Door Leakage Airflows

The testing apparatus for this experiment consists of a capture 

hood to encapsulate the door and capture airflow and a fan to 

impart a pressure on the hood and measure airflow. Readings are

taken at different pressure drops across the door and the data 

analyzed to identify a basic curve for predicting airflow 

through a typical apartment door in the building (see Figure 

10). These data are then combined with pressure monitoring results to infer the direction and 

magnitude of airflow across the apartment doors on a building-wide basis over time.
13

Figure 10. Setup for testing air leakage from a door frame

The testing apparatus is set up by constructing a capture hood from rigid polyisocyanurate foam 

board over the apartment entry door. This hood captures all airflow from the door frame and 

directs it to an opening where it is measured by a powered flow hood. A pressure difference 

across the door (A to B) is developed at several different levels. At each pressure difference, the 

powered flow hood measures the airflow. The powered flow hood is capable of both 

pressurization and depressurization tests but must be configured separately for either mode.

Door leakage in one of the test buildings in this study was measured using an alternate orifice 

plate method, because the above method had not been developed yet. The orifice plate method is 

described in more detail in Section 3.4.5 for the building in which it was used. Although the 

method was accurate, its setup was time consuming and cumbersome, leading to the need for an 

easier, faster way to measure leakage. The method described in this section is an improvement 

that was developed during the course of this study.

The two methods of measuring door leakage were not directly compared on the same building, 

but a check of the orifice plate method was made by comparing it to blower door results, as 

described in Section 3.4.5.

2.4.4 Pressure Monitoring

Airflow patterns in buildings are dependent on pressure differences. Understanding the major 

patterns of pressure fields is important to predicting the behavior of the airflow-related systems 

(mechanical, envelope, etc.) under various environmental and operational conditions. In most 

buildings during the winter, pressures prevailing on the exterior envelope are positive with 

E

Capture 

hood

B

Pressure 

inside 

hood

Pressure in 

corridor

Powered 

flow hood
14

respect to outside near the top of the building and negative at the bottom of the building, as 

illustrated in Figure 11. By comparing the results of flow testing with a “map” of pressure 

differences, the airflow through passive inlets and its variability can be predicted.

Figure 11. Typical pressure patterns in a multistory building in winter 

An array of pressure sensors will be used to monitor pressure differences and predict general 

airflow patterns between key building areas. The following points will be measured with digital 

gauges and communicated to a central monitoring station: ground floor interior/exterior; 

apartments on lower, middle, and upper floors; and upper floor interior/exterior. A total of five

points will be measured over a 1–2 week period. This test will be conducted twice; once during 

the winter and once during the summer season to observe the operating difference.

The goal of characterizing airflow at various pressures through trickle vents, apartment entry 

doors, and OA dampers of PTAC units is a predictable model of airflow patterns through these 

leakage points over time. By comparing the results of airflow testing with a map of pressure 

differences, the airflow through passive inlets can be predicted. Prevailing pressures monitored 

over time will indicate expected airflows. 

2.4.5 Supply and Exhaust Fan Power Consumption

Fan electricity power consumption is estimated from product literature at the flow rates 

measured. The electricity use combined with the flow rate can be used to determine the relative 

fan efficiency in cubic feet per minute of air per Watt of electricity (CFM/W).

2.4.6 Factors Affecting the Measurement

In addition, environmental conditions play a large role in the operation of passive devices, the 

primary factors being wind and temperature. The role of wind is still clearly visible by the shortterm fluctuation of the pressure monitoring results; however, the specific effects are difficult to 

quantify because of its intermittency and high variability. In addition, local wind data were not 

collected as part of this study. 

Typically 

positive 

pressures

Typically 

negative 

pressures
15

There is no clear correlation between apartment location and flow through passive devices. In 

general, apartments on higher floors experience greater pressure variation due to wind; however,

the specific geometry and situation of the building on its site enhance or detract from the impact 

of wind speed and direction. Location of trees, open spaces, setbacks, and other buildings all 

affect the local environment and can result in significant variations in building pressures for 

apartments on the same floor. 

Occupant behavior is likely the largest influence on system behavior, but the effect is difficult to 

quantify or predict. Occupants may alter the operation of the ventilation system directly, for 

example by opening, closing, or blocking off vents, or turning on or off fans; or they may affect 

their operation indirectly by opening windows or adding or removing seals like weather-stripping

to doors.

Occupant interaction with the ventilation systems can become a major challenge to measurement 

efforts, as additional efforts must be made to either record alterations to the systems by 

occupants, or to analyze the data and understand where occupant behavior is an influence. In this 

study, efforts were made to educate residents on the aims of the study and the requirements for 

participation. For example, residents were asked not to operate their windows during the study, 

but since CARB was not able to monitor them directly, there is no way to be certain that they 

were not. Future studies will need to record occupant interaction more directly if they are 

conducted in occupied buildings. 
16

3 Results and Findings

The following subsections are organized according to the four MUA strategies discussed in the 

Technical Approach. The tests associated with each strategy and corresponding results are 

presented below.

3.1 Type 1: No Direct Make-Up Air Supplied to Apartments

3.1.1 Apartment Blower Door Testing 

Blower door tests were conducted on 35% of apartments pursuant to utility incentive and green 

building program requirements. The building is built with an average apartment airtightness of 

0.24 CFM50/ft2 of enclosure area (or 5 ACH50) (see Table 3).

Table 3. Type 1 Building Apartment Airtightness Metrics

Measure Unit Floor Area 

(ft2) CFM50 CFM50/ft2Enclosure ACH50

Low 770 488 0.17 3.61

Average 1,018 767 0.24 5.06

High 1,115 990 0.30 6.87

3.1.2 Supply and Exhaust Flow Testing

The exhaust airflow of the bathroom fans was measured using an orifice/pressure box (Exhaust 

Fan Flow Meter from The Energy Conservatory). The average airflow from the bathroom fans 

during normal operation was 34 CFM. The average airflows from the first and second baths in 

“boost” mode were 91 and 92 CFM, respectively. The results are summarized in Table 4. Some 

of the variation in flows is due to the fact that two models of fans were used. One is designed for 

30 CFM at low and 80 CFM on boost. The other fan type is designed for 50 and 130 CFM.

Table 4. Bathroom Exhaust Flows

Apartment 

(Floor)

Bath 1 Low Speed,

Measured/Design,

CFM)

Bath 1(Boost, 

Measured/

Design, CFM)

Bath 2 (Boost, 

Measured/Design, 

CFM)

1220 (1st) 31/30 Not tested 63/80

1221 (1st) 40/50 114/130 99/130

1227 (1st) 32/30 78/80 80/80

1229 (1st) 40/50 126/130 132/130

1248 (2nd) 30/30 68/80 79/80

1252 (2nd) 34/30 76/80 136/80

1255 (2nd) 30/30 81/80 Not tested

1257 (2nd) 32/30 83/80 74/80

1276 (3rd) 34/50 112/130 111/130

1283 (3rd) 35/30 78/80 72/80

1287 (3rd) 37/30 91/80 78/80

Average 34 91 92

The exhaust flow from the kitchen range hood was not measured due to the size of the hood and 

the fact that it operates only when switched on by residents. It is rated for 220 CFM.
17

The corridor and storage room supply flows and garbage exhaust flows were measured using a 

powered flow hood. Supply flows to the lobbies and server rooms and exhaust from janitor 

closets were not measured. Measured supply flows were 15%–60% below design airflow.

Table 5. Measured Corridor Supply Flows (CFM) by Floor

Location Floor 1 Floor 2 Floor 3 Design

1 84 72 62 100

2 64 72 55 100

3 87 83 74 100

4 86 76 86 100

5 71 81 82 100

6 93 83 93 75

7 59 93 70 75

8 100 66 72 100

9 75 80 80 100

CFM/Floor 719 706 674 850

CFM/Apartment 27.7 27.2 25.9 32.7

Table 6. Storage Room Supply Flows (CFM) by Floor

Location Floor 1 Floor 2 Floor 3 Design

Storage 1 84 67 51 150

Storage 2 85 65 53 100

CFM/Floor 169 132 104 250

Table 7. Garbage Room Exhaust Flows (CFM) by Floor

Location Floor 1 Floor 2 Floor 3 Design

Garbage 1 107 147 145 130

Garbage 2 150 111 143 120

Garbage 3 234 300+ 300+ 170

CFM/Floor 491 558+ 588+ 420

When measuring the exhaust flows of the garbage rooms, the flow exceeded the 300 CFM limit 

of the powered flow hood (and the design) in several locations. It is estimated that the flow 

ranges from 100 to 400 CFM for each of the three garbage rooms on each floor, for total exhaust 

flow per floor of 500–600 CFM, which is 20%–40% over design. The result is a net supply of 

approximately 11 CFM per apartment, which is significantly lower than the design rate. As a 

result, the corridor is less pressurized than the design specification.

3.1.3 Door Leakage Airflows

As described in Section 2.2.3, the test apparatus for this experiment consists of a capture hood to 

encapsulate the door and capture airflow, and a fan to impart a pressure on the hood and measure 

airflow. Using a smoke pencil, the airtightness of the seal around the doorframe was verified (see 

Figure 12). Readings were taken at different pressure drops across the door (A to B in Figure 13) 

and the data analyzed to identify a basic curve for predicting airflow through a typical apartment 
18

door in the building. These data are later combined with pressure monitoring results to infer the 

direction and magnitude of airflow across the apartment doors on a building-wide basis over 

time.

Figure 12. Typical door weather-stripping coverage

Figure 13. Concept for testing air leakage from a door frame (left)

and actual test in progress (right)

The test was performed in both pressurization and depressurization modes on seven doors. The 

results were often similar, with the pressurization test usually slightly higher. One reason for this 

might be that the weather-stripping on the doorframes is more effective in one direction than in 

another. That is, in one direction the weather-stripping is held closed by air pressure, while in the 

other direction it is pushed slightly open. It is also likely that due to the shape of the door gap, 

flow is greater in one direction than the other at an equivalent pressure. Results from a single 

door test are shown in Figure 14 for an example.

A

Capture 

hood

B

Pressure inside hood

Pressure 

in corridor

Powered flow 

hood
19

Figure 14. Results of air leakage test from a single door

Plotted together, the results from seven doors tested, both by pressurization and depressurization, 

are shown in Figure 15.

Figure 15. Results of air leakage test from all doors tested

The leakage values from all eight doors tested are given in terms of CFM50 below, with the 

results of their enclosure testing, also at 50 Pa, for context. Since the enclosure testing utilized 

the door for the blower door fan, the sum of the two columns represents the true total apartment

enclosure leakage.

R2 = 0.9957

R2 = 0.9982

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 80 90 100

Pressure (Pa)

Flow (CFM)

Air Leakage Characteristics of a Door Frame

Depressurization Test

Pressurization Test

0.0

5.0

10.0

15.0

20.0

25.0

0.0 20.0 40.0 60.0 80.0 100.0 120.0

Pressure (Pa)

Flow (CFM)

Air Leakage Characteristics of Seven Apartment Doors

Pressurization Depressurization
20

Table 8. Tabulated Results of Door Air Leakage Tests (CFM50)

Apartment 

(Floor) Depressurization Pressurization Average (CFM50)

Apartment 

Air Leakage 

(CFM50)

1222 (1st) 184 195 190 958

1252 (2nd) 131 143 137 616

1255 (2nd) 115 126 121 695

1257 (2nd) 238 274 256 488

1259 (2nd) 124 Not tested 124 980

1282 (3rd) 123 152 138 761

1283 (3rd) 176 184 180 700

1287 (3rd) 156 160 158 518

Average 156 176 165 767

The data show that most of the doors had an equivalent leakage area at 4 Pa of 10–11 in.

2

. Of the 

entire sample set measured through both pressurization and depressurization, the average leakage 

amounts to an equivalent leakage area of 12.7 in.

2

.

The flow from door leakage is calculated using the orifice flow equation.

Q = C∆P n (1)

Where:

Q is leakage through the door gaps (in CFM).

C is the flow coefficient.

ΔP is the pressure difference between inside and outside of the test system (in Pascals).

n is the pressure exponent.

Using a linear regression on the logarithm of pressure and flow readings, the pressure exponent n

and flow coefficient C can be derived for each door, measured by both pressurization and 

depressurization. When the average pressure exponent n and flow coefficient C from all the 

tested doors is determined, the performance of a typical door can be predicted at any commonly 

experienced pressure in the building over time. This information is used in conjunction with 

long-term pressure monitoring to infer the direction and magnitude of flow through these 

openings over time. The equation used to calculate flow through the door gaps is as follows:

Q = 22.366 * ∆P 0.5097 (2)

The results of this equation evaluated at a variety of building pressures are shown superimposed 

on the data in Figure 16.
21

Figure 16. Results of air leakage test from all doors tested and average door performance

The results of this testing show that air flows through the apartment door very easily in spite of 

efforts to weather-strip the gaps around the door frame. No door sweep was installed, so it is 

assumed that the majority of the air leakage is through the door undercut, measured to be 

approximately 3/8” along the bottom. If even a small pressure difference exists between the 

apartment and the corridor, air flows into or out of the apartment.

