Measure Guideline: 

Selecting Ventilation 

Systems for Existing 

Homes

R. Aldrich 

Consortium for Advanced Residential Buildings 

February 2014
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iii

Measure Guideline: Selecting Ventilation Systems for Existing Homes

Prepared for: 

Building America

Building Technologies Program 

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

Robb Aldrich 

Steven Winter Associates, Inc.

of the 

Consortium for Advanced Residential Buildings (CARB) 

61 Washington Street 

Norwalk, CT 06854 

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40342-03

February 2014 
iv

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v

Contents

Contents....................................................................................................................................................... v

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

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Abstract....................................................................................................................................................... ix

Progression Summary................................................................................................................................ x

1 Preliminary Planning............................................................................................................................ 1

2 Codes and Standards........................................................................................................................... 3

2.1 ASHRAE Standard 62.2-2010.............................................................................................3

2.2 ASHRAE 62.2-2013 ............................................................................................................5

2.3 Home Ventilating Institute...................................................................................................6

2.4 2012 International Residential Code and International Energy Conservation Code ...........6

2.5 Programs and Other Standards.............................................................................................7

2.6 What Rate is Right? .............................................................................................................7

3 Implementing Local Ventilation—Bathroom Exhaust....................................................................... 8

3.1 Assessment...........................................................................................................................8

3.2 Selecting Fans......................................................................................................................9

3.3 Fan Installation...................................................................................................................10

3.4 Controls..............................................................................................................................11

3.5 Measurement, Verification, and Commissioning ..............................................................11

4 Implementing Local Ventilation: Kitchen Exhaust.......................................................................... 12

4.1 Assessment.........................................................................................................................12

4.2 Selecting Fans....................................................................................................................13

4.3 Fan Installation...................................................................................................................13

5 Whole-Building Ventilation Considerations .................................................................................... 14

5.1 Installation and Integration With Existing Systems...........................................................14

5.2 Distribution and Mixing of Outdoor Air............................................................................14

5.3 Source of Outdoor Air .......................................................................................................15

5.4 Energy Implications...........................................................................................................16

5.5 Humidity ............................................................................................................................17

5.6 Operation and Maintenance ...............................................................................................17

5.7 Overview of System Types................................................................................................18

5.7.1 Exhaust Only..........................................................................................................18

5.7.2 Central Fan Integrated Supply Ventilation ............................................................19

5.7.3 Heat Recovery Ventilator or Energy Recovery Ventilator....................................20

6 Implementing Whole-Building Ventilation: Exhaust-Only.............................................................. 24

6.1 Assessment.........................................................................................................................24

6.2 Selecting Equipment ..........................................................................................................24

6.2.1 Fans........................................................................................................................24

6.2.2 Controls..................................................................................................................25

6.3 Installation..........................................................................................................................26

6.4 Measurement, Verification, and Commissioning ..............................................................26

6.5 Operation and Maintenance ...............................................................................................26

7 Implementing Whole-Building Ventilation: Central Fan Integrated Supply.................................. 27

7.1 Assessment.........................................................................................................................27
vi

7.2 Selecting Equipment ..........................................................................................................29

7.2.1 Controller...............................................................................................................29

7.2.2 Ducts and Dampers................................................................................................29

7.2.3 Air Handler Fan Motors.........................................................................................30

7.3 Installation..........................................................................................................................30

7.4 Measurement, Verification, and Commissioning ..............................................................31

7.5 Operation and Maintenance ...............................................................................................31

8 Implementing Whole-Building Ventilation: Heat Recovery Ventilators and Energy Recovery 

Ventilators ........................................................................................................................................... 32

8.1 Assessment.........................................................................................................................32

8.2 Selecting Equipment ..........................................................................................................34

8.2.1 Energy Recovery Ventilator or Heat Recovery Ventilator? ..................................34

8.2.2 Efficiency...............................................................................................................35

8.2.3 Power Consumption...............................................................................................36

8.2.4 Controls and Features ............................................................................................36

8.2.5 Cold Weather Performance....................................................................................36

8.3 Installation..........................................................................................................................36

8.4 Measurement, Verification, and Commissioning ..............................................................37

8.5 Operation and Maintenance ...............................................................................................38

References................................................................................................................................................. 39

Bibliography .............................................................................................................................................. 43

Appendix: Summary of Ventilation and Distribution Studies .............................................................. 44
vii

List of Figures

Figure 1. Simple example of increasing whole-building ventilation rates to offset lack of local 

ventilation in an existing home........................................................................................................... 4

Figure 2. Measuring exhaust flow with a flow hood and exhaust fan meter ........................................ 8

Figure 3. Exhaust ducts should always terminate outdoors—not in an attic “near” the outdoors. .. 9

Figure 4. Tight bends can reduce fan flow rates, increase noise, and increase power consumption.11

Figure 5. Some range hoods, like this one, do not vent outdoors. They simply circulate air back 

into the kitchen. Such hoods do not meet most ventilation requirements. ................................. 12

Figure 6. Simple schematic of CFIS ventilation..................................................................................... 19

Figure 7. Schematic of exhaust air and outdoor air passing through the heat exchanger in an HRV21

Figure 8. Example ventilation related costs for four systems in six different U.S. climates ............ 23

Figure 9. Many fans have adjustable speed settings (30–70 CFM in this fan) for continuous 

operation. When the bathroom is in use and more flow is desired, the fan can boost to 80 CFM 

for a preset time.................................................................................................................................. 25

Figure 10. Some devices—when wired in series with a fan—will reduce the speed and flow rate 

delivered. ............................................................................................................................................. 25

Figure 11. Many timer controls can be programmed to cycle fans on and off. For an equivalent 30 

CFM continuously, a 50-CFM fan can operate 36 minutes per hour. The fan can also be 

controlled on-demand........................................................................................................................ 25

Figure 12. This timer from can be manually programmed or users can enter in home information 

and the fan will operate to meet 62.2-2010 requirements. ............................................................. 25

Figure 13. Poor location selection for CFIS ventilation intake, as the homeowners placed their 

outdoor grill in this corner of the patio ............................................................................................ 27

Figure 14. At least three manufacturers offer CFIS kits: Aprilaire, AirCycler, and Honeywell. ........ 29

Figure 15. A motorized damper (left) and manual balancing damper (right) in CFIS systems......... 30

Figure 16. The Fieldpiece LER (left) and the Evergreen IM (right) are two replacement variablespeed motors designed for residential air handlers....................................................................... 31

Figure 17. While schematics such as these are often listed as installation options by ERV and HRV 

manufacturers, these configurations are NOT recommended. ..................................................... 33

Figure 18. If integrating an ERV/HRV with the heating or cooling duct system, CARB recommends 

at least one side of the ventilation system (either supply or exhaust) have a dedicated duct 

run. ....................................................................................................................................................... 34

Figure 19. An HRV installed in a cold-climate home ............................................................................. 35

Figure 20. Local ERV (Credit: Panasonic) ............................................................................................. 37

Figure 21. Local ERV (cover removed) installed in a home................................................................. 37

Figure 22. Summary of RAoA tests from (CARB 2010). In both test cases, all interior doors are 

closed with exhaust ventilation operating continuously. In Case 1, the mixing fan is running.45

Unless otherwise noted, all figures were created by CARB.

List of Tables

Table 1. Summary of Advantages and Disadvantages of Ventilation Systems.................................. 18

Table 2. Example Annual Energy Consumption and Operating Costs for Ventilation Systems in 

Several Climates................................................................................................................................. 23

Table 3. Approximate Airflow Rates With Varying TELs and Available Static Pressures................. 28

Table 4. Minimum Outdoor Air Fraction To Provide Mixed Air Temperature of at Least 60°F With 

70°F Return Air Temperature............................................................................................................. 29

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

Definitions

AHU Air handling unit

ASEF Apparent Sensible Effectiveness

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers

BPM Brushless, permanent magnet (motor)

CARB Consortium for Advanced Residential Buildings

CFIS Central fan integrated supply

CFM Cubic feet per minute

DC Direct current

ECM Electronically commutated motor

ERV Energy recovery ventilator (recovers both latent and sensible heat)

HRV Heat recovery ventilator (recovers only sensible heat)

HVAC Heating, ventilation, and air conditioning

HVI Home Ventilating Institute

IAQ Indoor air quality

IEQ Indoor environmental quality

IECC International Energy Conservation Code

in. w.g. inches of water gauge

IRC international residential code

LBNL Lawrence Berkeley National Laboratory

RAoA reciprocal age of air

TEL Total equivalent length

TRE Total Recovery Efficiency
ix

Abstract

This document addresses adding—or improving—mechanical ventilation systems to existing 

homes. The purpose of ventilation is to remove contaminants from homes either directly or by 

dilution. This report discusses where, when, and how much ventilation is appropriate in a home, 

including some discussion of relevant codes and standards. Advantages, disadvantages, and 

approximate costs of various system types are presented along with general guidelines for 

implementing the systems in homes.

CARB intends for this document to be useful to decision-makers and contractors implementing 

ventilation systems in homes. Choosing the “best” system is not always straightforward; 

selecting a system involves balancing performance, efficiency, cost, required maintenance, and 

several other factors. It is the intent of this document to assist contractors in making more 

informed decisions when selecting systems.

Ventilation is an integral part of a high performance home. With more air-sealed envelopes, a 

mechanical means of removing contaminants is critical for indoor environmental quality and 

building durability.

Published: February 2014

Author: Robb Aldrich, P.E.

Building America Team: CARB

Acknowledgments: This guideline draws on work by many researchers and practitioners

including Building America teams (Building Science Corporation, IBACOS, Florida Solar 

Energy Center, among others), national laboratories (the National Renewable Energy 

Laboratory, Oak Ridge National Laboratory, Lawrence Berkeley National Laboratory), heating, 

ventilation, and air conditioning contractors, home performance contractors, product 

manufacturers, and many others.
x

Progression Summary

Determine what levels of ventilation are appropriate 

or necessary for the home.

 Section 1, Preliminary Planning

 Section 2, Codes and Standards

Address local ventilation as appropriate.

 Section 3, Implementing Local Ventilation—Bathroom Exhaust

 Section 4, Implementing Local Ventilation: Kitchen Exhaust

Determine the most practical type of whole-building ventilation 

system.

 Section 5, Whole-Building Ventilation Considerations

Implement whole-building ventilation.

 Section 6, Implementing Whole-Building Ventilation: Exhaust-Only

 Section 7, Implementing Whole-Building Ventilation: Fan Integrated 

Supply

 Section 8, Implementing Whole-Building Ventilation: Recovery 

Ventilator and Energy Recovery Ventilator
1

1 Preliminary Planning

Before delving into the various technologies and types of ventilation systems, it’s worth 

considering details of the home, scope of the project, and project goals. The “best” or most 

appropriate ventilation systems—with respect to first cost, efficiency, indoor air quality (IAQ), 

etc.—will depend on many aspects of the home and the overall renovation or weatherization 

project. The Consortium for Advanced Residential Buildings (CARB) recommends reviewing 

the following questions before deciding on a ventilation system.

Do I need mechanical ventilation?

In short, the answer is almost always yes. Mechanical ventilation has become a practical and 

prescribed necessity in nearly all homes—certainly in efficient, high performance homes.

In any occupied home, contaminants are being generated. Contaminants come directly from 

people and pets (hair, dander, perspiration, etc.) and from occupant activities (cooking, 

showering, etc.). Contaminants also come from building materials and furnishings (paint, 

adhesives, etc.). Of course, limiting the sources of these contaminants is an excellent step in 

improving IAQ, but some contaminants are unavoidable. The purpose of ventilation is to remove 

contaminants and/or dilute them by bringing in outdoor air.

What code or program requirements must be met?

Many home improvement and weatherization programs have requirements related to ventilation.

Be clear on these requirements before choosing a system.

What is the scope of the renovation project?

The incremental cost and levels of effort needed to install ventilation systems can vary

dramatically with the overall scope of a project. In a gut rehabilitation, for example, ducts and 

fans can often be installed rather easily—often similar to new home applications.

In a much more modest project, running ducts and installing fans can require extra drywall or 

finishing work. Such costs can add up for complex systems, significantly increasing the cost of 

adding ventilation.

What heating, air conditioning, and ventilation systems currently exist in the 

home?

If ventilation systems exist in a home, these may already meet—or can be fairly easily modified 

to meet—ventilation goals and requirements. Some ventilation systems can also make use of 

existing forced-air heating and cooling systems.

What type of heating and air conditioning systems will be in the renovated home?

