Ventilation installation practices are usually based on ASHRAE 62.21 in the U.S., the only question being which version of this standard is used by the relevant program. ASHRAE 62.2 is a minimum standard, not a best-practice standard. As a fifteen-year member of the ASHRAE 62.2 committee, I am often asked about the best ways of exceeding the 62.2 standard for good indoor air quality. This article is an overview of best-practice ventilation design, installation, and maintenance for single-family detached residential buildings.
Natural Ventilation
Natural ventilation is outdoor air entering a dwelling unit through intentional openings driven by the forces of wind and thermal buoyancy (also called stack effect). For a number of reasons, including the advent of climate change and the importance of sustainability, it is important to design our new homes to take advantage of natural ventilation.
For centuries, natural ventilation was the only option for enhancing indoor air quality (IAQ) and for providing indoor cooling during warm weather. More recently, mechanical ventilation with its precise airflow and set-it-and-forget-it controls, has rendered natural ventilation second-rate. In order to manage IAQ, save energy, control our thermal comfort, and establish a closer connection to the outdoors, we should utilize the advantages of natural ventilation in our dwellings.
Best practices for natural ventilation:
- Site the house so that the long façade is perpendicular to the summer winds, but don’t forget to consider solar gain, especially if solar panels will be installed on the roof.
- Include wind-capturing facades and landscaping to funnel wind to inlets. It is important to determine prevailing winds during the summer, spring, and fall as part of this planning.
- Allow accommodation for night-purge ventilation. This is the process of utilizing the cooler night air for natural cooling and then closing the windows during the day.
- Plan for cross ventilation with an inlet and outlet in each occupiable room.
- Ensure that operable inlet and outlet openings are easily accessible to occupants.
Mechanical Ventilation
It is always wise to use properly certified and energy-efficient equipment for mechanical ventilation. Make sure the fans you use are certified by HVI2 and by Energy Star3. If they are, you can feel confident that the airflow and sound will meet or exceed the ASHRAE 62.2 standard.
Fan efficacy, or cfm/watt, is a value used in many energy codes and standards. For example, the IECC-2021 and Energy Star require a minimum efficacy of 2.8 cfm/watt for exhaust fans rated at less than a 90 CFM. The Standard Work Specifications (SWS)4 require a minimum efficacy of 4 cfm/watt (6.0201.1a). Many high-quality exhaust fans available today have efficacies greater than 15 cfm/watt. This high efficacy results from the direct-current (DC), electronically commutated motor (ECM), which also allows multiple speed settings and automatic compensation for increased static pressure5. The ASHRAE 62.2 standard does not specify an efficacy requirement.
Another advantage of DC-ECM fans is that they are quiet. Any fan should comply with the ASHRAE 62.2 sound requirements of one sone or less for a continuously operating fan and three sones or less for an intermittently operating fan, such as a bathroom ceiling fan with an occupancy-sensing control6.
The ASHRAE 62.2 standard specifies local ventilation and dwelling-unit ventilation. Local exhaust ventilation is intended to extract contaminants at their source; requirements for bathrooms and kitchens are prescriptive, with 50 CFM demand-controlled fans for bathrooms and 100 CFM range hoods for kitchens. On the other hand, dwelling-unit ventilation is sized for the dwelling, based on the floor area and the number of occupants. Dwelling-unit ventilation is intended to dilute indoor air with fresh outdoor air.
Best practices for local bathroom ventilation:
- 80 CFM demand-controlled fan in ceiling or wall. This higher airflow rate clears the moisture from a shower faster than a 50 CFM fan.
- As a way to keep installation costs down, some weatherization programs use double-duty fans in bathrooms to provide local and dwelling-unit ventilation. Better IAQ results from using a separate demand-controlled fan in each bathroom and a separate dwelling-unit ventilation fan located centrally.
- The pressure difference across the closed bathroom door with the exhaust fan operating should be less than three pascals. If it is more than this, consider relieving the pressure by undercutting the door, installing a grille in the door, or installing a jumper duct between the bathroom and an adjoining room. A ¾ to 1-inch gap beneath the bathroom door usually suffices for providing enough makeup air.
- When installing 62.2-compliant ventilation in an existing dwelling, the standard allows the use of the alternative compliance path (Appendix A of the 62.2 standard). Although this alternative path provides acceptable IAQ and lowers installation costs, avoiding the use of the alternative path is likely to result in better IAQ.