To evaluate the airflow during “typical” operating conditions rather than specific pressure 

differences, the airflow through the eight apartment doors was recorded under four scenarios:

first, with apartment exhaust fans “off” (although one bathroom, “Bath 1,” remains on at 

low continuous rate only, ~34 CFM); second, with one bath fan, “Bath 1,” on boost mode

(~91 CFM); third, with both bath fans on boost mode (~183 CFM); and fourth, all fans running

(~400 CFM), including the range hood operating on high.

While the design expectation was that the first scenario, with just the one bath fan running at 

low, should draw in 30 CFM of MUA from the corridor, three of eight units measured 0 CFM 

across the door in this condition. The average airflow through the apartment door due to the 

operation of an additional fan is shown in Figure 17 and ranges from 9 CFM to 61 CFM, while 

the exhaust fan airflows range from 34 CFM to 400 CFM, respectively. During the tests, the 

doors to the bathrooms and bedrooms were left open, offering the least restriction to airflow 

possible and the most airflow under the door.

0.0

5.0

10.0

15.0

20.0

25.0

0.0 20.0 40.0 60.0 80.0 100.0 120.0

Pressure (Pa)

Flow (CFM)

Air Leakage Characteristics of Seven Apartment Doors

Pressurization Depressurization Average
22

Figure 17. Incremental door airflow under various scenarios

It appears that the first bath fan (generally the “master bath” in this building) is responsible for 

more airflow when in boost mode, but more likely its operation diminishes the additional effect 

of another bath fan. The range hood appears to have quite an effect, likely due to its proximity to 

the apartment door. Although its airflow was not measured, it is rated for approximately 220 

CFM at its high-speed setting, compared to the ~92 CFM measured in each bathroom when in 

“boost” mode. 

Compared to the flow measured through the corridor door, it is clear that while some of the 

MUA for these exhaust fans comes through the apartment door as intended (~15%–25%), a good 

deal more comes from other places—probably a combination of the exterior envelope, duct 

leakage, and leaks from other apartments and the walls adjacent to the corridor. Based on these 

short-term results, it is clear that the 30 CFM of MUA supplied to the corridor intended for each 

apartment, does not manage to make it into the apartment, through the door undercut or other 

door frame leakage, while the whole-unit ventilation system is in typical continuous operation, 

but rather only when both bath fans are in boost mode (see Table 9).

Table 9. Average Apartment Supply and Exhaust Flows

Scenario

Measured 

Exhaust 

(CFM)

Door Leakage

Supply

(CFM)

Unaccounted

Supply

(CFM)

Base: Bath Fan 1 on Low 34 9 25

Both Bath Fans on Boost 183 37 146

3.1.4 Pressure Monitoring: Cooling Season

Pressure monitoring was conducted in August in five apartments, as highlighted in Figure 7. The 

pressure of the apartment with respect to the corridor was logged over a period of approximately 

2 weeks. A negative pressure indicates that the apartment was depressurized with respect to the 

corridor. Data samples were taken each minute. The average for each hour was determined and 

plotted in Figure 18.

9

25

37

61

0

10

20

30

40

50

60

70

Bath 1 Fan on

Low Only

(~34 CFM)

Bath 1 Fan

Boost Only

(~91 CFM)

Bath 1&2 Fan

Boost

(~183 CFM)

+ Kitchen

Exhaust

(~400 CFM)

Airflow (CFM) through Apartment Door 

Due to Operation of Exhaust Fans
23

Figure 18. Apartment pressure with respect to corridor—cooling season

Of the five apartments, four had an average pressure > 0, which indicates that air flows through 

the door predominantly, and surprisingly, from the apartment to the corridor. Apartment 1225, 

on the ground floor, was the only apartment that was consistently depressurized. The initial 

hypothesis was that all apartments would be slightly depressurized with respect to the corridor, 

but the opposite was true.

One possible explanation is that bath fans operate continuously at a low level and switch to boost 

mode when the occupancy sensor detects motion. Since the units were unoccupied, the fans were 

all most likely operating at the low level. This level exhausts considerably less air than the boost 

mode. In addition, although the corridors have ducted supply air (measured below design at 

about 700 CFM per floor), the garbage rooms and janitor closets on each floor collectively 

exhaust more than design, and almost the same volume of air (500–600 CFM). The garbage 

rooms are compartmentalized to a degree; however, it is likely that a portion of the air they 

exhaust is drawn from the corridors.

The door airflow relationship was then applied to the pressure monitoring data to quantify the 

airflow across the apartment door. The equation for the average door was used in conjunction 

with the data from the five apartments that were monitored. A portion of the results is plotted in 

Figure 19. Only 1 week is plotted to provide higher resolution so that the daily fluctuation are 

more visible. 

-2

-1

0

1

2

3

4

5

6

7/30 8/1 8/3 8/5 8/7 8/9 8/11

Pressure (Pa)

apt 1225

apt 1247

apt 1250

apt 1265

apt 1287
24

Figure 19. Airflow across apartment door. Positive values indicate

flow from apartment to corridor.

The flow across the apartment doors ranged from 24 CFM into the apartment to 40 CFM out. 

There was considerable variation between apartments and over time within individual units. 

Only one apartment maintained an average flow into the unit. The remaining four all exhibited 

net airflow from the apartment to the corridor. This confirms the short-term results—that the 

intended MUA supplied to the corridor for the apartments does not manage to make it into the 

apartment, through the door undercut or other door frame leakage, while the whole-unit 

ventilation system is in typical continuous operation. It also unexpectedly identified airflow from 

the apartments into the corridor.

The pressure of the apartment with respect to outdoors was also measured. The results are shown 

in Figure 20.

-30

-20

-10

0

10

20

30

40

50

7/30 7/31 8/1 8/2

Flow (CFM)

apt 1225

apt 1247

apt 1250

apt 1265

apt 1287
25

Figure 20. Apartment pressure with respect to outdoors—cooling season

Again, the pressure fluctuates considerably, from –7.5 Pa to +12 Pa. This is likely a result of 

external conditions—wind, temperature, and opening and closing of stairwell and corridor 

doors—and the fact that the apartments, while meeting the current airtightness thresholds for 

various programs, are not sufficiently compartmentalized with respect to air movement to be able 

to draw in MUA from its intended source.

3.1.5 Pressure Monitoring: Heating Season

Pressure monitoring was conducted again in late November in five apartments. Four of the five 

apartments were the same as those used for the cooling season monitoring. In both seasons, the 

apartments were unoccupied. The pressure of the apartment with respect to the corridor was 

logged over a period of approximately 2 weeks. A negative pressure again indicates that the 

apartment was depressurized with respect to the corridor. The average hourly pressure for each 

apartment with respect to the corridor is plotted in Figure 21.

Of the five apartments, only one, Apartment 1287, was consistently depressurized with respect to 

the corridor. Unlike the consistently depressurized apartment from the cooling season 

monitoring, this apartment was located on the top floor of the building. The other four 

apartments were either neutral or slightly pressurized. This indicates, with the exception of 

Apartment 1287, there is no MUA being provided from the corridor via doorframe leakage or the 

door undercut. In fact, there is small net flow of air from many apartments into the corridor. 

The pressure in an apartment with respect to the ambient outdoor pressure was also measured in 

two apartments. Apartment 1220 is located on the first floor and Apartment 1265 is on the third. 

The results are shown in Figure 22.

-10

-5

0

5

10

15

7/30 8/1 8/3 8/5 8/7 8/9 8/11

Apartment Pressure WRT Outdoors (Pa)
26

Figure 21. Apartment pressure with respect to corridor—heating season

Figure 22. Apartment pressure with respect to outdoors—heating season

-5

-4

-3

-2

-1

0

1

2

3

11/11 11/13 11/15 11/17 11/19 11/21

Pressure (Pa)

apt 1220

 apt 1247

apt 1250

apt 1265

apt 1287

-25

-20

-15

-10

-5

0

5

11/11 11/13 11/15 11/17 11/19 11/21

Pressure (Pa)

apt 1220

apt 1265
27

Both apartments are consistently depressurized with respect to the outdoors, indicating a net flow 

of air into the apartment from outside. The first floor unit exhibits a greater pressure difference 

across the exterior wall, which is likely a result of stack effect in the building.

The pressures with respect to outside and corridor for two apartments are shown in the following 

two figures. In both instances, there is negligible pressure across the apartment-corridor 

boundary and a significant pressure across the apartment-outdoor wall. This indicates that the 

majority of the MUA for these units is being drawn from outdoors, through the exterior wall.

Figure 23. Apartment 1220 pressure with respect to outdoors and corridor—heating season

Figure 24. Apartment 1265 pressure with respect to outdoors and corridor—heating season

-25

-20

-15

-10

-5

0

5

11/11 11/13 11/15 11/17 11/19 11/21

Pressure (Pa)

1220 wrt

outside

-10

-8

-6

-4

-2

0

2

11/11 11/13 11/15 11/17 11/19 11/21

Pressure (Pa)

1265 wrt

corridor
28

3.1.6 Fan Power Consumption

The performance specifications of each fan are summarized in Table 10 and Table 11. Due to the 

location of the exhaust fans in the units, the power could not be easily measured. As a result, the

rated power and rated flow were used and applied to the measured flow. The power consumption 

is assumed relatively constant over the operating pressures.

Table 10. Exhaust Fan Performance—Power and Flow

Fan Type Rated Power 

(W)

Rated Flow 

(CFM)

Measured 

Flow

(CFM)

Measured 

CFM/

Rated W

Type 1: Low 2.4 50 38 15.8

Type 2: Low 3.2 30 33 10.3

Type 1: Boost 11.9 130 117 9.8

Type 2: Boost 7 80 79 11.3

For the purposes of comparison, it can be assumed that the fans operate for the vast majority of 

the time at low. Therefore, the average performance for this system is 13.1 CFM/W.

The performance specifications of the supply fans are summarized in Table 11. The measured 

flows could not be reliably assigned to the appropriate fan; therefore, they are not included in the 

table.

Table 11. Supply Fan Performance—Power and Flow

Fan Type Rated Power 

(W)

Rated Flow 

(CFM)

Rated CFM/ 

Rated W

VU-1 1491.4 3200 2.1

VU-2 372.9 1050 2.8

VU-3 372.9 1000 2.7

3.2 Type 2: Make-Up Air Through Ducted Supply Air

3.2.1 Apartment Blower Door Testing 

Blower door tests were conducted on a sample of apartments in the two buildings. Both buildings 

have an average apartment airtightness of 0.30 CFM50/ft2 of enclosure area (see Table 12 and 

Table 13). The first digit of the apartment number also indicates the floor level; however, many 

fourth-floor apartments were two-story units, with living space on the fifth floor.
29

Table 12. Type 2 Building #1 Apartment Airtightness Metrics

Apartment Floor Area 

(ft2)

Wall Height 

(ft) CFM50 CFM50/ft2Enclosure ACH50

108 1,114 12 1,337 0.34 5.8

206 990 12 396 0.11 2.0

209 1,114 12 1,337 0.34 6.3

210 996 13 1,374 0.39 6.9

212 1,018 13 1,425 0.39 7.2

213 1,059 13 1,483 0.40 7.0

214 1,056 12 1,331 0.36 6.3

302 970 12 485 0.14 2.5

400 730 10 993 0.34 6.8

403 1,331 10 932 0.22 4.2

408 1,559 10 1,195 0.22 4.6

417 1,115 12 1,516 0.39 6.8

Average 1,088 12 1,150 0.30 5.5

Table 13. Type 2 Building #2 Apartment Airtightness Metrics

Apartment Floor Area 

(ft2)

Wall Height 

(ft) CFM50 CFM50/ft2 ACH50

102 1202 10.5 1,441 0.44 6.9

103 1,180 10.5 1,419 0.47 6.9

117 1,316 10.5 1,463 0.32 6.4

201 918 11.0 580 0.17 3.5

205 1,156 11.0 798 0.20 3.8

208 652 11.0 480 0.18 4.0

218 1,201 11.0 965 0.24 4.4

302 1,297 11.0 1,108 0.26 4.7

303 1,253 11.0 871 0.21 3.8

309 1,004 11.0 884 0.25 4.8

312 833 11.0 907 0.29 5.9

401 925 8.5 1,140 0.33 8.7

404 1,119 8.5 1,147 0.30 7.2

407 920 8.5 1,288 0.40 9.9

417 1,487 8.5 1,653 0.35 7.9

422 1,138 8.5 1,634 0.43 10.1

Average 1,110 10.0 1,122 0.30 6.2

3.2.2 Supply and Exhaust Flow Testing

The supply and exhaust flows were measured in a sample of apartments in both test buildings 

using a powered flow hood and compared to the design rates. The net differences between the 

supply and exhaust flows were also calculated, with negative values indicating that exhaust flow 

exceeded the supply flow, an expected result based on the design. The results are presented in 

Table 14 and Table 15. 
30

Table 14. Building #1—Actual and Design Supply and Exhaust Flow

Apt Kitchen Exhaust

(CFM)

Bathroom Exhaust 

(CFM)

Supply

(CFM)