If home improvements include changes to the heating and cooling systems, keep in mind 

possible synergies between space conditioning and ventilation. Also keep in mind combustion 

safety requirements.
2

Does the home have an attached garage, crawlspace, or basement?

When planning for ventilation systems, be mindful of drawing in air from spaces such as these 

which may contain higher levels of contaminants.

What is the project budget?

Installed costs for new ventilation systems can range from a couple hundred to several thousand 

dollars. As with many building systems, cost will very often be a factor in system selection. 

What are the project goals with respect to ventilation?

Other than codes or program requirements, are there any other ventilation requirements in the 

project? Are there areas of the home that have greater or specialized ventilation needs? Is there a 

desire or need to have outdoor air supplied to each room? Is a system that provides rigorous 

filtration desired? Are there any heat recovery goals?

What levels of maintenance are acceptable?

All ventilation systems require some form of maintenance. Some maintenance is as simple as 

wiping off the grille of an exhaust fan from time to time; other systems require quarterly 

changing or cleaning of filters and heat exchange media. Without proper maintenance, 

performance of systems can be dramatically reduced.

In what climate is the home?

The practicality, comfort, and energy implications of various ventilation systems can vary greatly 

depending on climate.
3

2 Codes and Standards

2.1 ASHRAE Standard 62.2-2010

A growing number of codes and home performance programs are starting to address ventilation 

in a more meaningful way. While requirements can vary considerably, ANSI/ASHRAE Standard 

62.2 is—or is becoming—the most common standard referenced for ventilation requirements in 

homes (ventilation in high-rise apartments is covered in Standard 62.1, ASHRAE 2010a). The 

most recent version of the standard 62.2 was published in 2013 (ASHRAE 2013), but Standard 

62.2-2010 (ASHRAE 2010b) is still referenced by many programs.

ASHRAE 62.2-2010 requires local ventilation in bathrooms (with capacity of at least 50 CFM 

intermittently or 20 CFM continuously) and kitchens (with capacity of at least 100 CFM 

intermittently or 5 ACH of kitchen volume continuously). These local exhaust systems are 

required to remove odors and pollutants—especially water vapor—from these key areas. While 

exhaust fans are common in many existing homes, older fans often perform very poorly—

exhausting a very small fraction of the design flow rates—and consume a large amount of 

electricity.

While local exhaust fans can remove water and other pollutants from these heavy source areas, 

ASHRAE 62.2-2010 also requires whole-building ventilation. Whole-building ventilation “is 

intended to dilute the unavoidable contaminant emissions from people, from materials, and from 

background processes” (ASHRAE 2010b). The required whole-building ventilation capacity 

(expressed in outdoor air flow rate) depends upon home size and number of bedrooms. This is 

summarized in Equation 1.

Qfan = 0.01Afloor+ 7.5(Nbr+1) (1)

where

Qfan = Whole-building ventilation flow rate (continuous) [CFM]

Afloor = Floor area [ft2

]

Nbr = Number of bedrooms

ASHRAE 62.2-2010 assumes a home will have a baseline level of infiltration—2 CFM/100 ft2 of 

floor area. If blower door tests of existing homes show infiltration rates above this level (per 

ASHRAE Standard 136, ASHRAE 1993), then the amount of whole-building ventilation can be 

reduced.

ASHRAE 62.2-2010 has a special provision for ventilation in existing homes (Appendix A). If 

adding or improving local ventilation (kitchen or bathroom) to meet ASHRAE 62.2-2010

requirements is impractical or beyond the project’s budget, the standard can be met by increasing 

the whole-building ventilation rate. To determine the amount by which the whole-building 

ventilation rate must be increased, the local ventilation deficit is summed. In the example shown 

in Figure 1, there is no kitchen ventilation (deficit of 100 CFM) and the existing bath fan delivers
4

only 30 CFM (deficit of 20 CFM). Having operable windows in these spaces can reduce the 

deficit by 20 CFM (per space—not per window). In the example below, the total local ventilation 

deficit is 100 CFM. If a contractor did not improve any local ventilation, the home could still 

comply with ASHRAE 62.2-2010 if the whole-building ventilation system capacity were 

increased by 25% of this deficit—25 CFM in this example.

Figure 1. Simple example of increasing whole-building ventilation rates to offset lack of local 

ventilation in an existing home

This addendum does not obviate the need for whole-building ventilation. Whole-building 

ventilation is always required by ASHRAE 62.2-2010 unless the envelope is extremely leaky. 

For the example home in Figure 1, blower door tests would need to show infiltration above 17–

20 ACH50 to avoid the need for baseline whole-building ventilation. Even then, local ventilation 

would still need to be addressed.

It’s important to reiterate that ASHRAE 62.2-2010 is a minimum standard for ventilation 

capacity. While many whole-building ventilation systems are designed to operate continuously 

or on a timer (e.g., several minutes during each hour), ASHRAE 62.2-2010 mandates system 

capacity only—not how home occupants actually use the system.
5

2.2 ASHRAE 62.2-2013

The primary change in the 2013 version of Standard 62.2 is the elimination of the built-in

“infiltration credit” discussed above. The new formula for calculating whole-building flow rate 

is:

Qtot = 0.03Afloor+ 7.5(Nbr+1) (2)

where:

Qtot = Total required whole-building ventilation rate [CFM]

Afloor = Floor area [ft2]

Nbr = Number of bedrooms

If a blower door test is done on a single-family home, however, it’s likely this ventilation flow 

rate can be reduced.

Qfan = Qtot—Qinf (3)

where Qinf is the “Effective Annual Average Infiltration Rate”. Calculating Qinf is described in 

the standard (ASHRAE 2013), but there are several online tools where these calculations can be 

done, such as:

• Residential Energy Dynamics, 

www.residentialenergydynamics.com/REDCalcFree/Tools/ASHRAE6222013.aspx

• Heyoka Solutions, 

www.heyokasolutions.com/ASHRAE_62_2_2013_WX_Whole_Building_Laminated_Sh

eet_p/ashrae6213wxwbcw.htm

Another critical difference in the 2013 version of ASHRAE 62.2 is in the treatment of 

multifamily buildings. In the examples below, for the two single-family homes, there are modest 

differences in the rates required by the two standards. For multifamily buildings, however, the 

rates required by ASHRAE 62.2-2013 are twice as high. The primary reason: ASHRAE 62.2-

2013 does not allow infiltration credits for multifamily homes.
6

Like the 2010 version, ASHRAE 62.2-2013 makes special provisions for existing buildings. If 

adding local ventilation is beyond the scope of the project, whole-building ventilation rates can 

be increased to compensate.

2.3 Home Ventilating Institute

The performance of fans—especially the delivered flow rates—are typically tested and verified 

per HVI (Home Ventilating Institute) Publication 916 (HVI 2009a). These test procedures—and 

the associated certification procedures in HVI Standard 920 (HVI 2009b)—provide standardized 

ratings for most mechanical ventilation fan products used in homes.

HVI also publishes guidelines on recommended ventilation practices in homes (HVI 2009c). For 

whole-building ventilation, HVI references rates from ASHRAE 62.2-2007. HVI’s 

recommended local ventilation levels are, however, typically higher than the minimum rates

outlined by ASHRAE 62.2. For kitchen ranges located against a wall, for example, HVI 

recommends 40–100 CFM/linear foot of range width (e.g., 100–250 CFM for a standard 30-in.

range). The 100 CFM minimum specified in ASHRAE 62.2 is on the bottom of this range. HVI 

recommends higher exhaust rates for “island” ranges.

For bathrooms smaller than 100 ft2

, HVI recommends exhaust fan capacity of 1 CFM/ft2. For 

larger bathrooms, HVI recommends exhaust capacity of 50 CFM for each shower, bathtub, and 

toilet and 100 CFM for each jetted tub.

2.4 2012 International Residential Code and International Energy Conservation 

Code

The 2012 International Residential Code (IRC) (ICC 2011b) and 2012 International Energy 

Conservation Code (IECC) (ICC 2011a) include both local exhaust and whole-building 

ventilation. Section 303.4 of the IRC stipulates that when air infiltration of a dwelling is less than 

Single-Family Example 1

2,500 ft2, 3 Bedrooms, 2 Stories

Albany, New York

Blower Door Results: 1,500 CFM50

Whole-Building Ventilation Required

ASHRAE 62.2-2010, Qfan = 44 CFM

ASHRAE 62.2-2013, Qfan = 42 CFM

Single-Family Example 2

1,200 ft2, 2 Bedrooms, 1 Story

Houston, Texas

Blower Door Results: 600 CFM50

Whole-Building Ventilation Required

ASHRAE 62.2-2010, Qfan = 34 CFM

ASHRAE 62.2-2013, Qfan = 45 CFM

Multifamily Example 1

1,400 ft2, 2 Bedrooms, 2 Stories

Albany, New York

Blower Door Results: 900 CFM50

Whole-Building Ventilation Required

ASHRAE 62.2-2010, Qfan = 29 CFM

ASHRAE 62.2-2013, Qfan = 65 CFM

Multifamily Example 2

800 ft2, 1 Bedroom, 1 Story

Houston, Texas

Blower Door Results: 600 CFM50

Whole-Building Ventilation Required

ASHRAE 62.2-2010, Qfan = 20 CFM

ASHRAE 62.2-2013, Qfan = 39 CFM
7

5 ACH50, ventilation requirements are very similar to those of ASHRAE Standard 62.2-2010

(Section M1507.3). One key difference in the requirements is that there is no infiltration credit in 

the IRC.

2.5 Programs and Other Standards

In addition to the standards and guidelines discussed above, many certification or incentive 

programs have standards for ventilation. ENERGY STAR® Homes version 3, for example, 

requires compliance with ASHRAE 62.2-2010 (EPA 2012). The Leadership in Energy and 

Environmental Design (LEED) for Homes program requires compliance with 62.2-2007 (which 

is very similar to 62.2-2010, USGBC 2008).

The Building Performance Institute (BPI) has announced it is updating its Building Analyst 

standards to reference ASHRAE 62.2 (BPI 2012a), but currently these standards reference the 

much older ASHRAE 62-1989 (BPI 2012b). If working with specific rating systems, 

weatherization programs, or utility initiatives, be clear about ventilation requirements with 

program managers.

2.6 What Rate is Right?

While ASHRAE 62.2 and similar guidelines have been accepted by many builders, designers, 

and certification programs, it’s important to remember that these standards cannot provide onesize-fits-all solutions. ASHRAE is very clear that Standard 62.2 outlines minimum standards for 

ventilation rate. For indoor environmental quality (IEQ) reasons, there are certainly situations 

where higher ventilation levels can be beneficial. Other building scientists maintain that the 

ventilation rates in ASHRAE 62.2 are too high for many situations (BSC 2013).

It’s certainly true that a simplified standard cannot determine optimal levels of ventilation for all 

situations. Kitchens in which a great deal of cooking takes place, for example, can benefit from 

higher exhaust levels to keep odors and moisture levels down. New homes with high levels of 

chemical off-gassing may benefit from higher whole-building ventilation levels. Conversely, 

homes with very low occupancy and/or contaminant generation could save significant amounts 

of energy by reducing ventilation rates. Very often, program requirements will determine 

ventilation system capacity. Designers and contractors, however, may choose to install systems 

with more versatility—where ventilation rates can be adjusted as needed.
8

3 Implementing Local Ventilation—Bathroom Exhaust

Installing bathroom exhaust fans has been standard practice for decades. Most often fans are 

installed in bathroom ceilings with a small duct (most often 4-in. diameter) that carries air to an 

outdoor termination of some type. While many bathrooms already have exhaust fans, many older 

fans do not perform at the desired or rated ventilation rates.

3.1 Assessment

If a bath exhaust fan already exists, evaluate how well it works. Measure the airflow using an

appropriate flow hood or other device (Figure 2). Discussion of measurement accuracy of these 

instruments is outside the scope of this document, but keep in mind not all flow hoods are equal.

A Lawrence Berkeley National Laboratory (LBNL) study documents accuracy of various 

instruments (Stratton et al. 2012). Inspect the fan grille for dirt and dust; remove the grille and 

inspect the fan blades and housing. Simply cleaning the fan housing and grille can sometimes 

dramatically improve performance. Of course disconnect power before working on any electrical 

equipment.

Figure 2. Measuring exhaust flow with a flow hood and exhaust fan meter

Locate the outdoor terminal of the duct run. Exhaust ducts should always terminate outdoors—

never in an attic or crawlspace, etc. (see Figure 3). If possible, measure the flow rate here at the 

outdoor termination as well. A large disparity in measured flow rates implies leakage. If 

possible, trace the exhaust duct run and inspect for disconnects, crimps, leaks, etc. If the fan 

appears in good working order, flow rates are acceptable, and the duct run terminates outdoors, 

upgrading the fan may not be necessary.
9

Figure 3. Exhaust ducts should always terminate outdoors—not in an attic “near” the outdoors.