- Make sure the integral backdraft damper is working properly after the fan is installed.
- If the fan is installed within an unconditioned space, insulate the fan housing with at least R-8 insulation (SWS 6.0201.1j).
- Encourage occupants to use this ventilation.
Best practices for local kitchen ventilation:
- Although the ASHRAE 62.2 standard does not require over-the-range, demand-controlled exhaust fans, this is the best choice for effectively ridding the dwelling of the hazardous contaminants of cooking. Although the “capture efficiency” of range hoods is not yet published for range hood exhaust fans, it will be soon.7 The greater the capture efficiency, the better. Capture efficiency is increased by:
- An increase in range hood airflow rate.
- Decreasing the distance between the range top and the exhaust fan.
- An increase in the range top temperature.
- Avoid the use of “down-draft” ventilation, which is usually integrated into the range top. The ASHRAE 62.2 standard requires the airflow rate of this type of fan to be at least 300 CFM. Additionally, its capture efficiency is extremely low.
- Provide makeup air if the range hood operation at any speed interferes with combustion appliance venting (SWS 6.0201.2j).
- When installing 62.2-compliant ventilation in an existing dwelling, the standard allows the use of the alternative compliance path (Appendix A of the standard). Although this alternative path provides acceptable IAQ and lowers installation costs, avoiding the use of the alternative path is likely to result in better IAQ.
- Encourage occupants to use this ventilation.
Best practices for other local ventilation:
ASHRAE 62.2 does not require half-bathrooms (toilet rooms),8 laundry rooms, hobby rooms, or workshops to have local exhaust ventilation. However, there are times when activities in these rooms result in odors and contaminants that would be reduced by a local exhaust fan.
As a way to enhance IAQ in a dwelling, consider:
- Installing a 50 CFM exhaust fan in a toilet room.
- Installing at least a 50 CFM exhaust fan in other rooms where IAQ would be enhanced by expelling contaminants at their source before they migrate to other rooms in the dwelling.
Best practices for dwelling-unit ventilation:
Although exhaust fans must be used for local ventilation to extract contaminates near their source, the primary purpose of dwelling-unit ventilation is for dilution, so it may be provided by exhaust-only, supply-only, or balanced ventilation. Each of these ventilation types has advantages and disadvantages.
– Best practices for exhaust-only dwelling-unit ventilation:
Exhaust-only ventilation is popular with designers and installers working with the Weatherization Assistance Program (WAP) because it is the least expensive ventilation to install. Ductwork is minimal, local and dwelling-unit ventilation can be provided by one exhaust fan, and controls are simple. It is also a better choice in cold climates than supply ventilation.
- Use a high-quality DC-ECM fan(s).
- Install in a central location for the best distribution of fresh air.
– Best practices for stand-alone supply dwelling-unit ventilation:
Supply-only dwelling-unit ventilation, which is a better choice in warm climates, is often used in the U.S. for new and existing dwellings. The two basic types are stand-alone supply fans directing filtered outdoor air into the dwelling and air handlers used for heating and/or cooling that include a duct running from the outdoors to the return plenum of the air handler. When the air handler fan runs, outdoor air enters the heating and/or cooling duct system.
- Ensure that incoming supply air does not cause discomfort.
- Maintain filters properly.
- Supply duct from outdoors should:
- Include a damper.
- Not take air from attics, crawlspaces or other buffer areas.
- Be at least 6 inches above grade, 10 feet from contaminant sources, above snow or flood line, and at least 18 inches above an asphalt-based roof (SWS 6.0301.1b).
- Install in a central location for the best distribution of fresh air.
– Best practices for air-handler supply dwelling-unit ventilation:
- Must be controlled to run to satisfy ASHRAE 62.2 even if no heat or cooling is needed,
- Supply duct from outdoors should:
- Include a motorized damper.
- Not take air from attics, crawlspaces or other buffer areas.
- Be at least 6 inches above grade, 10 feet from contaminant sources, above snow or flood line, and at least 18 inches above an asphalt-based roof (SWS 6.0301.1b).
- Minimize the length of the supply air duct and make sure joints are well sealed.
- Operating costs can be significantly reduced by using an integrated control motor, which runs at high speed for heating and cooling and at low speed for ventilation only.