Actual Design % Actual Design % Actual Design % Net

102 23 50 46% 14 25 56% 28 75 37% -9

105 17 60 28% 12 30 40% 37 90 41% 8

108 <8 60 <13% <8 30 <27% 23 90 26% 7

110 <8 60 <13% <8 30 <27% 63 90 70% 47

112 <8 60 <13% <8 30 <27% 25 90 28% 9

204 16 60 27% 6 30 20% 34 90 38% 12

209 25 60 38% 14 30 47% 46 90 51% 7

212 33 60 55% 22 30 73% 60 90 67% 5

213 31 60 52% 20 30 67% 63 90 70% 12

214 28 60 47% 13 30 43% 65 90 72% 24

301 19 60 32% 11 30 37% 47 90 52% 17

303 18 50 36% 6 25 24% 43 75 57% 19

304 14 60 23% 7 30 23% 11 90 12% -10

305 15 60 25% 11 30 37% 25 90 28% -1

309 30 50 50% 18 25 72% 45 75 60% -3

312 29 60 48% 21 30 70% 60 90 67% 10

401 Vented Range Hood 23 25 92% 67 75 89% 44

403 Vented Range Hood 16 25 64% 66 50 132% 50

404 DNT 50 NA 14 25 56% 57 75 76% NA

405 Vented Range Hood 14 25 56% 43 50 86% 29

407 23 50 46% 13 25 52% 66 75 88% 30

408 Vented Range Hood 15 25 60% 55 50 110% 40

Avg 40% 52% 62%
31

Table 15. Building #2—Actual and Design Supply and Exhaust Flow

Apt Kitchen Exhaust 

(CFM)

Bathroom 1 

Exhaust (CFM)

Bathroom 2 

Exhaust (CFM) Supply (CFM)

Act Des % Act Des % Act Des % Act Des % Net

101 13 50 26% 16 25 64% 15 25 60% DNT 85 NA NA

106 28 50 56% 15 25 60% 16 25 64% DNT 85 NA NA

110 18 50 36% 17 25 68% (No Bath #2) DNT 70 NA NA

113 27 50 54% 18 25 72% 20 25 80% DNT 85 NA NA

114 25 50 50% 15 25 60% (No Bath #2) 29 50 58% –11

116 26 50 52% 16 25 64% 15 25 60% 45 75 60% –12

201 27 50 54% 13 25 52% (No Bath #2) 42 75 56% 2

203 26 50 52% 20 25 80% 20 25 80% 47 75 63% –19

208 29 50 58% 18 25 72% (No Bath #2) 26 50 52% –21

210 24 50 48% 17 25 68% (No Bath #2) 23 50 46% –18

216 22 50 44% 20 25 80% (No Bath #2) 30 50 60% –12

301 23 50 46% 14 25 56% (No Bath #2) 27 50 54% –10

304 29 50 58% 17 25 68% DNT 25 NA 47 75 63% NA

307 27 50 54% 22 25 88% DNT 25 NA 47 75 63% NA

312 36 50 72% 17 25 68% (No Bath #2) 26 50 52% –27

401 25 50 50% 15 25 60% 15 25 60% 38 90 42% –17

404 23 50 46% 16 25 64% 12 25 48% 43 90 48% –8

405 29 50 58% 19 25 76% 16 25 64% 45 90 50% –19

406 27 50 54% 14 25 56% 16 25 64% 41 90 46% –16

409 18 50 36% 18 25 72% 19 25 76% 45 90 50% –10

413 36 50 72% 21 25 84% 20 25 80% 43 90 48% –34

418 29 50 58% 26 25 104% 23 25 92% 47 90 52% –31

422 27 50 54% 17 25 68% 18 25 72% 76 90 84% 14

Avg 52% 70% 66% 55%

Despite their best intentions to provide a dedicated source of MUA, Table 14 and Table 15

illustrate one of the challenges with designing and installing ducted ventilation systems: the 

performance of the system falls short of the design specifications without proper commissioning.

Neither exhaust nor supply measurements were close to the design flows, meaning that the 

recommended ventilation rates were not achieved. Exhaust airflow, in some units of Building #1 

and most units of Building #2, exceeded the supply airflow, as expected, which means there are

still unidentified sources of MUA. Although LEED for Homes Certification required designing 

the system to meet ASHRAE 62.2, there was no requirement for measured airflows to actually 

comply.

The results above were compared to TAB reports from contractors for both buildings. In the 

view of CARB investigators, the veracity of at least one TAB report is highly suspect for several 

reasons. First, it is highly suspect that the total flow is exactly the same as designed, which 

unfortunately is extremely unusual in most buildings. All of the individual register measurements 

for exhaust CFM were exactly 5 CFM above, 5 CFM below, or exactly on the target, which is 

also highly unusual. If the measuring device used was only capable of 5 CFM increments, it is 

not precise enough to be used for this type of low-flow balancing work. Lastly, CARB results 
32

show that measured flow decreases with increasing distance from the MUA fan, which is a 

pattern consistent with leaky ductwork. By contrast, the TAB results show no pattern at all, 

instead reporting nearly perfect results randomly distributed throughout. These factors point to 

clear fabrication of the reported results.

3.2.3 Supply and Exhaust Fan Power Consumption

The power ratings for the supply and exhaust fans are summarized in Table 16. Actual field 

measurements were not made due to access issues.

Table 16. Supply and Exhaust Fan Power

Fan Type Rated Power

(W)

Rated Flow

(CFM) CFM/W

Exhaust Central 1,119 3,200 2.9

Supply Central 1,492 3,200 2.1

3.3 Type 3: Make-Up Air Through HVAC Supply

3.3.1 Apartment Blower Door Testing 

Blower door tests were conducted on 15% of apartments pursuant to LEED program 

requirements. Apartments are numbered by floor (first through fourth) and letter. The building is 

built with an average apartment airtightness of 0.29 CFM50/ft2 of enclosure area (see Table 17).

Table 17. Type 3 Building Apartment Airtightness Metrics

Apartment Floor Area 

(ft2)

Wall Height 

(ft) CFM50 CFM50/ft2 ACH50

1C 625 9.33 657 0.30 6.76

1K 625 9.33 614 0.28 6.32

2B 625 9.33 636 0.36 6.54

2E 625 9.33 623 0.28 6.41

2F 475 9.33 526 0.24 7.12

2R 625 9.33 580 0.32 5.97

3R 475 9.33 537 0.24 7.27

4A 625 9.33 610 0.34 6.27

4F 475 9.33 471 0.22 6.37

Average 575 9.33 583.8 0.29 6.56

3.3.2 Supply and Exhaust Flow Testing

The exhaust flows from the bathroom and kitchen registers were measured using a powered flow 

hood. The average total exhaust airflow from the apartments tested was 57 CFM, compared to 

the design of 55 CFM. The results are summarized in Table 18.
33

Table 18. Apartment Exhaust Flow Test Results

Apartment Kitchen

(CFM)

Design

(CFM)

Bathroom

(CFM)

Design

(CFM)

Total

(CFM)

Design

(CFM)

2R 26 30 26 25 52 55

3R 19 30 19 25 38 55

3P 27 30 28 25 55 55

3E 25 30 29 25 54 55

4F 22 30 56 25 78 55

4P 34 30 31 25 65 55

Average 25.5 30 31.5 25 57 55

In the common corridor, there is one supply register per floor. This is designed to provide 

conditioned air to the corridors only, and is not intended to supply MUA to the apartments. They 

are located at the northwest end of each corridor, near Stair A. The construction drawings 

indicate 140 CFM from each of these registers. This equates to slightly more than 8 CFM per 

apartment. The actual airflow was not measured because a good seal with the powered flow hood 

was not possible due to the trim and molding around the register.

3.3.3 Door Leakage Airflows

Although not designed as a source of MUA to the apartment, flow through the door was 

measured the same way as described in Section 2.2.3.

The apartment doors have weather-stripping around the sides and top, but do not have a door 

sweep at the bottom. There is approximately a 3⁄8-in. gap between the bottom of the door and the 

threshold, as shown in Figure 25.

Figure 25. Apartment door—gap between door and threshold

The leakage values from seven doors tested are given in Table 19 in terms of CFM50, and the 

results of their blower door tests, when available. Compared to the building tested in Type 1, 

where the door undercut was somewhat intended to allow MUA into the apartments, these doors 

show 30% higher airflow across the door at 50 Pa.
34

Table 19. Tabulated Results of Door Air Leakage Tests (CFM50)

Apartment Depressurization Pressurization Average Apartment Air 

Leakage

2R 213 209 211 580

3R 220 203 211 537

3P 187 209 198

3E 276 255 266

4R 359 221 290

4F 113 150 132 471

4P 204 189 196

Average 225 205 215

Figure 26 depicts the results of the individual door tests, along with a dataset that represents the 

average of all the data.

Figure 26. Results of air leakage test from all doors tested and average door performance

The results of this testing show that air flows through the apartment door very easily and can be 

correlated to the pressure difference between the apartment and the corridor (see Section 3.3.6).

3.3.4 Packaged Terminal Air Conditioning Make-Up Air

Manufacturers of PTACs have for many years included “fresh air dampers” and other devices 

that passively admit OA and incorporate it into their airstream. These work on a similar principle 

to passive or “trickle” vents, in that airflow through them is dependent on building pressures.

More recently, manufacturers have also begun to offer fresh air kits, with a small dedicated 

supply air fan to provide a continuous flow of OA, regardless of main unit operation. Because 

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Pressure (Pa)

Flow Rate (CFM)

Air Leakage of Seven Apartment Doors

Pressurization Depressurization Average
35

the fans are small, it is likely that their performance will still be affected by prevailing pressures 

in the building.

To test the airflow through the PTAC fresh air kit, a custom-made device which combines a 

powered capture hood and an orifice plate was used. The device is shown in Figure 27 and 

represented diagrammatically in Figure 28. 

Figure 27. Testing apparatus for measuring airflow through PTAC fresh air kit (cover removed)

Figure 28. Diagram of apparatus for measuring airflow through PTAC fresh air kit

When the pressure in the hood (A) is zero with respect to the ambient pressure around it (B), the 

flow going into the hood equals the flow leaving the hood. No pressure is placed on the outlet of 

the object being tested, and the effect of placing the hood over the test object is minimized. In 

order for the pressure in the capture hood to reach zero, suction must be placed on the orifice 

chamber by the chamber fan. A pressure drop across the orifice plate develops (C to D), and this 

can be measured to determine flow. The goal is to isolate and capture airflow only from the 

dedicated fresh air kit, not from other leakage pathways in the PTAC such as gaps between the 

unit and its through-wall sleeve. 

The procedure was repeated with higher and lower pressures inside the capture hood (delta 

between A and B), to simulate various pressure environments in which the device must work.

Chamber fan Orifice chamber Flexible hose Capture hood

A

B

D C
36

Measurements were taken at a variety of pressures in two series; one with a 3-in.

2 test orifice and 

one with a 4-in.

2 test orifice. The results (shown in Figure 29) were similar, with good 

correlation between airflow and pressure.

Figure 29. Results from 3-in.

2 orifice (L) and 4-in.2 orifice (R)

The results of the 3-in.

2 and 4-in.2 orifice tests were then plotted together. The results are shown 

in Figure 30. 

Figure 30. Combined results from both tests

y = -0.0071x2 - 0.4638x + 5.7699

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

5 0 -5 -10 -15 -20 -25 -30

Airflow (CFM)

Test Pressure (Pa)

Airflow from PTAC Fresh Air Fan

3" orifice

4" orifice
37

The equation from the best-fit curve can be used to infer the airflow through the PTAC fresh 

air kit at any potential building pressure. When combined with long-term pressure monitoring,

the direction and magnitude of airflow through the PTAC fresh air kit can be inferred (see 

Section 3.3.6).

The pressure that the PTAC fresh air fan experiences is affected by the operating conditions of 

the apartment, in particular the operation of exhaust fans, PTAC fresh air fan, and PTAC blower

fan. For example, when the PTAC’s main blower is running, the pressure near the fresh air fan 

drops considerably, allowing the fresh air fan to deliver more flow. To measure the effect of 

these conditions, a static pressure probe was placed in the PTAC cabinet, at the outlet of the 

fresh air fan. This pressure and the pressure with respect to outdoors were recorded over time. 

This setup is shown in Figure 31. The right image shows the static pressure probe placement in 

the blower cabinet. The flow through the fresh air fan was then calculated using the equation 

developed in Figure 30 from the orifice plate/capture hood tests.

Figure 31. Setup for measuring pressures inside the PTAC blower cabinet

A summary of the various conditions and results of the tests is shown in Table 20.

Table 20. Effect of Apartment Operating Conditions on Pressure at Outlet of PTAC Fresh Air Fan

Nominal 

Operating 

Conditions

Exhaust 

Fans

Fresh Air 

Fan

PTAC 

Blower Fan

Average 

Pressure 

(Pa)

Flow

(CFM)

All Off OFF OFF OFF –1.5 6.5

Exhaust Fans On ON OFF OFF –1.4 6.4

OA Kit On ON ON OFF –2.2 6.8

Blower Low ON ON LOW –3.7 7.6

Blower High ON ON HIGH –5.6 8.6

During the time of the tests, under “all off” conditions, the area in front of the PTAC fresh air fan 

was slightly depressurized with respect to outside. This resulted in a flow of approximately 6.5 

CFM into the apartment. Although it was expected that the operation of the exhaust fans would 

result in further depressurization of the apartment, when the exhaust fans were turned on (via the 
38

circuit breaker) the pressure actually increased slightly. This unexpected result is likely due to 

variations in building conditions (opening and closing of doors) and atmospheric conditions 

(wind). When the PTAC fresh air fan was energized, the pressure in the cabinet fell with respect 

to outdoors. This is expected, as the fan is designed to draw air from outside into the blower 

cabinet; however, the increase in airflow was less than 0.5 CFM, and nowhere near the expected 

airflow of 40 CFM. Energizing the PTAC blower caused further depressurization of the sample 

zone, with lower pressures being correlated to the higher blower speed. This is the result of the 

fan drawing air from the lower region of the cabinet and pulling it upward. The result of the 

blower set to high is an increase of 26%, or approximately 1.9 CFM, over the “OA Kit On” 

condition. Thus, the operation of the apartment, in particular the PTAC blower, has a direct 

impact on the pressure the PTAC fresh air fan experiences, and therefore the amount of air 

delivered, which, at 8.6 CFM with all fans running, still falls very short of the design of 40 CFM.