If fan replacement is necessary, inspect the ceiling cavity for clearances, joist spacing, and 

mounting configurations. The electrical service to existing fans will often be adequate for newer 

fans, but as always refer to an electrician or qualified contractor to assess electrical issues.

If a bathroom does not have an existing exhaust fan, evaluate the effort required to install a fan 

and ducting. If a bathroom is located on the top floor beneath a vented attic, running electrical 

and ductwork may not be terribly difficult. On lower floors, installing a new duct run may be 

much more problematic. If the bathroom is on an exterior wall, a fan that exhaust directly 

through the wall is a possible solution. If installing a new fan is not practical or affordable, many 

standards (including ASHRAE 62.2) allow for higher whole-building ventilation rates where 

local ventilation cannot be installed.

3.2 Selecting Fans

Once the desired flow rates are known (refer to appropriate codes or guidelines above), it’s 

usually best to select an exhaust fan that meets these flow rates at pressures of 0.25 in. w.g.

Many fans list flow rates at 0.1 in. w.g., but most exhaust duct runs have much higher pressure 

drops.

ENERGY STAR rated exhaust fans are recommended for several reasons. The most obvious 

benefit is lower energy consumption. ENERGY STAR requirements for bathroom exhaust fans 

(with rated flow rates below 90 CFM) call for fans 

to deliver a minimum of 1.4 CFM/Watt. For fans 

with rated flows 90 CFM and above, ENERGY 

STAR fans must deliver at least 2.8 CFM/Watt 

(EPA 2003).

While these energy requirements represent great 

improvements over older fans, many current 

manufacturers have bathroom exhaust fans with 

draws of more than 10 CFM/Watt (seven times 

more efficient than ENERGY STAR minimum 

requirements). These very efficient fans typically 

use brushless, permanent-magnet motors (BPM 

While standards and guidelines vary, 

CARB generally recommends these 

exhaust fan rates for bathrooms:

Powder rooms 50 CFM

(lavatory and water closet)

Full bathrooms 80 CFM

(lavatory, water closet, and tub/shower)

Large bathrooms 110 CFM

(additional tubs, whirlpool, etc.)
10

motors). These motors are also referred to as DC (direct current) motors, ECMs (electronically 

commutated motors), or variable-speed motors. In addition to lower energy consumption, the 

variable-speed capabilities of these motors allow many fans to maintain design flow rates over a 

wide range of static pressures.

In existing buildings, exhaust duct runs are often concealed in walls or ceilings. It’s very difficult 

to determine the state of these runs, and many have twists and turns that result in higher pressure 

conditions. Fans with BPM motors and advanced controls are excellent choices for such retrofit 

applications because fans can increase speed and flow to largely overcome duct restrictions.

Operating at higher fan speed increases power consumption somewhat, but this is usually a small 

price to pay for meeting airflow targets.

One additional benefit of efficient bath exhaust fans is lower noise; ENERGY STAR fans must 

meet noise requirements of 2 sones or less for most products (EPA 2003).

3.3 Fan Installation 

When an older exhaust fan is present, replacing the fan—but keeping the existing duct—can be a 

relatively low-cost project. If possible, the duct run should be inspected for leaks, insulation, 

obstructions, etc. and repaired as necessary. At a very minimum, the outside terminus of the 

exhaust run should be located and inspected. Exhaust fans should vent directly outdoors—not to 

an attic, crawlspace, etc. If an existing fan is terminated inappropriately, a new outdoor 

termination must be installed. Outdoor terminations should be properly air sealed, flashed, and 

include proper screens or backdraft dampers to prevent insects and other pests from entering the 

ducts.

If a new duct must be installed, duct runs should be as short and as straight as possible to an 

appropriate outdoor termination. If an elbow is required to redirect the ductwork, it is 

recommended that 2–3 ft of straight run is provided directly off of the fan housing prior to the 

elbow. Refer to fan literature to determine the proper duct diameter; larger ducts generally result 

in better flow rates and are more forgiving of long or winding duct runs. Joints in the duct system 

should be sealed with mastic, and any ducts located outside of conditioned space (e.g., attic) 

should be insulated to prevent condensation of the humid exhaust air.

Ideally, replacing an existing exhaust fan will require minimal drywall and/or ceiling finishing 

work. However, the variability in size and configuration of these fans makes this hard to predict.

Refer to retrofit installation instructions provided by the manufacturer, and if possible select a 

new fan with a ceiling opening similar to—or slightly larger than—that of the existing fan. It’s 

also a good idea to inspect the ceiling cavity to make sure clearances are acceptable.

Some manufacturers have begun to offer fans specifically designed for the retrofit market (such 

as Broan’s Ultra models). In the right circumstances, electrical connections, duct connections, 

and installation of the fan housing itself can be done with minimal finish work.
11 

Most older and single-speed bath fans deliver 

their rated flow rate at pressures of 0.1 in. 

w.g. In reality, most exhaust duct runs induce 

pressures closer to 0.25 in. w.g. A typical fan 

(non-DC motor) rated at 50 CFM at 0.1 in. 

w.g. will operate closer to 23–31 CFM at 0.25 

in. w.g. Similarly, an 80 CFM fan will deliver 

only 48–57 CFM at 0.25 in. w.g.

DC or variable-speed fans are more forgiving 

of duct problems and will often maintain flow 

over a wide range of pressures. But keeping 

ducts short and straight—unlike in Figure 4—

will result in quieter, more efficient operation.

Figure 4. Tight bends can reduce fan flow rates, 

increase noise, and increase power 

consumption.

3.4 Controls

Bathrooms in general—and showers in particular—introduce substantial amounts of moisture 

into homes. The primary reason for bathroom exhaust ventilation is to remove much of this 

moisture before it causes problems. Turning on an exhaust fan when taking a shower is certainly 

a step in the right direction. Ideally, however, an exhaust fan would operate for a time after a 

bathroom is used. Instead of a conventional switch, a crank timer or a delay-off switch can be 

used to keep a fan running for an additional period of time. 

Many fan manufacturers have started to include more advanced controls into the fans 

themselves. In addition to integrated delay-off timers, some newer fans feature built-in 

occupancy sensors that turn on fans when a person enters the bathroom. Other fans feature 

humidity sensors that keep a fan running whenever humidity levels are above an adjustable 

threshold. While these features can certainly help control moisture levels, they also add some 

cost and maintenance to the exhaust fan. 

3.5 Measurement, Verification, and Commissioning

Commissioning of exhaust fan operation is usually very straightforward. First, verify that the fan 

actually turns on when manually switched on and/or when programmed to turn on based on 

humidity levels, timer, or other controls. Exhaust airflow rate is easily measured with a flow 

hood or flow meter (e.g., see Figure 2). If delivered flow rates do not match design rates, inspect 

the duct system (when possible) to see if there are any obstructions, if backdraft dampers are 

operating properly, etc. It is common to see the exhaust fan backdraft damper restricted due to 

screws being used to connect ductwork to the fan housing collar. Screws should not be used to 

make this connection. Instead, clamps plus mastic or approved foil faced tape should be used. 
12

4 Implementing Local Ventilation: Kitchen Exhaust

While kitchen exhaust ventilation is required by many codes and standards, there are few

requirements beyond meeting prescribed minimum flow rates (AHSRAE 2010, HVI 2012). One 

low-cost solution to meet this requirement is simply installing a bathroom-type exhaust fan with 

flow rates of 100 CFM or higher. Some manufacturers, however, state that their bathroom-type 

fans should not be used near stoves where exposed to grease and other cooking products.

More often, kitchen ventilation is provided by a range hood. Such hoods can often collect and 

remove cooking byproducts better than ceiling or wall-mounted fans (Singer and Delp 2010). In 

addition, range hoods are often equipped with grease filters that protect and extend the life of the 

fan itself. Many range hoods (such as the one in Figure 5) can either be connected to an exhaust 

duct or recirculate the air into the kitchen. Exhausting air to outdoors is strongly recommended; 

recirculating air removes few contaminants and does not comply with most ventilation standards.

Figure 5. Some range hoods, like this one, do not vent outdoors. They simply circulate air back 

into the kitchen. Such hoods do not meet most ventilation requirements.

4.1 Assessment

If a kitchen exhaust fan is installed in a home already, evaluate its performance. Inspect the fan 

grille and housing; in some instances simply cleaning accessible parts of a fan (air inlet, grease 

screens, backdraft damper, outdoor termination etc.) can sometimes dramatically improve 

performance. Trace the exhaust duct run and locate the outdoor termination. Measure the fan’s 

flow rate (if a range hood, this is often most easily done at the outdoor termination).

If a fan is not installed, assess the kitchen for how and where a fan could be installed. Range 

hoods—which remove moisture and odors directly above the cook top—are preferred (Singer 

and Delp 2010). If this is not practical, down-draft fans or even ceiling or wall exhaust fans 
13

located in the kitchen are possible (refer to manufacturer instructions when locating a fan). If 

none of these is practical, many standards (including ASHRAE 62.2) allow for higher wholebuilding ventilation rates where local ventilation cannot be installed.

4.2 Selecting Fans

As with bathroom exhaust fans, ENERGY STAR certified range hoods are recommended.

ENERGY STAR range hoods must deliver a minimum of 2.8 CFM/Watt with a maximum sound 

level of 2.0 sones (EPA 2003). Most ENERGY STAR rated fan hoods have capacities of 100–

200 CFM. For normal residential use, these flow rates are often adequate, though HVI 

recommends higher flow rates (see Section 2.3 above). While higher volume fans certainly do 

exhaust more moisture and odors, they can also cause pressure imbalances within the home.

Large pressure imbalances can cause comfort problems (such as drafts) and interfere with normal 

operation of other heating, ventilation, and air conditioning (HVAC) systems (especially 

combustion systems).

As discussed previously, hoods are much more effective at capturing and removing pollutants 

than wall- or ceiling-mounted exhaust fans. Research has found that capture efficiency varies 

widely from hood to hood (Delp and Singer 2012). Currently there is no standard for assessing 

capture efficiency of hoods, but work on this is underway.

The 2009 and 2012 IRC (ICC 2009, ICC 2012) require makeup air provisions with exhaust fans 

rated above 400 CFM. Such makeup air provisions can be costly and can cause comfort issues.

Unless there is a compelling need for higher rates of exhaust, exceeding HVI guidelines (40–100 

CFM/linear ft of cooking range) is not recommended.

4.3 Fan Installation

In the best scenario, when a range is located on an exterior wall, a range hood will exhaust 

directly through that wall. When this is not practical (because of the location of the outdoor 

terminal, etc.), the duct run should be as short and straight as possible. Refer to the 

manufacturer’s instructions on duct sizing, and seal all duct joints with mastic. If the exhaust 

duct must run in unconditioned space (such as an attic), it should be insulated.

In many instances, running power to a new range hood may be the most challenging part of the 

process. This should be considered during planning, and wiring and electrical connections should 

be done by a qualified electrician.

Outdoor terminations should be properly air sealed, flashed, and include proper screens or 

backdraft dampers to prevent insects and other pests from entering the ducts.
14

5 Whole-Building Ventilation Considerations

5.1 Installation and Integration With Existing Systems

In existing homes, installing whole-building ventilation systems is often more involved and 

costly than in new construction—especially with more complicated ventilation systems. The 

level of effort—and ultimately the cost—required to install a ventilation system in an existing 

home depends on many factors, including:

• Existing ventilation systems

• Existing ducted heating and/or cooling

• Accessibility to attics and/or basements

• Scope of the renovation/rehabilitation project

One of the most common types of whole-building ventilation is simple exhaust ventilation—

where efficient exhaust fans run continuously to remove indoor air. Bathroom exhaust fans are 

operable in many existing homes, but these older fans are usually not appropriate for wholebuilding ventilation (for efficiency, noise, and flow reasons as described below, Section 6). The 

existing duct runs and electrical connections, however, may possibly be reused with the new 

system.

In homes with existing forced-air heating and/or cooling, it’s possible that a central fanintegrated supply ventilation system (CFIS, discussed below in Section 5.7.2) can make use of 

the ducts and the central air handler. 