- Balance the positive pressure created by the air handler fan with a single-port exhaust fan which can also serve as a bathroom fan. Controls are available for this purpose, such as the AirCycler® Fan ConnectTM.
– Best practices for HRV and ERV balanced dwelling-unit ventilation:
Although balanced ventilation is more expensive to install, it is becoming more popular as dwellings become tighter and occupants learn more about IAQ. The choices for balanced ventilation are heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs). HRVs transfer sensible heat – think temperature – only, and ERVs transfer sensible and latent heat – think water vapor. Overall, we think balanced dwelling-unit ventilation is the best choice for providing good IAQ because of its ducted distribution, heat recovery, ability to filter outdoor air, and a neutral impact on dwelling pressure.
- Use a dedicated duct system for the HRV/ERV, rather than a duct system integrated with the dwelling air handler.
- Supply air to bedrooms and living areas.
- Exhaust air from bathrooms9 and other areas where odors or contaminants are produced.
- Do not exhaust air from kitchens; this should be done with a range-hood fan.
- Supply duct from outdoors should:
- Include a damper.
- Not take air from attics, crawlspaces or other buffer areas.
- Be at least 6 inches above grade, 10 feet from contaminant sources, above snow or flood line, and at least 18 inches above an asphalt-based roof (SWS 6.0301.1b).
- Balance the supply and exhaust air. Install balancing dampers if required to balance the airflow. If the system is integrated with the air handler, balance the airflows with the air handler running (SWS 6.0303.1p).
- HRVs usually have a slight advantage in colder climates.
- ERVs usually have a slight advantage in warm, humid climates.
- For HRVs make sure the condensate drain is installed and drains properly.
- Ensure filters are installed properly.
- In cold climates, ensure HRV/ERV has provisions for automatic defrosting. In climate zones 6B, 7, and 8 as defined by ASHRAE 62.2, do not install an ERV unless it is equipped with an automatic defrost device (SWS 6.0303.1q).
Controls
Proper controls for mechanical ventilation are important to insure good IAQ; this is probably the area of mechanical ventilation that is developing the most quickly, especially Wi-Fi controls linking ventilation to air-quality monitoring devices.10 It is best not to be a “first adopter” of new controls; wait until they have been on the market a while and have been accepted by others as reliable and worthy.
Best practices for ventilation controls:
· Dwelling-unit ventilation must either operate continuously or intermittently with a programmed, automatic controller.
· Ensure all programmable controls have flash (non-volatile) memories. This type of memory ensures that the programmed setting will not be lost during an electricity outage. Controls with flash memories are standard today, but it is a good idea to make sure.
· Locate the control for dwelling-unit ventilation in a slightly out-of-the-way place so that occupants don’t use it as a convenient ON/OFF switch. Label this control, as required by the ASHRAE 62.2 standard.
Ductwork
Ductwork is important for ventilation efficiency, proper operation, and durability. Unfortunately, it is often installed by a technician who is an expert in another field who doesn’t understand its importance. [more]
Best practices for ductwork design and installation:
· Minimize ductwork length and fittings in order to keep resistance to airflow as low as possible.
· Use ductwork with a smooth interior to keep resistance low. Flexible ductwork is acceptable to use for the first few feet at the fan, but then transition to hard, smooth duct material.
· Support ductwork at least every three feet or according to the manufacturer’s instructions.
· Consider using 6-inch rather than four-inch diameter duct for long runs to reduce resistance to airflow. If possible, follow the recommendations in Table 5-3 in ASHRAE 62.2.
· Slope the duct toward the outdoor termination when possible (SWS 6.0101.1b) so that condensation in duct is able to drain outdoors.
· Follow the Standard Work Specifications (SWS) for duct fastening specifications (SWS 6.0101.1d).
· Seal all duct connections with UL 181B or 181B-M specified materials (SWS 6.0101.1e).
· Insulate all ductwork installed outside of the thermal/pressure envelope to a minimum of R-8 (SWS 6.0101.1f) in order to reduce heat loss/gain and condensation.
Exhaust Terminations
Terminations are part of ventilation ductwork that can degrade an otherwise well-designed duct system; exhaust terminations are important. Be aware that a termination can significantly reduce the fan airflow because of high resistance to airflow; try to select one that has a low resistance, usually expressed as a short equivalent length.