3.3.5 Pressure Monitoring—Cooling Season

Pressure monitoring was conducted in July in five apartments: 2R, 3P, 3R, 4P, and 4R. These 

apartments are on floors 2, 3, and 4. Apartments P and R are adjacent to each other and located 

on the southern exposure of the building. 

The pressure at the outlet of PTAC fresh air fan with respect to outdoors was logged over a 

period of approximately 2 weeks. A negative pressure indicates that the apartment was 

depressurized with respect to outdoors. Data samples were taken each minute. The average for 

each hour was determined and plotted in Figure 32.

Figure 32. Pressure at outlet of PTAC fresh air fan with respect to outdoors

The data indicate that the apartments are depressurized with respect to outdoors for the vast 

majority of the time. In addition, it appears that the pressure fluctuates between +2 Pa and 

-20.0

-15.0

-10.0

-5.0

0.0

5.0

7/16 7/18 7/20 7/22 7/24 7/26 7/28

Pressure With Respect to Outside (Pa)

2R

3P

3R

4P

4R
39

–15 Pa. The data for a single day are shown in Figure 33 to better illustrate trends between 

apartments.

Figure 33. Pressure at outlet of PTAC fresh air fan with respect to outdoors, single day

It should be noted that the blower on the PTAC in apartment 3R was set to “High.” On this

setting, the blower runs continuously at high speed. The blowers in the other apartments were set 

to “Auto,” which means they operate in response to calls from the thermostats. In the previous 

section, it was found that the blower operation directly affects the pressure the fan experiences. 

The pressure monitoring data confirm those findings, as the readings from 3R were overall, 

consistently more negative than the other readings. 

In addition to monitoring pressure, the apartment temperature and PTAC operation was recorded 

for each of the five apartments. To measure PTAC operation, a temperature data logger was 

placed on the cooling side of the unit. A drop in temperature for this logger was inferred to mean 

the PTAC was in operation. Figure 34 is a plot of pressure, apartment temperature, and PTAC 

temperature for apartment 4R over 1 day.

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00

Pressure With Respect to Outside (Pa)

2R

3P

3R

4P

4R
40

Figure 34. Apartment 4R—pressure, apartment temperature, and PTAC temperature

As expected, PTAC temperature (green line) and apartment temperature (blue line) are directly 

correlated. When the apartment temperature reaches approximately 82°F, the PTAC turns on and 

runs until the apartment temperature falls to about 78°F. This cyclic operation happens 

throughout the day. The more interesting relationship is between the PTAC operation and 

pressure at the fresh air kit fan (red line). It was observed that the two are also correlated. When 

the PTAC is off, the pressure fluctuates around –2 Pa. When the PTAC is operating, the pressure 

falls to –7 to –9 Pa. Again, this confirms that PTAC operation has a direct effect on pressure. 

The minor variations in pressure are most likely due to fluctuations in atmospheric conditions, 

predominately wind.

An additional pressure monitoring station, consisting of two DG-700 manometers and a laptop 

running Teclog, was established in apartment 3P. This station was used to measure the ambient 

pressure of the apartment with respect to outdoors and with respect to the corridor. It was also 

used to measure the pressure of apartment 3R with respect to the corridor. Data were collected 

every minute. The hourly averages are plotted in Figure 35.

-16

-14

-12

-10

-8

-6

-4

-2

0

2

0

10

20

30

40

50

60

70

80

90

100

0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00

Pressure (Pa)

Temperature (F)

PTAC

Temp
41

Figure 35. Apartment pressures with respect to outdoors and corridor

In order to better visualize the trends, a single day is shown in Figure 36, below.

Figure 36. Apartment pressures with respect to outdoors and corridor—single day

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

7/16 7/18 7/20 7/22 7/24 7/26 7/28 7/30

Pressure (Pa)

3P WRT Outside

3R WRT Hall

3P WRT Hall

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00

Apartment Pressure (Pa)

3P WRT Outside

3R WRT Hall

3P WRT Hall
42

Both apartments maintain a slight depressurization with respect to the corridor. When the results 

of the pressure monitoring are combined with the findings from the door leakage and PTAC 

tests, pathways of air movement into and out of the apartment can be established. In the two 

apartments where both of these parameters were measured, significantly more air is exhausted by 

the bath and kitchen fans, than is supplied through door leakage and PTAC fresh air fan

combined. This means that again, despite the best intentions of the high performance building 

program and the design team, additional air is being drawn from unintended spaces, such as 

adjacent apartments, interstitial cavities, and unintended holes in the exterior wall. 

Table 21 presents the supply flows as a percentage of the total flow being exhausted from the 

apartment. Although it was the intended source for MUA, the PTAC fresh air fan is responsible 

for a relatively small percent of the MUA. The majority comes from a combination of flow under 

and around the door and unknown sources of infiltration. Even though the door was not intended 

as a pathway for MUA, it actually provides a higher percentage than the intended source, and 

even a higher percentage than the door in the Type 1 building, where it was the intended source.

Table 21. Average Apartment Supply Flows as Percentage

Apartment PTAC OA Door Leakage Unknown

3P 13% 41% 46%

3R 23% 48% 30%

Table 22 compares the design flow through the PTAC fresh air fan to the average measured

hourly flow. 

Table 22. Actual and Design Supply Flows From PTAC Fresh Air Fan

Apartment PTAC OA Actual

(CFM)

PTAC OA Design

(CFM)

Actual/Design

%

3P 7.1 40 18%

3R 8.6 40 22%

4R 7.1 40 18%

2R 6.6 40 17%

4P 6.9 40 17%

The overall average installed performance of the fresh air fan was approximately 20% of the 

design specification. The average hourly flows across the five apartments ranged from 4.9 CFM 

to 14.3 CFM. This represents 12%–36% of the design.

3.3.6 Pressure Monitoring—Heating Season

Pressure monitoring was conducted again in December in the same five apartments. The 

apartments were monitored for approximately 2 weeks. All five apartments were occupied 

during the heating season monitoring period. The PTACs and PTAC blowers were all initially 

set to “auto” mode. The tenants were asked to keep the blowers on auto; however, no request 

was made regarding the PTAC operation. As a result, some tenants manually switched the PTAC 

off during warmer periods, which means that the blower was also off for these times.
43

The pressure at the outlet of the fresh air kit with respect to outdoors was recorded in the five 

sample apartments. The sampling period spanned 2 weeks, including the Thanksgiving holiday.

Unfortunately, it became apparent that several factors must have been at play during this period, 

such as occupancy of the apartment and by extension, the PTAC blower settings, thermostat set

points, and possible open windows. This made interpreting the data difficult and led CARB to 

limit the analysis to a shorter period when it was certain those variables had been accounted for.

The behavior of the apartments mirrors more closely that of the summer monitoring period. The 

snapshot of the data is presented in Figure 37. A negative pressure indicates the cabinet is 

depressurized and with respect to outside.

Figure 37. Pressure at outlet of PTAC fresh air fan with respect to outdoors—heating season

The results indicate that the outlet of the fresh air fan was negatively pressurized with respect to 

outdoors, which means air is being drawn in though the system. The performance of the systems 

is relatively consistent, with a maximum variation between apartments of 4 Pa. The subtle 

variation in pressure over the course of the day can most likely be attributed to environmental 

conditions.

In addition, the pressure between the apartment and corridor was logged in two apartments. The 

results are presented in Figure 38 and Figure 39.

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

16:00 22:00 4:00 10:00 16:00

Pressure (Pa)

Apt 2R

Apt 3P

Apt 3R

Apt 4P

Apt 4R
44

Figure 38. Apartment 3P pressures with respect to outdoors and corridor—heating season

Figure 39. Apartment 3R pressures with respect to outdoors and corridor—heating season

In both cases, the PTAC and apartment follow similar trends; however, the apartment is slightly 

more pressurized than the PTAC. This is to be expected as the PTAC blower and fresh air fan 

reduce the pressure in the PTAC cabinet.

These data were combined with the equations obtained for airflow across the door and PTAC at 

various pressures, to develop a more complete understanding of air movement in these two 

apartments. These results are summarized in Table 23.

-7

-6

-5

-4

-3

-2

-1

0

1

2

12/4 0:00 12/4 12:00 12/5 0:00 12/5 12:00 12/6 0:00 12/6 12:00 Pressure (Pa)

Apt 3P PTAC...

3P wrt Hall

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

11/25 16:00 11/26 0:00 11/26 8:00 11/26 16:00 11/27 0:00 Pressure (Pa)

Apt 3R PTAC...

3R wrt Hall
45

Table 23. Average Apartment Exhaust and Supply Flows (CFM)

Apartment Exhaust PTAC OA Door Leakage Unknown

3P 55 6.6 22.6 25.8

3R 38 8.5 14.9 14.6

Similar to the results from the cooling season, the PTAC OA kit is responsible for a relatively 

small amount (12%–22%) of MUA. The majority comes from the corridor door and 

miscellaneous sources of infiltration.

3.3.7 Supply and Exhaust Fan Power Consumption

The power ratings for the supply and exhaust fans are summarized in Table 24. Actual field 

power measurements were not made due to access issues. The measured flow was determined 

during the cooling season.

Table 24. Supply and Exhaust Fan Power

Fan Type

Rated 

Power

(W)

Rated 

Flow

(CFM)

Measured

Flow

(CFM)

Measured CFM/

Rated W

Supply PTAC Fan 2.4 40 8.6 3.6

Apartment Exhaust Fan 36 55 57.0 1.6

3.4 Type 4: Make-Up Air Through Passive Vents

3.4.1 Apartment Blower Door Testing 

Blower door testing was conducted in nine apartments. The results show that apartments in this 

building are generally well compartmentalized, with an average total leakage rate of 0.10 

CFM50/ft2 of apartment enclosure. The results of the test are summarized in Table 25. 

Table 25. Type 4 Building Apartment Airtightness Metrics

Apartment # Beds Flow 

(CFM50)

Apartment 

Volume 

(ft3)

ACH50

2B 2 312 6600 2.8

2L 1 248 5496 2.7

3C 2 244 7376 2.0

3G 3 310 9120 2.0

4J 2 387 6504 3.6

4M 1 209 7920 1.6

5B 2 249 7376 2.0

5F 3 423 10840 2.3

5K 2 356 5616 3.8

Average 2.5
46

3.4.2 Supply and Exhaust Flow Testing

Each apartment in this building is equipped with a single inline exhaust fan serving multiple 

registers, one in each bathroom and kitchen. Each register is outfitted with an automatic 

balancing damper to limit the airflow from each register and balance airflow among them. 

Exhaust rates were measured from each register in 14 apartments. The results are summarized in 

Table 26 below.

Table 26. Type 4 Exhaust Flow Measurements

Apartment # of 

Bedrooms

Exhaust Flow (CFM) Total 

Flow 

(CFM)

Design 

CFM

Kitchen Bath 1 Bath 2

1A 2 7 32 – 39 105

1D 3 21 40 21 82 125

2A 2 33 23 22 78 125

2B 2 34 19 37 90 125

2D 2 37 21 13 71 125

2F 3 35 18 20 73 125

2L 1 52 25 – 77 125

3C 2 40 24 25 89 125

3G 3 25 24 19 68 125

4J 2 42 23 – 65 105

4M 1 41 23 – 64 105

5B 2 37 15 16 68 125

5F 3 35 20 9 64 125

5K 2 54 30 – 84 105

Average 27 72

The average flow rate from an exhaust register was 27 CFM. The design calls for 20 CFM from 

bathrooms and 85 CFM from kitchens for a total of 105–125 CFM per apartment, depending on 

the number of bathrooms. Measured airflows in kitchens average 44% of design, while those 

from bathrooms average 113% of design. Difficulties specifying and constructing very small 

multibranch exhaust systems can account for much of this variation. In both one-bathroom and 

two-bathroom units, the same exhaust fan was specified but at different static pressures.

Automatic balancing dampers and newer fans that self-correct for installation variances may be 

able to help limit imbalances. In this building, automatic balancing dampers were used in both 

kitchen and bathroom registers. In kitchens, maximum flows were 68% of design. The dampers 

were either incorrectly specified or were unable to limit flows in bathrooms to design levels.

Flows in bathrooms were up to 200% of design of 20 CFM.

3.4.3 Passive Vent Testing

The test building uses trickle vents, a type of passive vent, to provide fresh air to apartments

based on the pressure difference between inside and outside. A photo of one type of trickle vent 
47

is shown in Figure 40 (left) below, and another one shown in a typical installation in a window 

frame in the Figure 40 (right).