When there is no existing duct system—or at least no duct system appropriate for the desired 

ventilation system—installing new ducts and running electrical service to fans can be invasive

and costly. These costs can be minimized when there is access from basements, crawlspaces, or 

attics. If ducts or electrical lines must be run between floors or in existing walls, sections of wall 

board must generally be removed and replaced. If other aspects of the renovation do not 

necessitate this level of intrusion, this invasive work can dramatically increase the cost of 

ventilation installation and integration.

5.2 Distribution and Mixing of Outdoor Air

Local, source-control ventilation is properly targeted toward areas where contaminants, 

especially moisture, are frequently and intensely generated. Whole-building ventilation is 

intended to reduce contaminants throughout the entire home by diluting them with outdoor air. 

Certainly directly removing (or diluting) contaminants in every room in a home is ideal, but the 

installed costs of such systems can be quite high—especially in retrofit projects. Some 

ventilation strategies make use of point-source or local ventilation systems to meet wholebuilding ventilation requirements (e.g., a bath exhaust fan running continuously). There is not yet 

broad consensus in the industry about if—or, more accurately, when—point-source, wholebuilding ventilation is an IEQ liability. Researchers are certainly looking into this question, and 

summaries of several studies are described in the Appendix.
15

When considering the question of ventilation and distribution, it’s important to keep in mind that 

the primary purpose of ventilation is to reduce indoor contaminant levels. Ventilation systems 

reduce contaminants by diluting them with outdoor air. The important question to ask about 

ventilation effectiveness, therefore, is not “Does fresh air get to all parts of the home?” but 

rather, “Are contaminants in all parts of the home reduced effectively?” These two questions are 

closely related, but they are not necessary the same.

While research is ongoing, there are some consistent findings from past studies that use tracer 

gas tests to approximate contaminant diffusion (see the Appendix for more information and 

references). Several of these studies compare point ventilation (such as a local exhaust fan) to 

distributed ventilation (such as CFIS or distributed energy recovery ventilator/heat recovery 

ventilator (ERV/HRV) systems).

• Point ventilation provides fairly uniform dilution of indoor air within a home when:

A. Interior doors are open

B. A central furnace or air handler operates intermittently

C. A mechanical “mixing” system operates continuously at low volume.

In these three situations, air change rates in all spaces appear to be similar to air change 

rates when fully ducted/distributed ventilation is installed. The volumes and duty cycles 

required in configurations B and C above require more investigation.

• When doors are closed and there is no central air handler operation, air change rates with 

point ventilation vary much more throughout a home. It is common for some areas 

(especially some bedrooms) to experience air change rates 50% lower than those of areas 

where point ventilation systems are located.

A summary of some of these studies can be found in the Appendix. Unfortunately, CARB cannot 

provide definitive advice on when or how important it is to distribute outdoor air throughout a 

home. Researchers are still investigating this issue. In the meantime, another Building America 

team (Building Science Consortium) has proposed increasing whole-building ventilation rates by 

25% when air is not actively distributed (for new homes only, Bailes 2013).

5.3 Source of Outdoor Air

Outdoor air brought into a home as part of a ventilation system should be—as much as 

possible—free from contaminants. With active air intakes, codes generally require intakes be at 

least 10 ft from sources of contaminants (ICC 2011c). Manufacturer installation instructions may 

also provide clearances from combustion exhausts, operable doors and windows, ground or snow 

level, etc. As many exhaust-only ventilation systems do not have dedicated air intakes, the 

concern about the source of outdoor air may be more pressing with these systems.

In larger buildings, ventilation systems often include active makeup air systems to balance air 

exhausted from buildings. Newer residential codes are also starting to require makeup air in 

some cases. In the 2009 IRC, for example, any local exhaust fans with capacity of 400 CFM or 

higher must include make-up air provisions (ICC 2009).
16

In smaller, existing homes, makeup air provisions are rare. By design or by default, most exhaust 

ventilation systems force infiltration through building envelopes. While this may not be ideal, 

there is not widespread consensus as to whether this is acceptable or unacceptable practice in 

particular circumstances. There are, however, several configurations that most agree are 

problematic:

• Depressurizing the home can interfere with natural-draft combustion appliances.

• In a home with an attached garage, forced infiltration can enter a home through the 

garage. With the air can come traces of car exhaust, fuel, or solvents that may be stored in 

the garage (CARB 2011).

• In a home with a moist, moldy crawlspace or basement, makeup air coming through such 

a space may bring mold spores, humidity, or other undesirable elements.

• In some cases, depressurizing a home can exacerbate introduction of radon or soil gas 

through the foundation.

These can be serious issues, and in some of these situations exhaust ventilation—without

makeup air provisions—should be avoided. In many cases, however, these issues can be 

addressed with common-sense solutions:

• Eliminate natural-draft combustion appliances.

• Meticulously air seal the garage from the living space and/or install ventilation in the 

garage that eliminates the positive pressure with respect to the living space.

• Create a well-sealed, dry, conditioned crawlspace or basement.

• Install a radon mitigation system if soil gas is a problem.

Most homes require exhaust ventilation, if only intermittently, to meet local ventilation 

requirements. The concerns above become somewhat more pressing when exhaust ventilation is 

used for whole-building ventilation and runs continuously.

5.4 Energy Implications

Mechanical ventilation systems have two key energy impacts:

• Electricity used to operate fans and ventilation equipment.

• Thermal energy required to condition outdoor air introduced into the space.

Electrical fan energy varies widely. The power range for most bathroom exhaust fans is 5–40 

Watts. Power consumption for many HRVs and ERVs ranges from 30–200 Watts. The fan in a 

central air handler or furnace—used in some ventilation strategies—can use 200–1,000 Watts. 

These are very general ranges, and power consumption certainly varies with airflow and system 

configurations. Power consumption also, not surprisingly, varies with system cost. Typically 

ventilation products with lower power consumption use variable-speed, BPM fan motors; these 

often carry a cost premium.
17

The second point—conditioning the introduced outdoor air—is climate-dependent. HRVs and 

ERVs can certainly mitigate this effect. The sensible effectiveness of ERV/HRV heat exchangers 

typically ranges from 55%–95%. Again, the higher values typically come with a higher cost. 

More extreme outdoor temperatures mean greater benefits from heat recovery. In humid 

climates, the latent transfer of ERVs can help reduce moisture introduced by a ventilation 

system. See Table 2 in Section 5.7.3 for example energy implications.

5.5 Humidity

As discussed above, bringing outdoor air into a home causes the heating or cooling systems to 

work harder to condition this air. When humid air is brought into a home, however, air 

conditioners often cannot remove all of this additional moisture.

This problem is exacerbated in efficient homes: as sensible cooling loads (i.e., energy needed to 

lower the air temperature) become smaller, air conditioners are smaller and/or need to run less 

frequently. When air conditioners run less frequently, less moisture is removed. In hot, humid 

climates, this can lead to real challenges. When ventilation systems bring more outdoor into 

homes, additional dehumidification systems may be needed to keep indoor humidity at 

comfortable and healthy levels.

Many researchers have examined—and continue to examine—this conundrum: how to provide 

adequate fresh air, keep indoor humidity at acceptable levels, and minimize cost and energy 

needed for dehumidification systems. Several references have much more information and 

recommendations on this topic (BSC 2009; Chandra et al. 2008; Rudd et al. 2005; Rudd 2013;

Shirey et al. 2006).

5.6 Operation and Maintenance

One important—though sometimes overlooked—aspect of a ventilation system is the level of 

maintenance required. All mechanical systems require some maintenance, but maintenance 

required on ventilation systems can vary greatly. On one end of this spectrum, local bathroom 

exhaust fans have very modest requirements. Typical maintenance instructions are—at least once 

per year—to clean/wipe the exhaust fan grille, vacuum dust and dirt from the fan body, and wipe 

the fan body clean (Panasonic 2011).

Some of the higher performing ERVs, by contrast, require checking or cleaning filters, heat 

exchange media, and exterior duct terminations quarterly. While exhaust fan maintenance 

instructions may consist of four or five short bullet points, instructions for checking and cleaning 

ERV components can be several pages. This increased level of maintenance is not a design flaw; 

it is simply necessary with more complex systems with greater functionality.

CARB has worked with several builders and developers who were very sensitive to the level of 

maintenance required in their homes. Ventilation systems are often of special concern, as a 

malfunctioning ventilation system in a tight, efficient home can lead to moisture, mold, and other 

problems. If systems are not maintained, ventilation flow rates will decrease over time—

eventually to a small fraction of the design rates. To minimize these risks, some builders actively 

choose ventilation systems with lower maintenance requirements. Lower maintenance systems 

also tend to have lower first costs; this certainly factors into decision processes.
18

5.7 Overview of System Types

While there are many types of ventilation systems and devices, three main types of systems 

predominate in U.S. homes: exhaust ventilation, CFIS ventilation, and balanced ventilation using 

an ERV or HRV. Table 1 and Section 5.7.1 through Section 5.7.3 outline some of the major 

system characteristics, advantages, disadvantages, and costs. Implementation of each of these 

three types of systems is described in the following sections.

Note that information in this section is general and approximate. The cost ranges presented are 

costs to contractors or installers. Many specifics, especially installation requirements and costs, 

can vary significantly from home to home and from region to region.

Table 1. Summary of Advantages and Disadvantages of Ventilation Systems

Exhaust Only CFIS ERV/HRV

Pros

Low first cost

Simple installation

Low maintenance

Low electrical energy

Low first cost

Often easy to install

Low maintenance

Outdoor air filtered and 

distributed

Heat recovery

Air can be filtered and 

distributed

Potential for low energy 

costs

Balanced (pressures)

Cons

No control of air intake

No distribution of outdoor air

No filtration

High energy use

Higher first cost

Higher maintenance

Installation may be more 

difficult

5.7.1 Exhaust Only

With exhaust only systems, exhaust fans operate continuously (or on timers) to remove air from 

the home. Fresh air is introduced through induced infiltration. Most often these systems make 

use of an exhaust fan that provides local ventilation as well (most often in a bathroom).

5.7.1.1 Pros

• Exhaust-only, whole-building ventilation is one of the most common systems employed 

in homes, especially existing homes. The key reasons for this are its low cost and 

simplicity. Nearly all homes have exhaust fans, and upgrading one (or more) fans to an 

efficient model designed for continuous operation is very straightforward.

• Exhaust fans that use very little electricity are available (5–12 Watts for 50–80 CFM).

• Exhaust fans also require very little maintenance. Typical maintenance instructions are 

to vacuum or wipe the fan grille from time to time. In this regard, exhaust-only 

ventilation can be very reliable.

5.7.1.2 Cons

• Depressurizing the home draws in air from outside the home. The key question is: where 

does this makeup air come from? Are contaminants being introduced? Exhaust-only 

ventilation should not be used in homes with atmospheric combustion appliances, homes 

where makeup air comes from damp, moldy crawlspaces or basements, homes with 
19

attached garages (that are not well air-sealed from the home), or other situations where 

contaminants are likely to be introduced continuously.

• Exhaust-only ventilation is typically not distributed. A single exhaust fan removes air 

from one location, and makeup air enters where it will. With such a system, it is likely 

different parts of the home will be ventilated to different degrees—especially when 

interior doors are closed. When doors are open—or when there is a forced-air system 

operating within the home—differences in air changes throughout the home are minimal 

(see Appendix).

5.7.1.3 Cost

Efficient exhaust fans themselves range from $100–$250—depending on rated flow rates, special

features, etc. If installed as an upgrade (e.g., in a bathroom that already has an old exhaust fan, 

power, ducting, etc.), installation costs may be $100. If installed in a location which did not 

previously have a fan, costs can be much higher. If installed in a ceiling beneath an accessible 

attic, installation can be $200-$400. If drywall or finishes must be removed and repaired, costs 

can be substantially higher.

5.7.1.4 Energy Implications

A 10-Watt fan running year-round consumes 88 kWh. At $0.11/kWh, this costs $10/yr. As this 

system has no heat recovery, outdoor air brought into the building must be conditioned. Costs to 

do this vary with climate and HVAC equipment (see Table 2), but these costs are usually much 

larger than cost of electricity to operate the fan.

5.7.2 Central Fan Integrated Supply Ventilation

CFIS systems make use of an existing forced-air heating or cooling system (see Figure 6). A duct 

is run between the return plenum and outdoors. CFIS controllers are programmed to turn on the 

air handler fan and open the motorized damper. Outdoor air is drawn into the return plenum, 

mixed with return air, and distributed throughout the home.

Figure 6. Simple schematic of CFIS ventilation
20

5.7.2.1 Pros

• Along with exhaust ventilation, CFIS is one of the most simple and affordable systems

to install in existing homes.