Best practices for exhaust terminations:
· Always terminate outdoors, never in an attic, crawlspace, or garage.
· Select a termination device with:
o Bird and insect screens with openings no less than ¼ inch and no more than ½ inch (SWS 6.0101.2a).
o A low effective length (low static pressure drop)
· Install terminations (SWS 6.0101.2a):
o At least 3 feet from a property line.
o At least 3 feet from an operable opening to a dwelling.
o A minimum of 10 feet away from a mechanical intake.
o Above the snow line.
· General preference for termination locations should be gable end or dwelling wall, roof, and soffit, in order. If installed in a soffit, seal all soffit vents within 6 feet of termination (SWS 6.0101.2a).
Commissioning Ventilation
Commissioning is the process that confirms that each part of the ventilation system is installed as designed, meeting all specifications and requirements. For example, is the ductwork the size it was designed to be, have the airflow rates been satisfied with the specified fans, and are the controls working as they should? Most parts can be verified by visual inspection or with test equipment, such as airflow measurement devices. A few items, such as the sone rating of an exhaust fan, cannot be verified in the field, so must be acquired from the fan specifications.
Best practices for commissioning mechanical ventilation systems:
· Make and use a checklist for commissioning your ventilation system installations. This provides consistency for your work and helps commissioning technicians avoid oversight.
· Visually inspect all parts of the system. If there is a design specification for the ventilation system, obtain a copy and verify each detail.
· Measure the airflow rates of all local and dwelling-unit ventilation devices. It’s a good idea to close the house up before verifying airflow. Make sure you use the correct device for the fans that need measuring. Some devices, such as the Exhaust Fan Flow Meter from The Energy Conservatory, will only measure exhaust flow, not supply flow.
· If an HRV/ERV is part of the ventilation system, make sure the supply and exhaust airflows are balanced. Adjust balancing dampers if necessary.
· Ensure that all the controls are working as intended.
· For supply fans and HRV/ERV units, ensure the appropriate filters are in place with no air bypass.
· For some programs, the job inspector is the person commissioning the ventilation system. During this visit, the inspector should adjust the final dwelling-unit ventilation rate based on ASHRAE 62.2 procedure, which is partially based on the post-weatherization (for an existing dwelling) or finalized construction (for a new dwelling) infiltration rate (CFM50 from a blower door test). The CFM50 value is entered into the RED Calc ASHRAE 62.2-2016 tool to find this final dwelling-unit ventilation rate. Dwelling-unit ventilation may be adjusted in one of two ways; with a variable-speed control or with a programmable intermittent timer:
o If using a variable-speed control, for the example house below with a 1200 CFM “Measure leakage @ 50Pa”, set the speed control to provide at least 47 CFM, the “Required mechanical ventilation rate, Qfan” highlighted with yellow in the screenshot of the RED tool below.
o If using a programmable intermittent timer, for the example house below with a 1200 CFM “Measure leakage @ 50Pa”, set the timer to run the dwelling-unit ventilation for at least 28 minutes each hour, the “Fan run-time per hour” highlighted with blue in the screenshot of the RED tool below. This run time assumes the “Fan capacity” is 100 CFM.
Maintenance, Service, and Occupant Education
We all know that equipment is only as good as its maintenance. Fan blades and filters get dirty, ductwork can collect water (condensate), insect screens can become blocked, controls can malfunction, and HRV/ERV airflow can become unbalanced. Eventually, ventilation systems need service, such as cleaning, tuning, and repairing. Perhaps most importantly, the dwelling occupants must know how to operate the ventilation systems and know when and who to call for service. Although in most parts of the country, there are robust networks of heating and cooling system service providers, this is not the case for ventilation systems. Because of this, homeowners must be educated to fill this service void.
About ten years ago, your author attempted to gather data for the average service life of ventilation equipment. Although this data is
available for heating and cooling equipment, it is very difficult to find reliable information for ventilation products. The designer or installer should attempt to obtain reliable service life data from the manufacturers and ensure this information is included with the ventilation system documentation left with the occupants.
Best practices for maintenance, service, and occupant education:
- Inform occupants how to operate the ventilation system, including the controls.
- Provide how-to videos to your customers/clients on the topics of ventilation system maintenance. These can be available on your website or on one of the many video websites, such as YouTube or Vimeo.
- Provide occupants with operation manuals for ventilation devices.