Figure 40. Common trickle vent (left) and installation in a window frame (right)

Trickle vents are tested by their manufacturers to determine their performance at various 

pressures or wind speeds, giving designers information about how the devices can be properly 

applied in their buildings.

In the test building, trickle vents were tested to characterize their performance as installed. Five 

trickle vents were tested by inducing a pressure difference on either side and measuring the 

airflow. A capture box was sealed around each vent and a fan was used to create a pressure on 

one side. The pressure on the inside and outside aspect of each vent was measured with 

manometers and the calibrated fan was used to measure the airflow. A photo of the testing 

apparatus is in Figure 41.

Figure 41. Setup for testing airflow through passive vents

Fan

Capture hood

 
48

Five separate vents were tested by this method and the results are shown together in the chart 

below. Airflow through the vents was measured at a variety of pressures. A clearly recognizable 

pattern is shown. The first trickle vent was tested under both positive and negative pressures, and 

the results were almost identical. The remaining four vents were tested only under negative 

pressure, and only results from the negative-pressure tests are displayed in Figure 42.

Figure 42. Trickle vent flow at varying pressures

The results show that trickle vents in this building are capable of airflow to an apartment but that 

a significant pressure difference across the vent is required. For example, if 25 CFM of MUA is 

desired, a pressure difference of nearly 50 Pa is required, roughly equivalent to a 20 mile-perhour wind. Obviously consistent wind cannot be depended on, and using mechanical equipment 

to create a 50-Pa pressure difference would possibly present many unintended consequences.

The results show that the commonly used trickle vents installed in most buildings as currently 

built are unlikely to provide rates of supply air anywhere equivalent to exhaust rates. The trickle 

vents are too small or too few, and the pressures commonly developed are much too weak to 

draw enough air from them.

It should also be mentioned that trickle vents must be installed well to function as intended. In a 

building evaluated by CARB for other studies, the lack of measured performance from a trickle 

vent prompted an investigation of the installation. Upon removing the vent cover from the 

window frame, closer examination revealed significant blockage of the air pathway through the 

layers of the window frame, caused by poor manufacturing technique. Obviously defects like this 

negatively impact the function of the vents.
49

Figure 43. Cut for trickle vent in window frame (left), trickle vent removed.

Secondary holes drilled through frame to allow air passage (right)

The data gathered from testing airflow through the trickle vents in the test building are used to 

model how they function as a source of MUA over time. Using a linear regression on the 

logarithm of pressure and flow readings from all five vents of the same make and model, the 

pressure exponent n and flow coefficient C was derived and equation used to calculate flow 

through the vents is as follows:

Q = 2.768 * ∆P 0.551 (3)

3.4.4 Door Leakage Airflows

Each door between apartments and corridors has a door sweep, but the sweep is not always tight 

to the door saddle. In addition, the door jambs and header do not typically have weatherstripping. As a result, significant gaps allow relatively easy passage of air between apartments 

and corridors. Simple measurements of the space around the door frames showed approximately 

22 in.2 of gap around them. Equipment for testing the doors with a rigid capture hood the same 

way as other buildings in this study was not available, so doors were tested in two other ways to 

quantify the amount of leakage area.

Using a blower door procedure to depressurize the apartment through a window, the 

apartment/corridor door gap was repeatedly taped and untaped to observe the difference in 

measured flow through the blower door. A difference of approximately 205 CFM50 was 

recorded in tests on two apartments, roughly equivalent to 20 in.2 of leakage.

Another approach used a sharp-edged orifice and pressure measurements to estimate door gap 

size and airflow due to normal operating conditions in the apartment. A plastic shroud was taped 

over the door frame, and a plate with a sharp-edged orifice was fixed to the shroud. An 

illustration is provided in Figure 44.
50

Figure 44. Setup for testing airflow through door gap with a plastic shroud and orifice plate

The pressure difference between the inside of the apartment (B) and in the corridor (C) was kept 

at 1–2 Pa by adjusting the orifice size, reducing the restricting effect of the shroud on the door

but keeping airflow through it within a measurable range (see Table 27). The pressure difference 

across the plate was measured by subtracting the pressure inside the plastic shroud (A) from the 

pressure inside the apartment (B). Using a coefficient of discharge of 0.61 for the orifice plate, 

the flow at the measured pressure difference across the door yielded airflow estimates.

Table 27. Test Results Showing Calculated Leakage From a Plastic Shroud

Unit

Pressure 

Difference

Across Orifice

(A-B)

Corridor 

Pressure 

WRT Apt 

(C-B)

Orifice 

Size

(in.

2

)

Calculated 

Flow

(CFM)

Apartment 

Exhaust 

Fan 

Operation

Calculated 

Leakage 

Area

(in.

2

)

3A 4.0 1.2 27 60.0 Fans On 24.6

3B 1.3 1.7 2 2.5 Fans Off n/a

3C 3.1 1.2 27 52.8 Fans On 26.3

Measurements were taken in three apartments, two with their continuous exhaust fans running, 

and one with the exhaust fan disabled. All three apartments were neighbors on the same floor. 

The airflow measurements indicate that operating the unit exhaust fans has a significant effect on 

the amount of air being drawn from the corridor door in these apartments. In the apartments with 

bath fans running, 53–60 CFM is drawn through leakage around the door. In the apartment 

without the fan operating, only 2.5 CFM is drawn through leakage around the door, illustrating 

the difference that the local exhaust makes in depressurizing the apartment. Due to the small 

values recorded, it was not possible to calculate the leakage area of this door.

The area of the gap around the door can be determined by using the measurements above. By 

subtracting the measured pressure drop across the apartment boundary (C–B) from the pressure 
51

drop across the shroud (A–B), the pressure drop across the door itself (A–C) can be determined.

Using this pressure drop and the flow measured through the orifice, one can find the area of the 

door gap. The average area of leakage around the doors calculated by this method is 25.5 in.2

A = Q/Cd * (2∆P/ρ)

11.5 * 1.36697 (4)

Where:

A is the leakage area of the door

Q is leakage through the door gaps (in CFM)

Cd is the coefficient of discharge

ΔP is the pressure difference between inside and outside of the test system (in Pascals)

ρ is the air density (in kg/m3

)

3.4.5 Pressure Monitoring

Pressure monitoring was undertaken in several building spaces with the intent of understanding 

the airflow patterns between building spaces and between the building and outside. Data were 

gathered on 1-second intervals, and then averaged over every hour to give a general picture of 

pressure in the spaces over time. Data were collected during a shoulder season with a mix of 

warm and cool weather. Two of the apartments were unoccupied for at least 1 week, while the 

rest were occupied. Definite patterns emerge when comparing the occupied apartments to the 

unoccupied apartments.

Figure 45 shows the pressure in several apartments throughout the building as well as the 

hallway, all with respect to a single outdoor reference pressure. The chart shows vacant 

apartments were more negatively pressurized than those apartments with tenants living in them.

Apartment 4L was unoccupied and vacant for the entire monitoring period, while Apartment 4M 

was unoccupied until November 2, 2013, when a tenant moved in and occupied the space.

Figure 45. Pressure in several building spaces over time

*Apt. 4L was unoccupied for the entire monitoring time.

**Apt. 4M was unoccupied until November 2.

In this occupied building, both tenants and the resident superintendent were unaware of the 

presence or function of the trickle vents installed in their windows. Several complained of a 
52

“stuffy” environment, and preferred to use their windows to ventilate the apartment. In Figure 

46, one can see the direct effect of an open window on the pressure in the apartment. Apartment 

5H was observed with open windows at each visit, and the resident explained that he preferred 

more fresh air than could be provided by the trickle vent. The pressure in this apartment is 

essentially neutral with respect to outside, showing free exchange of air with the outside. 

Other occupied apartments kept their windows closed more habitually, according to interviews

with tenants. One can see a trend of generally lower pressure with respect to outside in the 

apartment on the third floor, while apartments on higher floors have somewhat higher pressures.

This concurs with a general pattern common to buildings in cooler weather, as stack effect 

increases pressure on upper floors while lowering it on lower floors with respect to outside.

Figure 46. Pressure data logged in building spaces over a 24-hour period

Pressures between apartments and their neighboring hallways were also recorded (Figure 47). It 

appears that vacant apartments are significantly more depressurized relative to the hallway than 

occupied apartments. This indicates that air predominantly flows into apartments from the 

corridors in unoccupied apartments. This concurs with earlier testing that measured the flow of 

air through the corridor door in an unoccupied apartment was 53–60 CFM. In occupied

apartments, open windows obviously dominate the airflow dynamics. In this study building as 

well as many others, residents open windows to combat localized overheating in their 

apartments.
53

Figure 47. Pressures in apartments with respect to hallway

*Apt. 4L was unoccupied for the entire monitoring time.

**Apt. 4M was unoccupied until November 2.

The equation for an orifice opening can be used by applying the pressures measured through 

logging, to the area of the door leakage measured in previous tests

Q = A * Cd * (2∆P/ρ)

0.5 * 1.36697 (5)

Where:

Q is leakage through the door gaps (in CFM).

A is the leakage area of the door

Cd is the coefficient of discharge

ΔP is the pressure difference between inside and outside of the test 

system (in Pascals)

ρ is the air density (in kg/m3)

This equation was used to calculate the airflow through the door at any given time if the pressure 

is known. Applying this to the pressure logging data, the bulk flow of air into or out of the 

apartment can be determined. Table 28 shows these data for three apartments. Data from 

apartment 4K, in which windows were opened occasionally, are in contrast to the data from 

apartments 4L and 4M, which were unoccupied with their windows closed.

The data show that the two unoccupied apartments are drawing significant air from the corridors, 

while the occupied Apartment 4K in fact is sometimes delivering air into the corridor. The 

pressure in this apartment fluctuates widely, and flows are significant in either direction, possibly 

caused by the operation of the windows. At times, the apartment draws a significant amount of 

air from the corridor (up to 39 CFM) while at other times, it sends air in the other direction (up to 

45 CFM into the corridor). Since neighboring apartments on the same floor are consistently 
54

negatively pressurized with their windows closed, it is assumed that the cause of the wild swings 

of pressure and airflow in the other direction are the result of opening and closing windows.

Table 28. Summary Flow Data From Apartment/Corridor Doors

Apt 4L Apt 4M Apt 4K*

Average 55 66 –13

Maximum 65 78 39

Minim 44 55 –45

Tested Condition Unoccupied Unoccupied Occupied

*Negative numbers indicate flow from apartment into corridor

The logged data from Apartment 4K are quite variable, illustrating the impact of opening 

windows on the pressure dynamics of the building. In the chart below, there is an obvious 

correlation between the pressure in the apartment with respect to outside and with respect to the 

corridor. Most notably, when the pressure in the apartment with respect to outdoors jumps 

toward zero, it indicates an open window and the pressure with respect to the hallway increases 

to positive territory. These events are marked by air flowing from the apartment into the corridor 

for a time, as indicated by the directional arrows in Figure 48. In this building, apartments with 

their windows open may vent air into the corridor, while apartments with their windows closed 

draw air from the corridor.

Figure 48. Pressures in apartment with respect to hallway and outdoors

Comparing corridor door airflows to those originating from trickle vents, it is clear that more 

identified MUA originates from corridors in this building than from outside, or at least through 

trickle vents from the outside (see Table 29). The average trickle vent delivered about 5 CFM at 
55

the pressures commonly experienced in apartment 4L, an unoccupied apartment. In apartment 

4M, another unoccupied apartment, the average flow was almost 6 CFM per trickle vent.

Table 29. Summary Flow Data From Trickle Vents

Trickle Vent Flows (CFM)

Apartment Apt 4L Apt 4M Apt 4K Apt 3F Apt 5A Apt 5H

From Single Trickle Vent

Average 5 5.9 1.9 2.5 1.1 –0.1

Maximum 3.9 4.8 –4.5 –0.5 –4.5 –1.6

Minimum 5.8 6.7 4.3 3.9 2.8 2.9

From All Trickle Vents in Apartment

# Beds 1 1 2 3 2 1

# Trickle Vents 3 2 4 5 3 2

Total Flow 15.1 11.9 7.4 12.6 3.3 –0.2

Tested Condition Unoccupied Unoccupied Occupied Occupied Occupied Occupied

Data from Apartment 4M are taken from the unoccupied period only.

Negative numbers indicate flow from the apartment out the trickle vents to outside.

In summary, for apartments in which no windows were opened during the monitoring period, 

airflow from the corridor into the apartments dominates the MUA pattern (see Table 30). These 

flows are several times higher than those measured from trickle vents.

Table 30. Identified Sources of MUA in Apartments (CFM)

Apt 4L Apt 4M Apt 4K

Air From Corridor Door 54.8 66.1 –13.2

Air From Trickle Vent 15.1 11.9 7.4

Measured Apartment Exhaust 36 38 49

Tested Condition Unoccupied Unoccupied Occupied

3.4.6 Exhaust Fan Power Consumption

The exhaust fans used in this building are Panasonic WhisperLine FV-10NLF1, nominally 120-

CFM fans. Product literature lists fan efficiency at no less than 3.3 CFM/W, resulting in a 

maximum power consumption of 21.8 W at the average measured exhaust flow rate.
56

4 Discussion

Of the four strategies tested, all delivered varying degrees of MUA; however, none consistently 

delivered their design flow rates. That does not imply that these strategies are ineffective; it 

simply means that the installations tested for this study were not executed properly. The 

following discussion aims to provide criteria and specifications needed for each strategy to 

deliver design flow rates and then provides a side-by-side comparison of the four. 