• CFIS systems distribute outdoor air to all parts of the home.

• Aside from keeping the air intake free from debris, a CFIS system requires little 

maintenance beyond maintenance of the central heating and cooling system.

• Outdoor air is filtered (through the central air handler filter).

5.7.2.2 Cons

• One key limitation is that this system is viable only in homes with forced-air heating or 

cooling systems.

• The largest drawback of CFIS is the high electricity consumption from using the air 

handler fan for modest ventilation needs. This strategy should be considered only when 

the air handler has an ECM blower.

• If the central duct system is leaky, operating costs can be much higher.

• There is potential for comfort problems if cool mixed air blows on occupants during the 

winter (or warm, humid air blows on them during the summer).

5.7.2.3 Cost

If the air handler is accessible in a basement or attic—and a duct can be fairly easily run from 

outdoors—total CFIS cost may range from $500–$900 (including controls, motorized damper, 

and installation). If installing the outdoor air duct is involved or requires removal of drywall, 

refinishing, etc., costs can increase dramatically.

5.7.2.4 Energy Implications

As mentioned above, CFIS systems can consume tremendous amounts of electricity. If using an

efficient AHU fan motor (300 Watts) and run an average of 8 h/day for ventilation (i.e., in 

addition to operation needed for space conditioning), a CFIS would consume 876 kWh/yr. At 

$0.11/kWh, this costs $96/yr. Most central air handlers have motors that are not this efficient; it 

is not uncommon for draws to be two or three times higher.

Basic CFIS systems have no heat recovery, and outdoor air brought into the building must be 

conditioned. See Table 2 for examples of these cost implications. If CFIS is used in a system 

with considerable duct leakage, these thermal conditioning costs can be much higher.

5.7.3 Heat Recovery Ventilator or Energy Recovery Ventilator

HRVs and ERVs are balanced systems; this means they exhaust air and supply outdoor air 

simultaneously (see Figure 7). These two airstreams cross in a heat exchanger, so during the 

winter, much of the heat in the exhaust stream is transferred to the supply stream (the reverse is 

true in the summer). HRVs transfer sensible heat only; ERVs also transfer moisture (latent heat).
21

Figure 7. Schematic of exhaust air and outdoor air passing through the heat exchanger in an HRV

5.7.3.1 Pros

• Perhaps the most obvious benefit of ERVs and HRVs is the heat recovery that reduces 

conditioning implications of ventilation. The level of heat recovery (efficiency or 

effectiveness) varies considerably, however, among various products.

• Depending on how the systems are designed and installed, ERVs/HRVs can distribute 

air to many areas in a home.

• These systems don’t induce a large pressure (either positive or negative) on the building.

• Outdoor air can be filtered.

5.7.3.2 Cons

• Of the systems discussed here, these are typically the most expensive and can have more 

involved installation procedures.

• These systems typically require higher maintenance. Filters and heat exchange media 

typically need to be cleaned or replaced regularly.

• While heat recovery offers savings, some systems have high electricity consumption.

As with heat recovery effectiveness, however, this varies tremendously from product to 

product.

5.7.3.3 Cost

The range of ERV and HRV equipment costs is quite wide. Costs are generally proportional to 

heat transfer effectiveness and electrical efficiency. Until recently, costs for core hardware 

ranged from approximately $400–$2,000. Not surprisingly, the units with lower electricity

consumption (> 1 CFM/Watt) and higher heat recovery effectiveness tend to be most expensive.

Recently, some higher end European products have become available in the U.S. market. These 
22

systems boast even lower electrical consumption (near 3 CFM/Watt) and higher heat recovery 

effectiveness but with price tags of $3,000–$5,000.

Installation costs vary tremendously; key factors include:

• Location of the core unit. If installed in an accessible space (such as a basement), core 

equipment and duct connections may be fairly simple.

• If ducts can be run in an open space (basement or attic), wall and ceiling finishes may be 

left mostly undisturbed.

• More and longer duct runs translate into higher installation costs.

• Need for a condensate drain and/or pump.

Some contractors have found installation costs of $1,000–$1,500 when the system is entirely 

installed in a basement, attic, or other accessible space (i.e., very little ceiling or wall removal or 

finish work needed). Rules-of-thumb fail with more complex installations requiring ceiling, wall, 

or floor removal, construction of chases, refinishing, etc.

5.7.3.4 Energy Implications

One of the main benefits of ERVs/HRVs is heat recovery. In colder climates, the savings from 

heat recovery are more pronounced. The electrical power consumption of these systems can also 

vary significantly; in milder climates, the electricity costs can actually be greater than the 

thermal energy savings. More information on ERV/HRV efficiency ratings is presented in 

Section 7.2.

Table 2 shows example ventilation-related energy costs of four systems in six different climates.

The values in this table were generated using BEopt v2.0.0.4; they represent costs induced using 

a ventilation system delivering 50 CFM continuously (except for the CFIS system that provides 

the equivalent but runs intermittently). The heating and cooling systems are consistent with the 

Building America Benchmark (Hendron and Engbrecht 2010), specifically 78% AFUE gas 

furnace and 13 SEER air conditioner. The home is one story, has three bedrooms, and 1,200 ft2

of conditioned floor area. There is no duct leakage to the outdoors (especially relevant for CFIS 

system). All costs are annual and relative to a home with no whole-building ventilation. Costs 

are also represented in Figure 8.
23 

Table 2. Example Annual Energy Consumption and Operating Costs for Ventilation Systems in 

Several Climates

Location 

Energy Rates Exhaust, 10 W CFIS, 8 h/day

$/kWh $/therm kWh Therms Cost

($) kWh Therms Cost($)

Chicago 0.11 0.85 182 107 111 985 29 134

San Francisco 0.14 0.97 114 55 69 894 21 144

Atlanta 0.10 1.53 264 47 98 914 21 123

New York 0.18 1.33 187 81 142 1,023 30 277

Phoenix 0.10 1.46 469 11 64 1,105 5 121

Orlando 0.12 1.76 419 6 59 1,252 3 151

Location 

HRV, 55 W, 70% 

ASEF

ERV, 30 W, ASEFa, 

55% TREb

kWh Therms Cost

($) kWh Therms Cost($)

Chicago 534 42 95 270 18 45

San Francisco 498 33 100 272 22 59

Atlanta 648 22 98 357 13 55

New York 562 36 150 296 17 77

Phoenix 733 5 83 387 3 43

Orlando 850 3 103 487 1 59

a Apparent Sensible Effectiveness

b Total Recovery Efficiency

Figure 8. Example ventilation related costs for four systems in six different U.S. climates
24

6 Implementing Whole-Building Ventilation: Exhaust-Only

6.1 Assessment

Refer to Section 5 above to determine if an exhaust-only ventilation system is appropriate for 

your building. While these systems are very simple and low cost, they have some significant 

limitations, and there are situations where exhaust-only strategies are not appropriate.

Before installing fans in a home, evaluate the exhaust fans already in place. Locate the inlets and 

exhausts for each fan and, if possible, trace and inspect the duct runs. Measure the flow rates—

both at the inlets and outlets if practical.

Based on existing conditions and target whole-building ventilation rate, determine the best 

locations for exhaust fans. In the simplest situations, a single fan—using an existing exhaust duct 

run—can meet the entire ventilation rate required. In larger homes or when higher flow rates are 

needed, using multiple fans can be more practical and more effective in removing contaminants 

from an entire home. If a 3,000-ft2 home, for example, requires 100 CFM of whole-building 

ventilation, using two 50-CFM fans may be a better solution than a single 100-CFM fan.

If existing exhaust runs are not sufficient, identify pathways where new exhaust duct runs can be 

installed. These are usually most easily installed on the top floor of a building where ducts can be 

run through an attic to a termination at the soffit, gable, roof, etc. Exhaust ducts should never 

terminate in an attic; exhaust air should always be ducted to the exterior.

6.2 Selecting Equipment

6.2.1 Fans

While some older, existing exhaust fans may be acceptable for local exhaust ventilation, older 

exhaust fans are typically not appropriate for whole-building ventilation for several reasons:

1. Many older exhaust fans simply do not deliver desired airflow rates—or even their rated 

airflow rates. Poor installation may be largely to blame.

2. Newer fans can be much more efficient. While ENERGY STAR fans were recommended 

for local ventilation (Section 3.2), this is much more important when fans are intended to 

operate frequently or continuously. In fact, using fans that greatly exceed ENERGY 

STAR performance standards is strongly recommended. Many newer exhaust fans can 

operate at 6–9 CFM/Watt while the ENERGY STAR minimum is only 1.4 CFM/Watt

(EPA 2003, for exhaust fans rated below 90 CFM). Several manufacturers, for example, 

make 50–CFM exhaust fans that consume 6–8 Watts.

3. ENERGY STAR fans also have noise requirements. If an older (or non-ENERGY 

STAR) fan is intended to operate continuously, the noise generated may make residents 

turn off the fan for long periods of time. This can obviously defeat the purpose of 

continuous ventilation.

4. Finally, older fans likely were not designed to run continuously and have dramatically 

reduced longevity when compared to newer, ENERGY STAR fans.
25

6.2.2 Controls

In the simplest exhaust-only systems, fans are intended to run continuously at their rated speed.

When a home’s ventilation target is 50 CFM (or lower), one 50-CFM fan operating continuously 

can suffice. If the target flow rate is somewhat lower—say 30 CFM for a smaller home—

operating a 50-CFM exhaust fan continuously can be overkill. The additional 20 CFM of forced 

infiltration can be a substantial energy liability, especially in colder climates. Several control 

strategies and devices are available to address this issue. Some examples of these are shown in 

Figure 9 through Figure 12. These are presented as examples only; this is certainly not an 

exhaustive list nor does CARB necessarily recommend these specific devices over others. There 

are many ways to control the fan speed and/or period of operation to achieve the desired rates.

Figure 9. Many fans have adjustable speed 

settings (30–70 CFM in this fan) for continuous 

operation. When the bathroom is in use and 

more flow is desired, the fan can boost to 80 

CFM for a preset time.

Figure 10. Some devices—when wired in series 

with a fan—will reduce the speed and flow rate 

delivered.

(Credit: Tamarack)

Figure 11. Many timer controls can be 

programmed to cycle fans on and off. For an 

equivalent 30 CFM continuously, a 50-CFM fan 

can operate 36 minutes per hour. The fan can 

also be controlled on-demand.

(Credit: AirCycler)

Figure 12. This timer from can be manually 

programmed or users can enter in home 

information and the fan will operate to meet 

62.2-2010 requirements.

(Credit: Honeywell)
26

6.3 Installation

See Section 3.3 for guidance on installation of exhaust fans.

6.4 Measurement, Verification, and Commissioning

After fans are installed, adjust controllers (if necessary) to deliver the desired levels of 

ventilation at the proper times. Measure the exhaust rate flow from the fan. There are several 

ways to measure exhaust flow, but a flow hood or dedicated exhaust measurement device (e.g.,

Figure 2) are two fairly simple and reliable methods. Most variable-speed fans should deliver 

flow rates very close to the nameplate rates. If flows are much lower, it’s possible there is a 

restriction in the duct system. Check the duct runs and outdoor air hoods to remove restrictions if 

possible.

Finally, ensure that the controls operate as intended (e.g., fans boost to high speed, timers turn 

on/off at proper times, etc.)

6.5 Operation and Maintenance

As discussed above, exhaust fans require minimal maintenance. To maintain proper flow rates, 

however, most manufacturers do recommend periodic vacuuming of the exhaust fan grille and 

wiping/cleaning the fan housing with a damp cloth. Refer to operation instructions for details. On 

the outside of the home, the exhaust outlet should be checked periodically for obstructions or 

debris.
27

7 Implementing Whole-Building Ventilation: Central Fan 

Integrated Supply

7.1 Assessment

Clearly, CFIS systems are only appropriate in homes with forced-air heating or cooling systems.

Examine the air handler and return plenum location. Make sure the area is accessible and that 

there is adequate room to install an outdoor air duct.

Locate a good air intake location. The air intake should draw in clean outdoor air without 

restrictions. Outdoor air should not be drawn from an attic, basement, garage, or crawlspace.

Intake should be well above the ground and snow level, away from garages or parking areas, and 

away from any exhaust termination. The 2012 International Mechanical Code (ICC 2011c) 

typically mandates intakes be at least 10 ft from contaminant sources. The intake location should 

also be close to the return plenum.

Figure 13. Poor location selection for CFIS ventilation intake, as the homeowners placed their 

outdoor grill in this corner of the patio

Map the duct run. Make sure a duct can be run from the return plenum to the outdoor air intake 

location within the project’s scope. Duct runs should be as short and as straight as practical to 

provide unrestricted airflow.