- This information should include make, model, size, design airflow rate for all equipment, and control strategy for all equipment.
- Include maintenance schedules occupants are able to perform, such as filter inspection and replacement, fan blade inspection and cleaning, inlets, and outlet terminations inspection, etc.
- Because HRV/ERV ventilation systems are more complex, additional scheduled inspection is required, such as filters, condensate drains, exterior inlets/outlets, and defrost systems.
- Inform occupants of red-flag items to watch for that indicate a need for service, including increased noise levels, decreased airflows, or malfunctioning controls.
- Make sure occupants are aware of service providers they can contact.
- If in the future the dwelling envelope is tightened, the altered dwelling-unit ventilation requirement should be determined based on the retrofitted envelope.
Although most ventilation systems are based on ASHRAE 62.2, this standard is a minimum standard, as it is intended to be. Exceeding the requirements of 62.2 is acceptable, as long as IAQ is enhanced and money is not wasted. It is wise to install equipment with capacities that exceed 62.2-required airflow rates so that there is enough capacity to provide for additions to the dwelling, increases in the number of occupants, or new recommendations for increased outdoor air during the next pandemic.
Selected References
ASHRAE. Standard 62.2 User’s Manual (based on ANSI/ASHRAE Standard 62.2-2016), 2017, ASHRAE, Atlanta, GA.
Raymer, Paul. Residential Ventilation Handbook: Home Ventilation Management, 2nd ed. 2017, Salty Air Publishing, Falmouth, MA.
Notes
[1] ASHRAE Standard 62.2 is titled Ventilation and Acceptable Indoor Air Quality in Residential Buildings. This standard is updated every three years, the latest version being 62.2-2022. The DOE-funded Weatherization Assistance Program uses ASHRAE 62.2-2016. The sections regarding ventilation in the International Residential Code are based on ASHRAE 62.2-2010.[2] HVI, the Home Ventilating Institute (www.hvi.org) certifies the airflow and sound ratings of exhaust and supply fans and HRV (heat recovery ventilators) and ERV (energy recovery ventilators). Check out their online director of currently certified residential equipment at https://www.hvi.org/hvi-certified-products-directory/.[3] Energy Star-rated ventilation products must meet efficiency standards. They maintain lists for exhaust and supply fans at https://www.energystar.gov/products/ventilation_fans. EnergyStar maintains lists for Canadian HRV/ERV products at https://oee.nrcan.gc.ca/pml-lmp/index.cfm?action=app.search-recherche&appliance=HERV. Energy Star in the U.S. does not maintain a lists for HRV/ERV products.[4] The Standard Work Specifications (SWS) are used by many weatherization programs in the U.S., including the Weatherization Assistance Program (WAP). These Specifications are maintained by the National Renewable Energy Laboratory (NREL) funded by the U.S. Department of Energy. Please refer to https://sws.nrel.gov/using-standard-work-specifications.[5] As the static pressure imposed by ductwork increases – say from a dirty insect screen – the fan speed increases to supply the set CFM airflow rate, at least up to a point. This increase in speed also reduces the efficacy (cfm/watt) of the fan.[6] See the fan specifications for the sone rating. If a fan has multiple speeds, the higher speeds will have higher sone ratings. Note that in-line fans are not rated for sound (sones) because they are not installed in a room, but in line with the duct that serves the room.
[7] See ASTM E3087-18 Standard Test Method for Measuring Capture Efficiency of Domestic Range Hoods, October 2018. https://www.astm.org/e3087-18.html[8] A half-bathroom is defined in ASHRAE 62.2-2022 as a toilet room: “a room containing a toilet, water closet, urinal, or similar sanitary plumbing fixture and, frequently, a lavatory but not a bathtub, shower, spa, or similar source of moisture.” ASHRAE 62.2-2025 will have a requirement for a 50 CFM exhaust fan in a toilet room of a newly constructed dwelling.[9] HRVs have an advantage over ERVs when exhausting moisture-laden air from bathrooms; ERVs will recycle much of this moisture back into the dwelling, whereas HRVs will not.[10] Some manufacturers are now marketing ventilation controls that are Wi-Fi-connected to IAQ monitors. Although this technology is in its early stages, we think it will develop quickly and, perhaps, dominate within the next decade. The ASHRAE 62.2 committee is currently considering an IAQ procedure for controlling ventilation devices.