4.1 Type 1: No Direct Make-Up Air Supplied to Apartments

This type of ventilation strategy is commonly found in multifamily construction. Local exhaust is 

provided with no provision for MUA, which is assumed to come from the exterior envelope as in 

low-rise residential construction. Alternatively, MUA may be provided to a “pressurized”

corridor and assumed to transfer to the apartment through door leakage around the apartment 

door. In the test building, the common area ventilation system was intentionally oversized to 

provide OA for the corridors and the apartments. In practice, more air than expected was 

exhausted from common areas, and almost balanced the supply. The corridors were essentially 

unpressurized by the common area ventilation systems. Still, testing indicated that approximately 

25% of the MUA for apartment exhaust was being drawn from the corridor through leakage 

around the door, while the remainder came from other sources of infiltration. It was outside the 

scope of this study to identify all sources of infiltration; however, it is most likely a combination 

of infiltration from the exterior, from other apartments, and from interstitial spaces. 

With so many potential sources of MUA, it is not accurate to assume that one source

predominates. While some factors that influence air movement can be controlled, most cannot. 

As a result, this strategy does not consistently deliver adequate MUA. In addition, as 

construction moves toward tighter buildings, the source of MUA will shift toward the weakest 

pressure planes, which will likely be interior spaces.

4.2 Type 2: Make-Up Air Through Ducted Supply Air 

This strategy of ventilation was effective at delivering MUA to many apartments. The percent of 

supply to exhaust ranged from 38% to 105%. However, it failed to meet design rates in most 

cases. This highlights the need for close oversight during construction and a thorough 

commissioning process.

Though challenges exist, it is possible to design and construct this system to meet the specified 

flow rates. Numerous factors, including poor ductwork design, duct leakage, undersized fans, 

poor installation, and imbalanced risers, can contribute to the subpar performance. A skilled and 

motivated ventilation contractor is imperative in creating a properly functioning system. 

Independent testing and verification is also important in helping to establish a system that 

performs properly. While a TAB contractor may be used, it is equally important to verify the 

results. There was considerable discrepancy between CARB’s field measurements and the TAB 

report, which consistently reported the system as performing within 5%–10% of design. CARB

measurements showed that the system consistently underperformed by 30%–60%. Discussion in

Section 3.2.3 gives more detail on the possible reasons for this discrepancy.
57

4.3 Type 3: Make-Up Air Through HVAC Supply 

This somewhat novel ventilation strategy is innovative but ultimately was not effective at 

delivering MUA to apartments. The strategy provided supply flows that were approximately 

15% of the MUA. The balance was split roughly in half between leakage around the door and 

other sources of infiltration. Despite the poor performance, this strategy still offers promise. 

Three changes in the system design would significantly improve the performance. The first is to 

construct a tight, well-compartmentalized building. The building tested for this study was 

relatively tight; however, the gap beneath the apartment doors significantly reduced the overall 

apartment tightness. This is generally not taken into account when the apartment is tested 

because the blower doorframe is typically placed in the door opening. Installing weatherstripping and a door sweep would help to ensure that the door does not contribute to air 

infiltration. A tighter apartment results in a greater pressure differential that the MUA fan 

experiences. This results in greater flow through the fan.

The last two improvements pertain to the design of the MUA kit. The path from outside to 

inside, through the kit, is rather long and subject to substantial pressure drops. Moving from 

outside to inside the following components are encountered: exterior damper, mesh screen, 

several feet of corrugated plastic tubing, one foot of insulated ductwork, and a 1-in. thick filter. 

Reducing this path and the number of pressure drops would result in higher flow rates.

Last and most importantly is the fan itself. The one used in the MUA kit is a simple 12-V 

computer fan. There is a wide variety of these fans available with an equal array of performance 

curves. The fan curve could not be obtained for the particular model used in the MUA kit; 

however, qualitative testing showed that it did not perform well under high static pressure. 

Future versions of this system should consider using a fan that is designed to operate under 

higher static pressure. Ideally, the fan should be sized for the operating characteristics of the 

system. It is likely that if all three improvements are made, the performance will improve and 

substantially more MUA will be delivered. It is conceivable that design level ventilation could be 

achieved with an improved system

4.4 Type 4: Make-Up Air Through Passive Vents 

This type of ventilation strategy requires near-complete control over all elements of air 

movement in a building in order to function well. Apartments must be substantially airtight so 

that alternate pathways for air movement can be minimized. It is beyond the scope of this study 

to quantify what levels of airtightness are necessary for typical passive vents to function 

properly, but the results of this study show that even in apartments of better-than-average 

airtightness, performance from passive vents is difficult to ensure.

Testing showed that at an average apartment airtightness of about 0.10 CFM50/ft2

, continuous 

in-unit exhaust fans drawing 36–45 CFM were able to depressurize the apartment to about 3–4 

Pa. This is enough to draw 11.5 CFM from trickle vents compared to 41 CFM of exhaust, 

resulting in controlled MUA ratio of 28% (see discussion of “controlled MUA fraction” in 

Section 4.5 below). Through pressure monitoring, it became immediately clear that opening 

windows in an apartment completely eliminates any such depressurization.
58

This may be a moot point, because opening the window clearly promotes significant air 

exchange between the apartment and outside, and hopefully this means that fresh air makes its 

way into the apartment. However, it also became apparent from pressure monitoring that the 

airflow between the apartment and the corridor was highly affected by the opening and closing 

of windows, and that the direction of airflow is not easily predictable or controllable. This means 

that an apartment with open windows could end up supplying polluted air to the corridor and to 

other units, or that it could draw polluted air from the corridor inward, depending on the 

prevailing pressures in the building at that time.

4.5 The Relationship Between Apartment Airtightness and Make-Up Air 

The Type 4 strategy of providing ventilation, while probably the lowest cost in terms of 

construction and operation, is also probably the most difficult to execute well, requiring aboveaverage levels of attention to air-sealing details. The apartment airtightness necessary is not 

commonly seen in multifamily residential construction.

To illustrate this point, consider a building with trickle vents. In order to keep trickle vents to a 

reasonable size, the apartment in which they are installed must be at a sufficient negative 

pressure to draw air from the vent, and therefore the apartment must be relatively airtight to 

develop that pressure. This shows that a balance must be struck between realistic targets for 

depressurization and trickle vent size (see Table 31). The apartments monitored for this study 

were maintained at a negative pressure of 3–4 Pa. In order to deliver 30 CFM to an apartment at 

a pressure of 4 Pa, a trickle vent must have 25 in.2 of open area. This is akin to having a 2-ft wide 

window open by 1 in. For comparison, the trickle vents tested in the Type 4 building had an 

opening of about 4 in.2 per vent.

Table 31. Area of Trickle Vents Required at Various Pressure Differences To Deliver 30 CFM

Pressure Difference

(Pa)

Trickle Vent Area 

Required

(in.

2

)

Minimum # Common 

Trickle Vents Required 

at 4 in.

2 Each

3.0 29.1 8

4.0 25.2 7

5.0 22.6 6

6.0 20.6 6

7.0 19.1 5

8.0 17.8 5

9.0 16.8 5

10.0 16.0 4

Obviously, caution should be exercised before specifying 25-in.

2 engineered “holes” in 

the exterior envelope, but this illustrates the tradeoff that must be weighed before system 

selection.

As an example, to meet ASHRAE 62.2-2010 standards for a studio apartment in this building, 30 

CFM of ventilation would be required. The exhaust fan used in this building is capable of 
59

working against 155 Pa static pressure at that flow rate. Given that ductwork, registers, and 

exterior louvers in the exhaust system each detract from that figure, the amount available to 

depressurize the apartment as needed to make trickle vents function is not large.

In a basic blower door test, all sides of a test space are depressurized to an equal degree (see 

Figure 49). In a single-family home, the leakage quantified relates fairly directly to indooroutdoor airflow and energy use. In multifamily buildings, however, the leakage figures 

determined from this process are not directly indicative of either airflow patterns or energy 

impacts.

Figure 49. All sides of an apartment contribute to leakage in a blower door test

Not all surfaces of an apartment leak equally under the artificial conditions of a blower door test, 

and this is even truer in normal operating conditions. Though data indicating the leakage rates of 

separate surfaces are difficult to obtain, it is safe to say that interior demising walls leak 

considerably more than exterior walls or typical floors or ceilings, especially those made of 

monolithic slabs of concrete. Still, even though demising walls are leaky, they do not often 

experience the major pressure differences that induce leakage if they are served by equally

operating mechanical systems (see Figure 50). In the test building, every apartment had a 

relatively similar rate of exhaust and a similar operating pressure. Air does not move across the 

shared apartment walls if the pressure in both apartments is the same. The leakage and MUA

therefore come mostly from the exterior or the corridor/apartment wall.

Figure 50. Pressure differences on select surfaces in normal building operation

-50 Pa

-5 Pa

-5 Pa

-5 Pa
60

The tests performed in this study to quantify corridor door leakage found that the doors leaked 

about 200 CFM50. Converting this to leakage at an apartment’s typical operating pressure of 

–4 Pa, this equates to about 57 CFM, about the same value as the tests that measured door 

airflow under actual operating conditions. Clearly the corridor door can be by far the biggest 

point-source of MUA in the apartment, whether intentional or not.

Of course, if any apartment comes under different operating conditions because of a malfunction 

or change in the operation of another apartment, air movement occurs across the shared walls.

For example, if the exhaust fan in a smoker’s apartment stops working, not only is the smoke not 

exhausted from the apartment, but the working exhaust systems in nearby apartments draws that 

smoke into their own apartments. Compartmentalization is very important to prevent this 

situation.

Creating an apartment that is capable of producing flow from trickle vents and only trickle vents 

would prove to be quite difficult. In Table 32, a theoretical scenario based on test results from 

the test building is shown. Current leakage levels and distributions are shown, compared to target 

levels for a scenario in which trickle vents could work. Leakage sources from the blower door 

test are identified, along with assumptions for the share of the total leakage due to each envelope

component. From leakage tests done on these building components, the following have been 

identified. The door and trickle vents combined account for almost as much leakage as the entire 

envelope combined.

Table 32. Leakage Sources of Tested Apartment

Leakage Test Results CFM50 Source

Envelope 300 Blower door test

Door 200 Door leakage test

Trickle Vents 75 Depressurization test

Total 575

Identifying the share that each surface of the apartment accounts for of the envelope leakage 

figure, the estimated contribution of each is given below in CFM50. As mentioned before, not all 

envelope leakage paths identified during the blower door test allows air through in actual 

building operation. For example, the pressure difference between apartments is close to zero 

during normal building operation so no MUA is taken from this space. In the table below, those 

surfaces which contribute to air movement during normal building operation are indicated by an 

asterisk. The share of each building component in actual operation is shown in the table as well.

The pressure that a typical apartment experiences in this building as currently built is about 4 Pa.

At this pressure, the various envelope components pass considerably less air, shown in the 

columns labeled “Actual Operation.” The total air exchanged through all envelope components is 

106 CFM.

The target envelope leakage numbers required to obtain most MUA from trickle vents are 

substantially lower. Leakage from all other building components must be minimized so that the 

greatest pressure is exerted on the trickle vents. They should be the leakiest component of the 

envelope. In the scenario in Table 33, the total envelope leakage including leakage from the 
61

corridor door and the trickle vents is 179 CFM50, which is extremely airtight. Such an airtight 

apartment would elevate the pressure in the apartment to perhaps 10 Pa, drawing 76 CFM of 

MUA total, with the majority coming from trickle vents.

Table 33. Current and Proposed Building Characteristics for Properly Functioning Trickle Vents

Current Target

Total Leakage Actual 

Operation Total Leakage Actual Operation

Leakage 

Source Share CFM50 Pressure CFM Share CFM50 Pressure CFM

Floor 2.5% 14 0 0 3% 10 0 0

Ceiling 2.5% 14 0 0 3% 10 0 0

Ext. 

Wall* 7% 40 4 11 7% 20 10 9

Corridor 

Wall* 10% 58 4 16 13% 40 7 15

L Wall 15% 86 0 0 17% 50 8 0

R Wall 15% 86 0 0 17% 50 0 0

Door* 35% 201 4 57 7% 20 7 7

3 Trickle 

Vents* 13% 75 4 21 34% 101 10 45

575 106 300 76

*indicates a surface that contributes to air exchange during normal building operation. 

Attaining tightness levels this high is beyond the reach of everyday construction practices. This 

building, as currently built, has blower door results that are considered very airtight. The average 

airtightness of apartments in the building is 2.5 ACH50, and the leakage ratio is 0.11 CFM50/ft2

of enclosure area. To put this in perspective, these levels are already approximately one third of 

the current requirement in the ENERGY STAR Multifamily High Rise program. The proposed 

level of airtightness for the envelope components is broken down in Table 34. Leakage from 

the envelope is minimized to 179 CFM50 for an 800-ft2 apartment, or 0.067 CFM50/ft2 of 

enclosure area.