Measure airflow and return static pressure. With the air handler in “fan only” mode (i.e.,

heating and cooling turned off but the fan turned on), measure the system airflow and the static 

air pressure in the return plenum.

AHU fan motor. Determine the type of air handler fan motor and/or measure the power 

consumption of the motor. Variable-speed motors (also called ECMs or BPM motors) typically 

use approximately 50% less electricity than older permanent, split-capacitor (PSC) motors.
28

While inefficient motors can provide adequate airflow, the energy consumption is substantial.

Using CFIS with PSC fan motors is not recommended.

Determine the Outdoor Air flow rate and operating schedule. The approximate outdoor air 

flow rate can be calculated from the measured return static pressure and the total equivalent 

length (TEL) of the duct run. Each linear foot of outdoor air intake duct counts as 1 ft of TEL, 

but bends and fittings also increase TEL. Many elbows, for example, add at least 15 ft to TEL; 

an air intake hood may add 30 ft. See ACCA Manual D (Rutkowski 2009) for more details about 

calculating TEL. Flow rate can be estimated using a duct calculator tool; examples are shown in 

Table 3.

Table 3. Approximate Airflow Rates With Varying TELs and Available Static Pressures

Approximate Airflow (CFM)

TEL 

(ft)

Return Static 

Pressure

(Pa)

6-in. 

Galvanized

6-in. 

Flexible

8-in. 

Galvanized

8-in. 

Flexible

25 –20 200 150 435 330

50 –20 140 105 310 220

100 –20 84 75 210 160

200 –20 66 54 145 105

50 –40 200 150 435 330

100 –40 140 105 310 220

200 –40 84 75 210 160

50 –60 250 175 540 380

100 –60 170 125 385 270

200 –60 116 85 250 175

The more outdoor air introduced into the system, the less time the air handler must operate to 

deliver outdoor air. A key limitation, however, is on the temperature of the mixed air (i.e., the 

mixture of return air and outdoor air) supplied to the home. Cool air can cause comfort problems 

during the winter, and many furnace manufacturers require that air passing over the heat 

exchanger be above a minimum temperature (55°–60°F is common; see manufacturer literature).

The mixed air temperature can be calculated by the ratio of air temperatures and flow rates.

Table 4 shows an example of maximum outdoor air flow fractions to keep mixed air at 60°F or 

above (assuming 70°F return air). For example, if total system airflow is 1,000 CFM and outdoor 

air is 10°F, outdoor air flow rate should be no higher than 170 CFM to keep mixed air 

temperature above 60°F. Some guidelines for CFIS systems cold climates (Pettit et al. 2013) 

state that outdoor air should not exceed 15% of the total system flow rate.
29

Table 4. Minimum Outdoor Air Fraction To Provide Mixed Air Temperature of at Least 60°F With 

70°F Return Air Temperature

Outdoor Air 

Temperature

Maximum Outdoor 

Air Fraction for 

Mixed Air > 60°F

40°F 33%

30°F 25%

20°F 20%

10°F 17%

0°F 14%

–10°F 13%

–20°F 11%

7.2 Selecting Equipment

7.2.1 Controller

There are several CFIS controllers on the market (see Figure 14). These controllers are available 

as discrete components, but many are also available as part of a kit that also includes a controller, 

motorized damper, and transformer. While these devices have very similar functionalities, make 

sure the device (or kit) used is compatible with the existing air handler and control system.

Figure 14. At least three manufacturers offer CFIS kits: Aprilaire, AirCycler, and Honeywell.

(Credit: Aprilaire, AirCycler, and Honeywell)

7.2.2 Ducts and Dampers

Duct size is covered in “assessment” above. Duct runs should be as short and straight as is 

practical. Ducts bringing cold outdoor air through conditioned space may cause condensation; to 

reduce this risk, insulate ducts running through conditioned space in colder climates.

Motorized dampers—which open to allow outdoor air in and close when the ventilation quotas 

have been met—are often available as part of a CFIS package or kit (see Figure 14). If a different 

size damper is needed, motorized dampers can certainly be acquired separately. Refer to 

manufacturer literature to ensure that dampers are compatible with the controller.
30

Figure 15. A motorized damper (left) and manual balancing damper (right) in CFIS systems

In addition to the motorized damper, a manual balancing damper is recommended in the outdoor 

air duct. This can be adjusted if necessary to fine-tune the volume of outdoor air introduced into 

the system.

7.2.3 Air Handler Fan Motors

As mentioned above, CFIS ventilation systems are not recommended with an inefficient AHU 

fan motor. For older furnaces or air handlers, there are at least two efficient fan motor 

replacements available (see Figure 16). While electricity consumption of these motors depends a 

great deal on HVAC system characteristics, these motors may consume 50% less electricity to 

deliver the same flow rate (Aldrich and Williamson 2013). If central air handlers do not have an 

efficient motor, and replacing the motor is not possible or practical, CARB recommends a 

different type of ventilation system.

7.3 Installation

A qualified HVAC contractor should install the controller, ducts, dampers, and other 

components. The location of the duct run, connection to the return plenum, and location of 

outdoor air intake should be planned beforehand (see Section 7.1 above).

Ducts should be sealed with approved foil-faced tape or mastic as appropriate (see Aldrich and 

Puttagunta 2011). In cold climates, ducts running through interior spaces should be insulated to 

prevent condensation. Outdoor air intakes should be equipped with screens to keep out insects 

and debris. As mentioned above, intakes should be well above snow level, but it’s preferable that 

they be accessible from the ground for cleaning and maintenance. Outdoor intakes should be 

integrated with siding and flashed properly to prevent water intrusion, and penetrations through 

exterior walls or ceilings should be appropriately sealed to limit infiltration.
31

Figure 16. The Fieldpiece LER (left) and the Evergreen IM (right) are two replacement variablespeed motors designed for residential air handlers.

7.4 Measurement, Verification, and Commissioning

After installation, commissioning of the system is critical. The first step is usually checking to 

ensure that when the controller calls for outdoor air, the air handler turns on and the motorized 

damper opens (see Figure 15). The outdoor air flow rate should then be measured. This can be 

accomplished in several ways, but two common methods are:

• A traverse of the outdoor air duct with a pitot tube or anemometer will provide air 

velocity. Velocity (feet per minute) times the cross-sectional area (square feet) of the duct 

will yield volume flow rate (CFM).

• At the outdoor air intake, a flow hood can measure airflow being drawn into the system 

(this typically requires little or no wind for accurate measurements).

Finally, the balancing damper and the controller should be adjusted so that the desired amount of 

outdoor air is introduced into the system at the desired schedule.

7.5 Operation and Maintenance

As always, refer to manufacturer instructions for maintenance. One of the appealing features of 

CFIS systems is low maintenance, but this does not mean “no” maintenance. One very common 

method of failure is clogging of the air intake; regularly check the air intake to make sure it is 

free from debris (leaves, grass clippings, trash, bird nests, etc.).
32

8 Implementing Whole-Building Ventilation: Heat Recovery 

Ventilators and Energy Recovery Ventilators

8.1 Assessment

Most HRVs and ERVs are housed in rectangular cabinets that contain fans, filters, heat exchange 

media, etc. Four ducts usually connect to this cabinet: outdoor air intake, exhaust to outdoors, 

outdoor air supply to the home, and exhaust from the home.

Determine location for equipment. The ERV/HRV should be located so that duct runs can be 

short and straight, power and control wiring can be connected easily, and the unit can be easily 

accessed for maintenance. Many ERVs/HRVs produce a fair amount of noise; keep this in mind 

if units will be located near sensitive areas (e.g., bedrooms). Basements and attics are common 

locations for ERVs and HRVs.

Determine outdoor air intake and exhaust locations. The outdoor air intake should draw in 

clean outdoor air. Outdoor air should not be drawn from an attic, basement, garage, or 

crawlspace. Intake should be well above the ground and snow level, away from garages or 

parking areas, and far from any exhaust termination. Per the International Mechanical Code, air 

intakes should be at least ten feet from contaminant sources (ICC 2011c), but different 

jurisdictions or programs may have more detailed guidelines.

To help justify the higher costs of ERVs and HRVs, many manufacturers suggest that bathroom 

exhaust fans can be eliminated. Instead of dedicated exhaust fans, the ERV/HRV can be used to 

continuously exhaust air from each bathroom. This often meets code or program requirements

for local ventilation (20 CFM continuously). While many people have used this strategy 

successfully, others have had problems. CARB has heard complaints from occupants where this 

strategy is used: “the bathroom never dries out” or similar.

While ERVs and HRVs all have mechanisms to deal with condensate, putting more moisture 

through the systems can lead to problems. With ERVs, much of this moisture is recaptured and 

retained in the home. Very often, this is not desired. When ERVs or HRVs are used in a home, 

CARB recommends separate bathroom exhaust fans be used to remove moisture quickly and 

directly.

Determine fresh air supply locations. As discussed above (Section 5), one of the advantages of 

an ERV or HRV system is the ability to distribute fresh air to several locations. While providing 

outdoor air to all spaces may seem ideal, it is rarely practical. Especially in an existing home, 

running ducts to all parts of a home is difficult. If an ERV is installed in a basement, for 

example, delivering fresh air to first-floor spaces may be straightforward; reaching second-story 

spaces may be quite challenging. Even a single supply register—located relatively far from 

exhaust registers—can provide better mixing of fresh air than local ventilation systems.

In their product literature, some manufacturers recommend integrating ERVs/HRVs with the 

central duct systems used for heating and cooling. This requires much less work, especially in an 

existing home: short duct runs can bring fresh air to the supply plenum and exhaust can be drawn 
33

from the return plenum (Figure 17). CARB has found that this strategy does not work well. 

When the central air handler fan is not running, the outdoor air follows the path of least 

resistance. In the first arrangement shown in Figure 17, the path of least resistance is for fresh air 

to move backwards through the air handler and be exhausted through the duct in the return 

plenum. In both arrangements; very little fresh air is introduced into the living space.

Figure 17. While schematics such as these are often listed as installation options by ERV and HRV 

manufacturers, these configurations are NOT recommended.

Turning on the air handler when the ERV/HRV operates can solve this short-circuiting problem, 

but this increases electricity consumption dramatically and is not recommended. If occupants use 

the air handler to heat or cool when the ERV/HRV is not operating, the ventilation system in 

these scenarios will introduce large amounts of duct leakage. CARB recommends that at least

one side of the ERV/HRV system (i.e., either the exhaust or return side) be ducted separately. In 

Figure 18, for example, air is exhausted through a dedicated run and tempered outdoor air is 

supplied to the return plenum. This can solve the short-circuiting problem but not the duct 

leakage problem when the ERV/HRV is not operating. A fully ducted system—where ventilation 

supply and exhaust ducts are separate from heating and cooling ducts—may offer the best 

distribution of outdoor air, but this is often impractical in retrofit applications.
34

Figure 18. If integrating an ERV/HRV with the heating or cooling duct system, CARB recommends 

at least one side of the ventilation system (either supply or exhaust) have a dedicated duct run.

Size ducts and map duct runs. To determine duct sizes, refer to manufacturer literature. For 

more detailed sizing calculations refer to ACCA Manual D (Rutkowski 2009), but for simple 

duct systems these calculations may not be necessary. For ERVs or HRVs with multiple or 

variable fan speeds, size duct systems for the higher flow rates. Even if the system is initially

intended to run at lower speed, occupants may choose to operate at higher speeds in the future. 

As always, keep duct runs as short and as straight as practical.

8.2 Selecting Equipment

8.2.1 Energy Recovery Ventilator or Heat Recovery Ventilator?

ERVs transfer moisture (latent heat) as well as temperature (sensible heat). HRVs transfer 

sensible heat only; they do not transfer moisture. Choosing one type of system or another is not 

always straightforward, but the key question to ask is: Are typical indoor moisture levels more 

comfortable than outdoor moisture levels? If so, an ERV may be a more practical choice.

The ERV/HRV question is easiest to answer in hot, humid climates. In Florida, for example, air 

inside comfortable homes is relatively cool and dry. Outdoor air is quite humid. When outdoor 

air passes through an ERV, the air will be somewhat cooled and dehumidified by the exhaust air.

This does not mean an ERV dehumidifies the home. On the contrary, running an ERV will 

increase the amount of humidity in the home, but the humidity increase will be less than if an 

HRV was used.