Table 34. Proposed Leakage Levels for Functioning Trickle Vent

Leakage Targets CFM50

Envelope 179

Door 20

Trickle Vents 101

Total 300

Though there is no specific threshold of apartment airtightness required, the proper functioning 

of the system under varying environmental and operational conditions is highly dependent on 

compartmentalization. The airtightness of the envelope must be minimized to avoid transfer of 

air between apartments when a window is opened in a neighboring apartment, for example. As 
62

demonstrated in this relatively tight building, opening the window in an apartment significantly 

affects pressure in its neighbors. The alternative to compartmentalization is to ensure that all 

apartments are held at the same pressure regardless of windows opening, fans turning on and off, 

etc., which is not practical. 

This discussion seeks to point out that although there is no specific standard for how airtight an 

apartment must be to benefit to the function of the ventilation system, in general tighter 

apartments afford more control over the source of MUA through ever-changing environmental 

conditions. Table 35 illustrates where MUA is sourced at two levels of airtightness in a model 

apartment. 

Table 35. Two Scenarios of Apartment Tightness and MUA Sources

Scenario 1

As Currently Built

Scenario 2

Proposed

Envelope 299 79

Door 201 20

Trickle Vents 75 101

Total Leakage 575 200

CFM50/ft2 0.11 0.03

# Times Tighter Than ENERGY STAR 3 10

CFM From Trickle Vent 21 45

CFM From Other 85 20

% MUA From Trickle Vent 20% 69%

% MUA From Other 80% 31%

In the first scenario, based on actual measurements of the building in this study, the total leakage 

of an apartment may be 575 CFM50, including leakage from the envelope, the corridor door, and 

trickle vents. This total leakage equates to 0.11 CFM50/ft2 of apartment envelope, about 3 times 

tighter than the current ENERGY STAR for Multifamily High Rise standard. 

In this scenario, three trickle vents, each 4 in.2 in area, bring in 21 CFM total, or 20% of MUA, 

while 85 CFM or 80% of MUA comes from other unidentified sources such as the shared walls, 

floor, ceiling, corridor door, and exterior wall. 

In the second scenario, the apartment is tightened by weather-stripping the corridor door so that 

it leaks only 20 CFM50, the trickle vents are enlarged to 13 in.2

, and the rest of envelope leakage 

is minimized to only 79 CFM50 (see Table 35). Under normal operating conditions, 45 CFM 

(69%) of MUA would come from trickle vents, while only 20 CFM (31%) comes from 

unidentified sources. 

Obviously making an apartment 10 times tighter than the ENERGY STAR standard will be no 

easy task, but if this threshold is ever reached by advancing the art and science of 

compartmentalization, there is good reason to hope that apartment exhaust will be made up in 

large part by fresh, controlled air sources.
63

4.6 Comparison of Different Strategies

The following section aims to provide a means of comparing the four MUA strategies. Each 

strategy was applied to two theoretical apartments: a 450-ft2 studio and a 750- ft2 two-bedroom

apartment. Based on ASHRAE 62.2-2010, 30 CFM and 45 CFM of whole-unit ventilation are 

required, respectively (ASHRAE 2010b). (ASHRAE 62.2-2013 requires slightly higher levels; 

however, the units evaluated were constructed prior to 2013 so the 2010 rates were used for this 

comparison.) The fan performance metric (supply CFM/W) that was developed for each strategy 

was applied to the two theoretical apartments to determine the electricity requirement necessary 

to provide the required MUA. The results are presented in Table 36.

Table 36. Comparison of Ventilation Strategy Fan Power Applied to Two “Sample” Buildings

Strategy Fan Type

Total Fan 

Power*

(Rated W)

Measured 

Airflow 

(CFM)

CFM/W

Total Fan 

Power (W) 

for Studio 

(30 CFM)

Total Fan 

Power (W) 

for 2-

Bedroom 

(45 CFM)

Type 1: No Direct 

MUA Supplied 

To Apartments

Individual 

apartment 

exhaust fan

3.2** 9.7 3.0 9.9** 14.8**

Type 2: MUA

Through Ducted 

Supply

Central 

supply 1,119 1,792 1.6

42.3 63.5

Central 

exhaust 1,492 1,898 1.3

Type 3: MUA

Through HVAC 

Supply

PTAC 

supply fan

2.4 4.1 3.6

15.8 20.0

Apartment 

exhaust fan 36 57 1.6

Type 4: MUA

Through Trickle 

Vents

Individual 

apartment 

exhaust fan

21.8** 11.5 0.5 56.9** 85.3**

*Values estimated from manufacturer or design literature.

**Type 1 and Type 4 both use the same type of fan. The variation in power is due to individual product selection. 

Had the fan from Type 1 been used in Type 4, the values would be significantly lower.

The data in the Table 36 should be viewed with the understanding that it contains a mix of both 

tested values and manufacturer design values. Due to difficulty in accessing several of the fans, 

all of the values for fan power were taken from manufacturer literature. The values for airflow 

were all directly measured or derived from measurements. In the case of Type 1: exhaust only, 

the fan depressurizes the apartment and results in flow from the corridor into the apartment. 

Therefore, the measured airflow represents the amount of air coming through the door undercut. 

For Type 3: HVAC supply, the total fan power is the sum of the existing exhaust fan and the 

power required by the PTAC fan to supply the requisite amount of fresh air. These values were 

determined from tests conducted on studio apartments in the building, which have one kitchen 

exhaust, one bath exhaust, and a single PTAC supply. Values for one- and two-bedroom units 

would be slightly different due to variations in the number of baths and PTACs; however, these 

units were not tested under this study.
64

Comparing one system to another with finality is difficult because none of the buildings actually 

performed as designed. In the case of passive vents, the proper functioning of the vents is very 

dependent on the airtightness of the building envelope. The only building type in which verifying 

performance is relatively straightforward is Type 2: ducted supply, but even this was shown 

through measurements to fall considerably short of the target.

It should also be understood that these values are only valid for the equipment used in the 

buildings in this study, and should not be taken as universally representative of all buildings of 

the same type. For example, the mechanical equipment for Types 1 and 4 are substantially 

similar—small, unitized exhaust fans located in the bathrooms of each apartment—but because 

of a choice of fan equipment, those in the Type 4 study building consume considerably more 

electricity than those in the Type 1 building. Design choices have an impact on the performance 

of every project, and these data should be taken as examples only.

Type 1 has been included in the table for reference; however, it does not reliably deliver fresh 

air, as demonstrated in the tests. Types 2 and 3 include the power requirements of the supply and 

exhaust fans, since these systems require both fans to function properly. Of the three remaining 

strategies that have potential to reliably provide MUA, Type 3 performs the best with regards to

electric energy requirements; however, the power requirements needed for the supply fan would 

likely be greater than those noted in the table because a different fan would need to be used to 

deliver the needed flow. Types 2 and 4 require significantly more fan energy; however, the 

ventilation performance of Type 4 is likely to be more inconsistent and subject to environment 

factors than Type 2.

Although Type 2 requires more electric energy than Type 3, it has the potential to conserve 

thermal energy by tempering the supply of outdoor MUA with energy recovered from the 

exhaust air through the ERV. It should be noted that Type 2 does not require energy recovery. It 

is possible to provide ducted supply and separate exhaust without an HRV or ERV. Types 3 and 

4 both deliver MUA from the outside; however, the comparison between their thermal 

performances is not a simple process.

Table 37 shows a comparison between the total exhaust removed from the apartment and the 

supply air from known sources. The ratio of known supply air to exhaust air is the controlled 

MUA fraction. The MUA through ducted supply is the best performer of the group, with a 71% 

controlled MUA fraction. This means that every CFM of exhaust is matched by 0.7 CFM of 

fresh air supply. The worst performer of the group was the PTAC MUA supply, with every CFM 

of apartment exhaust matched by less than 0.1 CFM of supply. The two passive MUA strategies 

(Types 1 and 4) both had a controlled MUA fraction of about 28%. In addition, for reference 

only, the ASHRAE 62.2-2010 whole-building ventilation rate is included for the apartments. 

This value is determined from ASHRAE 62.2-2010 Table 4.1a and is based on the floor area and 

number of bedrooms in each apartment. One-bedroom apartments with less than 1,500 ft2 have 

OA ventilation requirements of 30 CFM, and two- and three-bedroom apartments with less than 

1,500 ft2 have OA ventilation requirements of 45 CFM. Although exhaust only strategies are 

accepted by ASHRAE 62.2, the ideal ventilation design would provide MUA from the outdoors 

that met these ventilation rates. Again, Type 2 is the best performer in this regard.
65

Table 37. Comparison of Supply Air Delivered by Each Ventilation Strategy 

Strategy

ASHRAE 62.2

Whole-Building 

Ventilation Rate 

(CFM)

Average 

Supply Air 

Delivered 

(CFM)

Average Total 

Continuous 

Exhaust

(CFM)

Average Controlled 

MUA Fraction

(Supply Delivered/

Exhaust)

Type 1: No Direct 

MUA Supplied to 

Apartments

30–45 9.7 34 28.5%

Type 2: MUA

Through Ducted 

Supply

30–45 37.9 53 71.4%

Type 3: MUA

Through HVAC 

Supply 

30 4.1 57 7.2%

Type 4: MUA

Through Trickle 

Vents 

30–45 11.5 41 28.0%

These data do not indicate the expected performance of every building that uses one of these 

strategies, but it is illustrative of the challenges in making each one work well. Clearly, 

construction quality of these particular buildings was a large factor in performance. CARB

suspects that better unit compartmentalization and better construction of the ventilation systems

would help the performance of each of these strategies. 

Table 38 summarizes the general aspects of each type of ventilation system studied in this report.

Material costs (not including installation labor) were estimated by the following methods:

• The cost of exhaust-only systems was estimated based on wholesale prices for 

continuously operated exhaust fans such as the Panasonic WhisperGreen fan, the model 

used in the study building.

• The cost of ducted ERV/HRV systems was estimated based on conversations with the 

developer of the two buildings in this study. The figure includes the main air handler cost 

and associated ductwork.

• The cost of PTAC OA fans was based on comments by the manufacturer on the 

additional cost for the optional package added to their typical PTAC unit.

• Costs for the trickle vents are based on comments by several developers in the New York 

City area who regularly use the devices, but does not include the cost of installation in a 

window.

Fan energy estimates are based on manufacturer’s data relating fan performance in CFM with 

power consumption. The power consumption is based on 30 CFM of exhaust being removed and 

30 CFM of fresh air being delivered. These estimates are based on manufacturer’s data of the 

equipment used in the study buildings, and the actual power consumption of a real building is

different based on system design, product selection, and construction.
66

• For Type 1, Exhaust Only, fan power is the estimate of a typical in-unit exhaust fan such 

as the Panasonic WhisperGreen at the design levels of ventilation used in the study 

building. No dedicated supply is provided, so no power consumption for supply is 

accounted for.

• For Type 2, Ducted Supply, fan power was estimated based on manufacturer’s data for 

fan power consumption at design airflows for both supply and exhaust fans in the central 

rooftop unit.

• For Type 3, PTAC OA Fan, the estimate is based on the power consumption of the small 

fresh air supply fan as well as a typical in-unit exhaust fan, in a design similar to that of 

the study building.

• For Type 4, Trickle Vents, fan power is the estimate of a typical in-unit exhaust fan at the 

design levels of ventilation used in the study building. No dedicated supply is provided, 

so no power consumption for supply is accounted for.

This analysis of ventilation system efficiency is simplified, and it ignores factors such as heat or 

energy recovery efficiency of an HRV/ERV unit. It also ignores certain likely effects on building 

efficiency such as the interaction between infiltration and ventilation in buildings with exhaustonly systems. Discussions of these interactions (Roberson 2004) are beyond the scope of this 

study.

The controlled MUA fraction is the ratio of measured supply CFM from known sources to 

measured exhaust CFM. The ideal ratio would be 1 CFM supply to 1 CFM exhaust, with every 

unit of air drawn from the apartment replaced by a unit of fresh air of known origin. Passive 

systems and those that rely on pressure differences to draw air through building spaces (Types 1

and 4) are at a disadvantage to ducted or forced air systems (e.g., Types 2 and 3), because they 

rely on very tight interzone air barriers and clear passive MUA pathways to function. In all 

likelihood, the controlled MUA fraction of passive systems will never be 100%, meaning there 

will always be air drawn from unintended spaces such as other apartments, hallways, and other 

building spaces.