Outside of hot, humid climates, selecting ERVs or HRVs is not as straightforward. Some 

manufacturers provide recommendations or even maps of the country designating which type of 
35

system is most appropriate, but the recommendations vary from manufacturer to manufacturer. A 

few example situations are below.

• In relatively leaky homes in colder climates, indoor air during the winter is often 

uncomfortably dry—so dry that many people use humidifiers. An ERV can help retain 

moisture within the home, improve comfort, and possibly eliminate the need for 

humidifiers.

• In very airtight homes in cold climates, especially smaller homes with lots of moisture 

generation, indoor air humidity can be uncomfortably high. In this a case, an HRV will 

help reduce indoor humidity.

• In hot, dry climates there is a similar trend: larger or leakier homes with low moisture 

generation may benefit from ERVs (to retain indoor moisture), while smaller, tighter 

homes with higher occupancy may benefit from HRVs (to reduce indoor humidity 

levels).

There is not a hard-and-fast rule about which type of system is more appropriate. Especially in 

mixed climates, there are many situations where an ERV may be more beneficial during cooling 

season and an HRV more appropriate during heating season. Installers should use their judgment 

when selecting systems.

Figure 19. An HRV installed in a cold-climate home

8.2.2 Efficiency

See the previous section for information on when an ERV or an HRV may be more appropriate.

HVI publishes standards for testing and rating efficiencies of ERVs and HRVs. Apparent 

Sensible Effectiveness (ASEF) is the most commonly published rating for heat recovery during 

cold weather. Look for ASEF values above 80%; the most efficient systems have values above 

90%. Total Recovery Efficiency (TRE) is listed for ERVs, and it accounts for sensible and 

latent heat recovery during cooling season. For best performance, look for TRE values above 

50%. A full listing of certified equipment is available on HVI’s website (HVI 2013).
36

8.2.3 Power Consumption

Power consumption can vary widely, from below 30 Watts to 200 Watts. Systems with low 

power consumption often have variable-speed or ECM motors that often allow for better control 

of ventilation rates. While systems with low power consumption (> 1 CFM/Watt) are often more 

expensive, they are strongly recommended. In addition to the lower electricity consumption and 

better controllability, they also usually generate much less noise.

8.2.4 Controls and Features

Most ERVs and HRVs are available with programmable timers of various types. These are 

usually quite versatile and can be set to various schedules. In ERVs or HRVs with variable-speed 

fans (ECMs or BPM motors), the flow rate can often be adjusted by changing the fans’ speed.

Such fans can often be manually boosted to high speed when more air is desired.

The basic timer or variable speed controls are usually all that is required to meet code or program 

ventilation rates, but some systems offer advanced features (often at a premium). Some examples 

are:

• Controls with carbon dioxide and/or humidity sensors can boost speed at higher 

concentrations (which usually indicate higher occupancy).

• Some systems can boost speed and/or bypass the heat exchanger to take advantage of 

nighttime cooling.

• Higher levels of filtration are sometimes available (including HEPA) to remove airborne 

allergens and other pollutants.

8.2.5 Cold Weather Performance

With cold outdoor temperatures, moisture in the exhaust air will often condense in the heat 

exchanger. Under more extreme conditions, this moisture can freeze. Most ERVs and HRVs 

have condensate drains and frost-protection mechanisms. However, some of these defrost 

devices or controls must be ordered as add-on features. When selecting a system, make sure you 

have the necessary features to protect against frost in your climate.

8.3 Installation

Installation should be done by a qualified professional. Follow manufacturer instructions for 

proper installation. Be sure to install the ERV/HRV unit where it can be easily accessed for 

maintenance, where power can be safely run to the unit, and where the system’s noise will not 

disturb occupants. Install condensate drains as required by manufacturers.

All duct runs should be as short and straight as is practical. Be sure outdoor penetrations are 

properly air sealed and flashed and have the appropriate screens for pest protection. Ideally, these 

penetrations will be easily accessed from the ground so occupants can make sure they stay free 

of debris.

Duct should be sealed (see Aldrich and Puttagunta 2011), and the ducts running to and from the 

outdoors should be insulated. If the E/HRV is installed in unconditioned space (e.g., in a vented 

attic) the ducts carrying exhaust air and tempered outdoor air should also be insulated.
37 

Figure 20. Local ERV (Credit: Panasonic)

Most of the HRVs and ERVs discussed 

here are ducted systems—i.e., two ducts 

run from the unit to the indoors, and two 

ducts run from the unit to outdoors. Cost 

for installing these duct systems is often a 

large part of total system cost. To help 

address this challenge, Panasonic has 

introduced a local ERV called the 

WhisperComfort. 

This unit is installed in a ceiling—similar 

to a bathroom exhaust fan. The unit 

requires two outdoor air ducts (one intake 

and one exhaust), but the unit exhausts 

directly from and supplies directly to the 

room below. 

Tests have showed this device to be quite 

effective (CARB 2011). Compared to 

conventional ERVs, this system can 

dramatically simplify installation and 

reduce costs. There are of course 

limitations: heat recovery is only 

moderately efficient, outdoor air is not 

distributed throughout the home, and it is 

not recommended for cold climates. Still, 

CARB believes it may be a practical 

choice for many rehab projects.

Figure 21. Local ERV (cover removed) installed in a 

home.

8.4 Measurement, Verification, and Commissioning

As with all ventilation systems, it’s important to measure the delivered airflows and make sure 

they meet specifications or requirements. Flow rates can be measured in several ways. At 

exhaust registers, a flow hood or exhaust meter (e.g., Figure 2) can measure flow rates. Flow 

hoods are also useful to measure flow rates at supply registers. In addition to providing a check 

of total system flow, measuring flow rates at each register is the only practical way to check 

system balancing. 

If indoor supply or exhaust registers cannot all be accessed (e.g., if air is supplied to the return 

plenum of the central heating and cooling system), flow hoods can be used at the terminations 
38

outdoors. This is usually practical only if the terminations are accessible from the ground and if 

outdoor conditions are calm (i.e., no wind).

As described for CFIS systems, a duct traverse with a pitot tube or anemometer can provide 

velocity and flow rate. Some ERV and HRV units are, in fact, equipped with pressure taps so 

that flows can be measured by connecting a manometer directly to the ERV/HRV itself.

If flow measurements do not match specifications or design targets, adjust the controls if 

appropriate. If the desired flow rates are hard to achieve, inspect the duct system, registers, and 

exterior terminations for obstructions. If controls have advanced features (e.g., timers, flow 

boosts, etc.) ensure that these functions are working properly.

8.5 Operation and Maintenance

As always, refer to manufacturer instructions for proper maintenance. Maintenance requirements 

of HRVs and ERVs are often more rigorous than for other ventilation systems. Many systems 

require periodic cleaning and/or replacement of heat exchange media, filters, and other parts 

prone to collect dust and dirt. In addition, outdoor air inlets should be checked periodically to 

make sure they are free from debris (leaves, grass clippings, trash, etc.).

Some devices are equipped with indicator lights that alert occupants when it’s time for 

maintenance. If this is not present, residents should be meticulous in maintaining a schedule.

Without maintenance, flow rates can drop dramatically; CARB has seen airflow reduced by 50% 

over twelve months in a system that was not maintained (outdoor intakes blocked with 

grass/trash, filters and media never cleaned). If possible, checking airflow rates over time (as 

recommended during commissioning) is an excellent practice to ensure the desired airflow is 

being consistently delivered.
39

References

Aldrich, R.; Puttagunta, S. (2011). Measure Guideline: Sealing and Insulating Ducts in Existing 

Homes. Norwalk, CT: Consortium for Advanced Residential Buildings. 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/meas_guide_seal_duc

ts.pdf. Accessed July 24, 2013.

Aldrich, R.; Williamson, J. (2013). Evaluation of Retrofit Variable-Speed Furnace Fan Motors. 

Golden, CO: National Renewable Energy Laboratory. (in publication)

ASHRAE. (1993). ANSI/ASHRAE Standard 136-1993, A Method of Determining Air Change 

Rates in Detached Dwellings. Atlanta: American Society of Heating, Refrigerating and Air 

Conditioning Engineers, Inc.

ASHRAE (2010a). ANSI/ASHRAE Standard 62.1-2010, Ventilation for Acceptable Indoor Air 

Quality. Atlanta: American Society of Heating, Refrigerating and Air Conditioning Engineers, 

Inc.

ASHRAE. (2010b). ANSI/ASHRAE Standard 62.2-2010, Ventilation and Acceptable Indoor Air 

Quality in Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigerating 

and Air Conditioning Engineers, Inc.

ASHRAE. (2013). ANSI/ASHRAE Standard 62.2-2013, Ventilation and Acceptable Indoor Air 

Quality in Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigerating 

and Air Conditioning Engineers, Inc.

Bailes, A. (2013). “Resistance May NOT Be Futile in the Residential Ventilation Wars.” Green 

Building Advisor. www.greenbuildingadvisor.com/blogs/dept/building-science/resistance-maynot-be-futile-residential-ventilation-wars. Accessed July 29, 2013.

BPI. (2012a). “BPI to Reference ASHRAE Standard 62.2-2010.” Malta, NY: Building 

Performance Institute. www.bpi.org/news_expansion.aspx?selectedID=964. Accessed June 15, 

2012.

BPI. (2012b). Technical Standards for the Building Performance Professional. v1.4.12. Malta, 

NY: Building Performance Institute. www.bpi.org/Web%20Download/BPI%20Standards/ 

Building_Analyst_Professional_1_4_12.pdf. Accessed June 15, 2012.

[BSC] Building Science Corporation, Florida Solar Energy Center/BA-PIRC, and IBACOS. 

(2013). Building America White Paper: Impact of Residential Mechanical Ventilation on Energy 

Cost and Humidity Control. Golden, CO: National Renewable Energy Laboratory. NREL-RB84.

[BSC] Building Science Corporation. (2009). “Supplemental Humidity Control for Hot-Humid 

Climates.” BSC Information Sheet 620. Westford, MA: Building Science Corporation. 
40

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/supplemental_humidi

ty.pdf. Accessed December 6, 2013

CARB (2007). “Completion of Performance Evaluations of Research Houses Targeting at 

Minimum 40% Whole House Energy Savings.” Deliverable for Task 13.B.2, NREL Task Order 

KAAX-3-33411-13. Norwalk, CT: Consortium for Advanced Residential Buildings.

CARB (2010). “Point-Source Heating Systems in Cold-Climate Homes: Wisdom Way Solar 

Village.” 2010 Annual Progress Report, Budget Period 3, Attachment FF. Submitted to NETL, 

December 2010. Norwalk, CT: Consortium for Advanced Residential Buildings. http://carbswa.com/articles/advanced%20systems%20research/room_heaters_adv_system.pdf. Accessed 

June 28, 2012.

CARB (2011). “Evaluation of the Performance of a Spot Energy Recovery Ventilator in a 

Retrofit Application.” Deliverable for Task 7.1, NREL Task Order No. KNDJ-0-40342-01. 

Norwalk, CT: Consortium for Advanced Residential Buildings.

Chandra, S.; Parker, D.; Sherwin, J.; Colon, C.; Fonorow, K.; Stroer, D.; Martin, E., McIlvaine, 

J.; Chasar, D.; Moyer, N.; Thomas-Rees, S.; Hoak, D. Beal, D.; Gil, C. (2008). An Overview of 

Building America Industrialized Housing Partnership (BAIHP) Activities in Hot-Humid 

Climates. FSEC-PF-439-09. Cocoa, FL: Florida Solar Energy Center. 

www.fsec.ucf.edu/en/publications/pdf/FSEC-PF-439-09.pdf. Accessed December 6, 2013.

Delp, W.W.; Singer, B.C. (2012). Performance Assessment of U.S. Residential Cooking Exhaust 

Hoods. LBNL-5545E. Berkeley, CA: Lawrence Berkeley National Laboratory. 

http://homes.lbl.gov/content/performance-assessment-us-residential-cooking-exhaust-hoods. 

Accessed December 1, 2013.

[EPA] Environmental Protection Agency. (2003). “Ventilating Fans Key Product Criteria.” 

ENERGY STAR, Washington, DC: Environmental Protection Agency. 

www.energystar.gov/index.cfm?c=vent_fans.pr_crit_vent_fans. Accessed December 31, 2012.

[EPA] Environmental Protection Agency. (2012). “ENERGY STAR Qualified Homes, Version 3 

(Rev. 06) Inspection Checklists for National Program Requirements.” Washington, DC: 

Environmental Protection Agency. 

www.energystar.gov/ia/partners/bldrs_lenders_raters/Bundled_Checklists_v68_2011-09-

01_clean_fillable_508.pdf?5044-c6d6. Accessed May 28, 2013.