In Table 38, the only system that was proven to deliver the designed amount of fresh air at a rate 

equal to the exhaust was the ducted supply system, though for any given apartment, that depends 

directly on the quality of the distribution system. This is borne out in the testing results, that 

show that for any given apartment, the controlled MUA fraction was anywhere between 40% and

105%. It should be reiterated that the table states results for the buildings tested in this study, and 

that a new building could fare better or worse depending on construction quality. 
67

Table 38. Comparison of Ventilation Strategies

Type 1

Exhaust Only

Type 2

Ducted Supply

Type 3

PTAC OA Fan

Type 4

Trickle Vents

Material Cost

(Estimated) $150–$200/fan $800–1,200/apartment $90 more than standard PTAC $40/vent

Fan energy per 

Studio Apartment

(24/7 Operation; 

Estimated)

87 kWh/year

(apartment exhaust fan)

371 kWh/year 

(supply and exhaust fans)

139 kWh/year

(OA fan and exhaust fan)

498 kWh/year*

(apartment exhaust fan)

Controlled MUA

Fraction

(Tested Results)

20%–45%

(from door gap) 40%–105% 5%–25% 15%–40%

System Pros

• Inexpensive

• Little to no 

maintenance required

• Supply air is provided 

in a controlled and 

measurable way

• Provides the 

opportunity for 

filtration and heat 

recovery or air

• Single, easily

monitored fresh air 

device

• Relatively inexpensive

• Provides some opportunity 

for filtration

• Supply air is drawn through 

heating/cooling element 

which tempers it, results in 

improved comfort

• Relatively inexpensive

• Little to no maintenance 

required

System Cons

• No designated source 

of supply air

• Results in air being 

drawn from corridor, 

other apartments and 

interstitial spaces

• May cause 

complaints of odors 

from other units

• Expensive material cost

• Annual operating costs 

are higher than other 

methods

• HRV/ERV requires 

routine maintenance

• Fan is not capable of 

operating against high 

static pressure

• Building conditions can 

overwhelm fan, resulting in 

reduced supply flow

• Filter requires periodic 

changing and is not easily 

accessible 

• Flow is subject to 

changing building 

pressures

• Requires very tight 

construction to function 

properly

• Building operation can 

result in low or reversed 

flows 

*This value is a result of product selection. Had a fan similar to Type 1 been used the value would be significantly lower.
68

5 Preliminary Conclusions

This project sought to answer the following research questions:

• What exhaust ventilation and corresponding MUA strategies (if any) are being 

implemented in high performance new construction multifamily buildings to meet wholeunit ventilation recommendations of ASHRAE 62.2-2010?

Through this research and CARB’s role in multifamily program implementation work in 

the region, CARB researchers believe that the majority of high performance, new 

construction, multifamily housing in the northeastern United States use one of four

general strategies for ventilation: 

1. Continuous exhaust only, with no designated supply or MUA source

2. Continuous exhaust with ducted MUA to apartments

3. Continuous exhaust with supply through a MUA device integral to the unit 

HVAC

4. Continuous exhaust with supply through a passive inlet device such as a trickle 

vent.

CARB researchers have also seen a subset of the first category, where the doors between 

interior corridors and apartments are undercut to allow MUA to flow from the corridor 

into the apartment. The general strategy is to pressurize the corridor using supply air and 

depressurize the apartments through continuous local exhaust. Though in many building 

codes this practice is essentially prohibited for reasons of fire safety, it still continues as 

either a designed MUA strategy or as a poorly enforced interpretation of building code. 

The third and fourth strategies, although not intentional, showed significant MUA

supplied through leakage around the door.

These four main system types are representative of common ventilation strategies in the 

northeastern U.S. green building market. Each of the study buildings had participated in a 

green or energy efficiency standard such as LEED or ENERGY STAR. One 

commonality of all of the study buildings was the requirement that each design contain 

some way for fresh MUA to enter the apartment. The range of buildings is not meant to 

be exhaustive, but it is representative of the building stocks that CARB has worked with, 

including low-rise, midrise, and high-rise building types.

• How can the air delivery performance of passive MUA strategies such as trickle vents, 

fresh air dampers, or door undercuts be measured and quantified in actual installations?

How variable is the performance and what are the key factors that affect it—apartment 

location, extent of compartmentalization, season, etc.? 

CARB’s approach centered on inducing pressures that are not present in normal building 

operation, for the purposes of measuring flows within the normal range of common 

testing equipment. The contrivances constructed for this purpose amounted to in-field 
69

laboratory-style testing of the passive devices through a range of conditions. This level of 

rigor is not practical for wide use. Given the inability to quantify installations in the field, 

thorough installation guidelines, which address building airtightness, exhaust levels, and 

other factors, should be developed to ensure passive vents perform as designed.

In the future, the in-field performance of passive MUA devices should likely be verified 

through qualitative methods—visual inspection and simple functional performance 

tests—rather than quantitative methods. Laboratory tests conducted by manufacturers or 

third parties should be relied on for design. In addition, clear specifications for the 

installation and operating parameters of these devices should be established by the 

manufacturers. Manufacturer documentation of minimum building requirements, such as 

apartment airtightness and local exhaust rates, should be provided and correlated to 

device performance. 

From the tests, there is considerable variation in the amount of MUA provided by passive 

devices. Since operation of these devices is based on a pressure difference, any factors 

that affect the pressure across the device directly affect the flow. The airtightness of the 

apartment plays a major role in the performance of these devices. A well-sealed and 

compartmentalized apartment is needed in order to establish an adequately pressurized 

space. 

In addition, environmental conditions play a large role in the operation of passive 

devices, the primary factors being wind and temperature. The role of wind is still clearly 

visible by the short-term fluctuation of the pressure monitoring results, however, the 

specific effects are difficult to quantify because of its intermittency and high variability. 

In addition, local wind data were not collected as part of this study. 

There is no clear correlation between apartment location and flow through passive 

devices. In general, apartments on higher floors experience greater pressure variation due 

to wind; however the specific geometry and situation of the building on its site enhance 

or detract from the impact of wind speed and direction. Location of trees, open spaces, 

setbacks, and other buildings all affect the local environment and can result in significant 

variations in building pressures for apartments on the same floor. 

• Do the MUA strategies for apartments, both passive and active, function as designed 

when installed in real-world buildings?

Active MUA strategies, such as dedicated supply air from a central rooftop unit, can be 

highly effective at delivering specified quantities of MUA to apartments; however, a 

thorough TAB process is required. In the units tested under this study, the performance 

varied from 12% to 132% of the design. This wide a range indicates the need for clear, 

well-defined specifications and careful construction oversight. In addition, 

commissioning and verifying system performance are also important to a wellperforming system. In the case of PTACs with MUA kits, the products need 

improvement in order to function as intended. With revision and product quality control, 

there is ultimately no technical reason that this design cannot work.
70

The performance of passive MUA strategies, such as door undercuts and trickle vents, is 

considerably more variable and, based on current practices, is not meeting the design

intent. It should be reiterated that door undercuts are not a reliable or safe method of 

providing MUA and are not recommended. Under specific conditions, it is possible for 

trickle vents to provide design-level supply; however, the challenge is to maintain the 

required conditions under a variety of environmental and operating situations. Since this 

strategy is passive by definition, it is difficult to implement them to perform as designed 

under all conditions that a building experiences over the course of a year. CARB found 

the performance ranged from about 10%–35% of design. 

While there is no technical reason that passive strategies cannot function, the designs are 

very difficult to execute in the real world, requiring levels of compartmentalization that 

are not common to typical construction. In addition, it is clear that there is a significant 

lack of understanding on the part of building designers as to what performance levels can 

be expected from passive vents when installed in a real building. For example, the trickle 

vents used in the study building have performance data available in units of wind speed, 

not pressure.

Little guidance is given in product literature on how to translate performance data of 

passive vents into a working design. If they are to be used effectively, much more data 

are needed to demonstrate the real-world performance of the vents, the typical pressures 

that can be expected in apartments, and guidelines for the required airtightness of 

apartments.

• Can specifications be developed to better ensure proper performance from all MUA

strategies?

Clear, well-defined specifications can be developed to ensure proper performance from

all MUA strategies, but this study did not yield enough data to conclusively define what 

those specifications should be. In the future, specifications should focus on air sealing, 

compartmentalization, ventilation system performance, and general construction quality. 

The airtightness of an apartment plays a major role in the performance of MUA systems, 

especially when passive strategies are employed. Further study should be conducted to 

determine the minimum level of air sealing necessary to ensure performance. This should 

be expressed as a common metric, such as CFM50/ft2

. In addition to air sealing of 

apartments, it is important to ensure compartmentalization of other building spaces. This 

is particularly important in maintaining pressure differentials and minimizing fluctuations 

due to environmental conditions. It may be difficult to provide a specific metric, but a 

qualitative standard and general guidance can be developed. 

In order for a MUA strategy to function properly, it is necessary that the exhaust system 

operate as designed. It was observed several times where exhaust fans were not 

performing at the design levels. CARB also found situations where the same model fan 

was installed in each apartment, yet the performance was drastically different. These 

issues are the result of poor design, poor installation, or a combination of the two. 
71

Thorough construction quality control, testing, and commissioning help to prevent these 

issues and ensure proper function of both exhaust and MUA systems. Along the same 

lines, strong construction oversight of the general process should be included as part of 

the guidance.

• What is the energy “penalty” of mechanical supply systems?

All ventilation strategies result in electricity and thermal energy penalties. As previously 

mentioned, the thermal analysis is beyond the scope of this study. The electricity penalty 

of ventilation is the result of the energy required to run the fans. Mechanical supply 

systems employ both supply and exhaust fans; therefore, the electricity requirements are 

greater than passive strategies, which only utilize exhaust fans.

The electricity requirements of each system are summarized in Table 36. The fan energy 

estimates are based on a mix of manufacturer’s data for electricity consumption and 

tested data for delivered CFM of fresh air. This leads to expected results, such as the 

mechanical supply (Type 2) requiring approximately 2.7 times more fan electricity than 

the passive strategy (Type 4). It also leads to an unexpected result where the MUA

through HVAC system (Type 3) requires slightly more electricity per CFM than the 

mechanical supply, due to the weak performance of the fresh air fan.

This estimate also does not take into account distribution issues of the ducted supply

system. For example, additional fan power would be required to increase pressure and 

flow in the distribution system to overcome duct leakage and deliver design airflows at 

the farthest registers where they are too low. Instead of analyzing this scenario, the total 

airflow of the system as measured in the field was compared to the rated electricity

consumption at design flow.

There are obvious shortcomings with this approach, but due to the difficulty of measuring 

fan electricity consumption, particularly of many small fans in buildings without central 

systems, a full comparison of field-measured flow and power consumption was not 

possible. Future studies may take this into account.

• Considering the choices for various ventilation approaches and the factors impacting their 

cost effectiveness—material cost, efficiency, and performance—under what conditions is 

each system optimal?

All approaches evaluated in this study have the potential to deliver effective ventilation, 

but each has benefits and drawbacks. Choosing which system is best for a particular 

building depends on more than its effectiveness. For a building design already including 

PTACs, the lowest first-cost option is certainly to use PTACs with an improved fresh air 

kit. For building designs that more readily allow for penetrations in the exterior walls, 

unitized exhaust fans and passive inlet vents may be the most appropriate. It also should 

be straightforward to construct a ducted supply system that delivers the appropriate 

ventilation, using established principles.
72

Material costs and operating costs for each approach in this study have been discussed.

Although Type 4 was the most expensive in terms of fan energy, it represents only one 

type of fan. A more efficient model could have been used, which would result in 

substantially less energy consumption. Considering that, the most expensive approach is 

then the centrally ducted supply design, whether through a supply-only unit or an HRV or 

ERV. Extra ductwork, equipment, and lost floor area are all costs not incurred by other 

approaches. On the other end of the spectrum, a system design in which no dedicated 

supply system is installed is the least expensive. In terms of operating costs, the same is 

basically true. Those systems that utilize only exhaust and have no dedicated supply are 

the least energy intensive to run. Those that have dedicated supply fans are the most 

energy intensive.

Performance of any of the ventilation approaches is highly dependent on thoughtful

design and quality construction. Tight and well-balanced ductwork is also critical, and 

can mean the difference between an efficient, high performance system and an expensive 

but ineffective one. Obtaining effective performance from a system with ducted supply is 

well understood, utilizing tried and tested principles. The duct system must be well 

designed and laid out, very tightly sealed, and well balanced. Obtaining top performance 

from other ventilation approaches is somewhat less straightforward. The current 

generation of PTACs with fresh air kits is not able to deliver the desired quantities of 

fresh air. Still, the approach is innovative and improving the design should make it more

effective.

For all ventilation systems, but particularly for exhaust-only systems such as those with 

no ducted supply or with passive inlet vents, controlling where fresh MUA originates is

difficult. This is borne out by the results of this testing, which found that the controlled 

MUA fraction, or the supply rate of MUA of known origin to the exhaust rate, ranges 

greatly from less than 50% to more than 150%, even in the same building.

The discussion in Section 4.5 on the necessity of compartmentalization does not apply 

only to buildings with passive vents. CARB believes that all buildings will achieve better 

control over airflow through compartmentalization and better HVAC construction. Any 

ventilation system studied in this report has the potential to serve a building well. Each 

strategy has benefits and costs that make it more appropriate for one building but not 

another. There is no “best” ventilation system but rather a collection of best practices for 

any given approach. Table 39 shows situations in which each type of ventilation system 

would be well suited.
73

Table 39. Optimal Conditions for Each Ventilation System Type

System Type Conditions Under Which Each System Is Optimal

Type 1.

Exhaust Only

• Not recommended for multifamily buildings with common 

hallways. Low-rise buildings with no common hallway; 

exterior entries

Type 2.

Ducted Supply

• Buildings where exterior ventilation penetrations are not 

allowable.

• Buildings where filtered fresh air is a design requirement. 

• Very tall buildings that must overcome significant stack effect.

Type 3.

PTAC OA Fan • Buildings with PTACs as basis of design.

Type 4.

Trickle Vents

• Buildings with very airtight apartments.

• Shorter buildings (4 or fewer stories) in which stack effect is 

less of an issue.
74

References

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Resources

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November. http://publications.gc.ca/pub?id=99748&sl=0
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