Hendron, R.; C. Engbrecht. (2010). Building America House Simulation Protocols. Golden, CO: 

National Renewable Energy Laboratory. DOE/GO-102010-2141. 

www.nrel.gov/docs/fy11osti/49246.pdf. Accessed July 23, 2013.

Hendron, R.; A. Rudd; R. Anderson; D. Barley; E. Hancock; A. Townsend. (2006). Research 

Report—0602: “Field Test of Room-to-Room Uniformity of Ventilation Air Distribution in Two 

New Houses.” Westford, MA: Building Science Corp.
41

HVI (2009a). HVI Airflow Test Procedure. HVI Publication 916. Wauconda, IL: Home 

Ventilating Institute. www.hvi.org/ratings/HVI916_1March2009.pdf. Accessed July 22, 2013.

HVI (2009b). HVI Product Performance Certification Procedure Including Verification and 

Challenge. HVI Publication 920. Wauconda, IL: Home Ventilating Institute. 

www.hvi.org/ratings/HVI920_1March2009.pdf. Accessed July 22, 2013.

HVI (2009c). Home Ventilating Institute (HVI) Indoor Air Quality (IAQ) Position Paper.. HVI 

Wauconda, IL: Home Ventilating Institute. 

http://hvi.org/publications/pdfs/HVI_IAQPositionPaper_01Oct09.pdf. Accessed July 22, 2013.

HVI. (2013). HVI Tested/Certified Home Ventilating Fans. www.hvi.org/proddirectory/. 

Accessed July 29, 2013. 

ICC (2011a). 2012 International Energy Conservation Code. Washington, DC: International 

Code Council.

ICC (2011b). 2012 International Residential Code. Washington, DC: International Code 

Council.

ICC (2011c). 2012 International Mechanical Code. Washington, DC: International Code 

Council.

Panasonic. (2011). “Ventilating Fan Instructions.” Secaucus, NJ: Panasonic Corp. 

http://panasonic.com/business/building-products/ventilation-systems/pdf/WhisperGreen3En.pdf. 

July 23, 2013.

Pettit, B.; K. Neuhauser; C. Gates. (2013). MassSave Deep Energy Retrofit Builder Guide. 

Somerville, MA: Building Science Corporation. https://www1.nationalgridus.com/Files/ 

AddedPDF/POA/DER_Guide_final_lores.pdf. Accessed July 23, 2013.

Rudd, A. (1999). “Measured Air Change Rates and Distribution Ventilation Air in a Single 

Family Home.” Research Report—9907. Westford, MA: Building Science Corp.

Rudd. A. (2013) “Expert Meeting: Recommended Approaches to Humidity Control in High 

Performance Homes.” DOE/GO-102013-3856. US Department of Energy. 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/expt_mtg_humidity_c

ontrol.pdf%20.

Rudd, A.; D. Bergey. (2013). Ventilation System Effectiveness and Tested Indoor Air Quality 

Impacts. Golden, CO: National Renewable Energy Laboratory. (in publication)

Rudd, A.F.; Lstiburek, J.W.; Ueno, K. (2005). Residential Dehumidification Systems Research 

for Hot-Humid Climates. NREL/SR-550-36643. Golden, CO: National Renewable Energy 

Laboratory. 
42

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/36643.pdf. Accessed 

December 6, 2013.

Rudd, A.; Lstiburek, J. (2000). “Measurement of Ventilation and Interzonal Distribution in 

Single-Family Homes.” ASHRAE Transactions, 106 (2).

Rutkowski, H. (2009). Manual D Residential Duct Systems (Third Edition). Arlington, VA: Air 

Conditioning Contractors of America.

Shirey, D.; Hugh, H.; Raustad, R. (2006). Understanding the Dehumidification Performance of 

Air-Conditioning Equipment at Part-Load Conditions. FSEC-CR-1537-05. Cocoa, FL: Florida 

Solar Energy Center. www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1537-05.pdf. Accessed 

December 6, 2013.

Singer, B.; Delp, W. (2010). Experimental Evaluation of Installed Cooking Exhaust Fan 

Performance. LBNL-4183E. Berkeley, CA: Lawrence Berkeley National Laboratory.

Sherman, M.H.; Walker, I.S. (2010). Impacts of Mixing on Acceptable Indoor Air Quality in 

Homes. LBNL-3048E. Berkeley, CA: Lawrence Berkeley National Laboratory. 

http://eetd.lbl.gov/node/51162. Accessed December 6, 2013.

Stratton, J. C.; Walker, I.S.; Wray, C.P. (2012). Measuring Residential Ventilation System 

Airflows: Part 2 - Field Evaluation of Airflow Meter Devices and System Flow Verification. 

LBNL-5982E. Berkeley, CA: Lawrence Berkeley National Laboratory. 

http://homes.lbl.gov/content/measuring-residential-ventilation-system-airflows-part-2-fieldevaluation-airflow-meter-devi. Accessed December 1, 2013.

[USGBC] United States Green Building Council. (2008). LEED for Homes Reference Guide. 

Washington, DC: U.S. Green Building Council.
43

Bibliography

Barley, C.D., R. Anderson, R. Hendron, E. Hancock. (2007). A Test Protocol for Room-to-Room 

Distribution of Outside Air by Residential Ventilation Systems. National Renewable Energy 

Laboratory, NREL/TP-550-31548. www.nrel.gov/buildings/pdfs/31548.pdf. Accessed May 31, 

2011.

[DOE] U.S. Department of Energy, Energy Efficiency & Renewable Energy. (2010). Guide to 

Home Ventilation. Washington, DC: US DOE. 

http://energy.gov/sites/prod/files/guide_to_home_ventilation.pdf. Accessed July 29, 2013.

Holladay, M. (2009). “Designing a Good Ventilation System.” Musings of an Energy Nerd. 

Green Building Advisor. www.greenbuildingadvisor.com/blogs/dept/musings/designing-goodventilation-system. Accessed June 25, 2012. 

National Center for Healthy Housing. (2009). Improving Ventilation in Multi-family Buildings 

that Do Not have Fan-Powered Ventilation. Columbia, MD: National Center for Healthy 

Housing. www.nchh.org/Portals/0/Contents/Factsheet_Ventilation%201.pdf. Accessed July 31, 

2013.

NPCC (2011). Residential Weatherization Specifications. Portland, OR: Northwest Power and 

Conservation Council, Regional Technical Forum. 

www.nwcouncil.org/energy/rtf/subcommittees/res/Approved%20WX%20Specs%208%2030%2

02011.pdf. Accessed June 25, 2012.

Reardon, J.T.; Shaw, C.Y. (1997). “Evaluation of Five Simple Ventilation Strategies Suitable for 

Houses without Forced-Air Heating.” ASHRAE Transactions, 103, (1), pp. 731-744.

Rudd, A.; Lstiburek, J.; Townsend, A. (2009). “A Method for Modifying Ventilation Airflow 

Rates to Achieve Equivalent Occupant Exposure.” ASHRAE Transactions, v. 115, 2. 

Russell, M.; Sherman, M.; Rudd, A. (2005). “Review of Residential Ventilation Technologies.” 

LBNL 57730. Berkeley, CA: Lawrence Berkeley National Laboratory.

Sakaguchi, J.; Akabayashi, S. (1998). “Experimental Study on Air Exchange Efficiency in 

Houses with Central Ventilation Systems.” Proceedings, Roomvent ‘98, 6th International 

Conference on Air Distribution in Rooms, Stockholm, Sweden, June 14-17, 1998.

Sherman, M.; Walker, I. (2007). Air Distribution Effectiveness for Different Mechanical 

Ventilation Systems. Lawrence Berkeley National Laboratory, LBNL-62700. 

http://eetd.lbl.gov/ie/pdf/LBNL-62700.pdf. Accessed June 14, 2011.

Minnesota Sustainable Housing Initiative. (2013). Heat and Energy Recovery Ventilators (HRVs 

& ERVs). Minneapolis, MN: Minnesota Sustainable Housing Initiative. 

www.mnshi.umn.edu/kb/scale/hrverv.html. Accessed July 31, 2013.
44

Appendix: Summary of Ventilation and Distribution Studies

Below are summaries of several research efforts to evaluate the impact of distributing outdoor air 

or mixing air within a home. The list is certainly not exhaustive.

• Rudd (1999) documents tests of two supply-only ventilation systems in new Las Vegas, 

Nevada homes. The tests find that when the ventilation systems are integrated with the 

central air handler (such as with a CFIS system), all areas in the home experience similar 

air change levels. Local supply ventilation did not provide even levels of air changes 

throughout the home; this was especially pronounced when interior doors were closed. 

When the air handler was operated in conjunction with local ventilation, disparities in air 

change levels were substantially reduced.

• Rudd and Lstiburek (2000) includes the results from above as well as results from testing 

ventilation systems in a large new home in Minneapolis, Minnesota. During tests of local 

exhaust ventilation used as whole-building ventilation, some areas of the home 

experienced air change rates (reciprocal age of air, RAoA) of nearly 50% less than the 

average air change rate in the entire home. When the central air handler was operated 

periodically in conjunction with the exhaust fan, RAoA values were much more uniform. 

In tests of CFIS ventilation, local RAoA values differed from the home average by less 

than 10%.

• Hendron et al. (2006) documents tests in two new homes in Sacramento, California. The 

tests compared exhaust-only ventilation (using a local exhaust fan) and supply ventilation 

using the central air handler and duct system. Ventilation systems were set to meet 

airflow required by ASHRAE 62.2-2004. Multi-point tracer gas tests consistently showed 

that central fan-integrated supply ventilation provided the lowest variability in age of air 

from room to room (RAoA differences of 0.02 h-1 or less). Tracer gas tests for exhaustonly systems provided the following insights:

o With all doors closed, RAoA values could be quite disparate. In the most extreme 

case, RAoA values differed by a factor of 2.5 between two spaces in the home.

o When doors are open, age-of-air uniformity is much greater. During this test, the 

highest measured RAoA was 15% higher than the lowest RAoA.

o When the central air handler was operated on a 33% duty cycle (without 

introducing additional outside air), RAoA values were quite uniform—very nearly 

as uniform as the central AHU-integrated supply ventilation.

• CARB (2007) describes multipoint tracer gas test results in a Massachusetts home with 

exhaust-only ventilation using bathroom fans. Results were similar to those described 

above: when interior doors were open, RAoA values were very similar; when doors were 

closed, secondary bedrooms had RAoA values nearly double those of central spaces

(Figure 20). Tests also included preliminary tests of trickle-vent effectiveness.
45

Figure 22. Summary of RAoA tests from (CARB 2010). In both test cases, all interior doors are 

closed with exhaust ventilation operating continuously. In Case 1, the mixing fan is running.

• The Massachusetts home described in another study (CARB 2010) also had an exhaustonly ventilation system. This home, however, had a separate air “mixing” system. The 

mixing fan is simply an efficient exhaust fan installed in the ceiling of a central space. 

This fan moves approximately 20–30 CFM continuously from the central space to each 

bedroom. Without this fan running, bedrooms with closed doors experienced RAoA 

values nearly 50% lower than in central spaces. With the mixing fan running, however, 

bedroom RAoA values were only 10%–15% lower than in central spaces.

• An LBNL paper (Sherman and Walker 2010) reviews several past studies to gauge the 

IAQ impacts of mixing indoor air. The study concludes that overall, additional mixing of 

indoor air will not substantially affect IAQ. In some cases added mixing can reduce 

occurrences of extreme contaminant exposure. To minimize average exposure to 

contaminants, however, added mixing is not recommended.

• Testing of a Las Vegas home is described in (CARB 2011). Two different whole-building 

ventilation systems were installed in this home: an exhaust-only system and a local, 

ceiling-mounted ERV. With the local ERV, a key concern was to evaluate “shortcircuiting” potential as the supply and exhaust ports are located in the same ceilingmounted device. Tracer gas tests showed that this was not a problem in this home.

Tracer gas tests showed that neither local ventilation system provided equal air change 

rates throughout the home unless interior doors were open or the central air handler fan 

was operating. In the test with the exhaust-only system running with all doors closed, the 

utility room showed RAoA values nearly twice as high as in other spaces; this is 

concerning as the utility room borders the garage.
46

• In two new homes in Texas, BSC researchers tested three ventilation systems and found 

the exhaust only system resulted in more disparate air change rates and higher 

concentration of volatile organic compounds when compared to the CFIS and ERV 

systems (Rudd and Bergey 2013).
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