ASHRAE 62.2 Morsels – 2

Cover of ASHRAE 62.2-2016

As a fifteen-year member of the ASHRAE 62.2 committee, I am often asked about the more difficult or contentious details of the 62.2 Standard. These questions range from replacing kitchen wall fans and the pros and cons of double-duty fans, to tuning in the dwelling-unit ventilation at the final inspection. This is the second of two articles including a collection of these questions from users of the 62.2 Standard and the RED Calc 62.2 tools, along with my best answers

Replacing Kitchen Wall Fans

Recently I was contacted about replacing an exhaust wall fan in a kitchen. There are many of these out there with a pull chain and gasketed cover on the exterior. When the pull chain is released, the exterior cover opens and the fan turns on; it’s a pretty good design, however it does have some disadvantages. The biggest problem is that the fan is not over the range, as a range hood is, so it doesn’t capture most of the contaminants from cooking. It will extract contaminants that have made it to the body of the kitchen, but it is not likely to capture much.

If there is no exhaust fan in a kitchen when you arrive, the best option is to install a vented range hood over the electric or gas range. Of course, sometimes it’s not possible to install a vented range hood, so you can either install a through-the-wall exhaust fan or use the ASHRAE 62.2 alternative compliance path (see the article on this website “ASHRAE 62.2 Alternative Compliance Path” for more information). If you must satisfy the requirements of ASHRAE 62.2-2016 or a more recent version, any exhaust fan that is not a vented range hood must be at least 300 cfm (150 L/s), whereas a range hood installed directly above the range must be 100 cfm (50 L/s) or more.

The high airflow rate for a new through-the-wall kitchen exhaust fan is likely to be loud – usually at least 6 sones – and might impact the proper operation of vented combustion appliances, so make sure you perform thorough combustion safety testing after installation.

Pros and Cons of Double-Duty Exhaust Fans

It is common practice for the Weatherization Assistance Program (WAP) agencies to install double-duty exhaust fans, usually in the most central bathroom in a home. “Double-duty” refers to this practice because the exhaust fan serves as both the ASHRAE 62.2 requirements of a local exhaust fan for the bathroom and as the dwelling-unit ventilation (DUV) for the whole house. Combining two 62.2 ventilation requirements in one fan does comply with the Standard, but it is not the best practice. What are the pros and cons of double-duty fans?

First, a more detailed explanation of this double-duty practice. Based on ASHRAE 62.2 local ventilation must be supplied in bathrooms with a minimum of a 50 cfm (25 L/s) exhaust fan operated with a demand-controlled switch, or a minimum of a 20 cfm (10 L/s) exhaust fan operating continuously. These requirements are not dependent on the size of the bathroom or the dwelling, they are prescriptive rules.

Additionally, DUV is required to be supplied as either exhaust-only, supply-only, or balanced ventilation. DUV is sized according to the area of the dwelling and the number of occupants. The RED Calc 62.2 tools are intended to help with this sizing. In the case of double-duty ventilation in a bathroom, the DUV must be exhaust-only ventilation because it is being supplied by the same fan that is providing the local exhaust ventilation.

The trick to getting double-duty exhaust ventilation to provide the right ventilation rates for both the local and the DUV is the control. The high-quality bathroom exhaust fan must be set to run continuously at the required minimum DUV rate, say 40 cfm (20 L/s), and have a demand-controlled switch that boosts the fan exhaust rate to 50 cfm (25 L/s) to satisfy the local bathroom exhaust rate.

If you have been reading carefully, you noticed there is an easier way to do this for the example above. If the fan is operating at 40 cfm (20 L/s) for the DUV rate, it is simultaneously satisfying the local bathroom continuous exhaust rate of 20 cfm (10 L/s), so no boost switch is required. This fulfills the requirements of the 62.2 Standard. However, it is better to rid the bathroom of shower moisture and objectionable odors with a “boosted” airflow rate of 50 cfm (25 L/s) or higher, in my opinion.

Finally, what are the advantages of installing a double-duty exhaust fan? The biggest is that it is less expensive than installing a separate local exhaust fan in the bathroom and a DUV fan elsewhere. It also requires less time to install, which might be very important to completing the job on schedule. Because it is often difficult to terminate a fan properly outdoors, a double-duty fan eliminates one termination; this is especially important if you must terminate through the wall of a brick dwelling.

And the disadvantages? The biggest is that double-duty fans are not the best practice; it is always better for acceptable IAQ to install two separate fans, it’s just a matter of how much better. Additionally, the controls for the double-duty exhaust fan are likely to be more complicated and expensive and more difficult for the homeowner to understand.

What do I do with a Range Hood Not Ducted Outdoors?

Because un-ducted (or recirculating) range hoods do not comply with the ASHRAE 62.2 Standard, such a range hood can either be replaced by a new ducted range hood or, if possible, kept in place and ducted to the outdoors.[1]

If a recirculating range hood is replaced with a new unit, the replacement unit must comply with the ASHRAE 62.2 standard for airflow – a minimum of 100 cfm (50 L/s) – and a maximum sound level of 3 sones for a demand-controlled hood.

Before replacing a recirculating range hood, it is a good idea to examine the hood and installation instructions (you will probably have to find these online) to determine if it is possible to keep the hood in place and vent it properly to the outdoors with rigid ductwork. Many recirculating range hoods accommodate this conversion, but it is not always worth the effort. Before deciding to convert, consider:

  • The age of the existing recirculating range hood.
  • The condition of the fan. Most range hoods take a beating and are layered with grease.
  • Will you be able to route the duct to the outdoors?
  • Are you able to find the parts needed for the conversion?
  • Will the conversion cost less than replacing the recirculating range hood with a new ducted model?
  • Once converted with ductwork to the outdoors, will the airflow rate be adequate to remove most of the contaminants from cooking?

As a Weatherization Inspector, How Do I Tune In the Whole-House Fan?

One of the challenges of properly sizing the whole-house or dwelling-unit fan when complying with ASHRAE 62.2 is accurately estimating the post-weatherization blower door CFM50. This CFM50 must be estimated by the technician (usually the energy auditor) designing the ventilation system. Of course, the ventilation fan must be selected accordingly and installed DURING the weatherization process. Specifying the wrong fan can be costly and might even require a reinstallation. This mistake can be prevented with the cooperation of the post-weatherization inspector.

The post-weatherization flow rate of the dwelling-unit ventilation is determined by three values: the square footage of the dwelling, the number of bedrooms (number of occupants), and the infiltration credit, which is determined by the post-weatherization CFM50.[2] The first and second of these values do not change from pre- to post-weatherization, but CFM50 does. Using the RED Calc ASHRAE 62.2 tool, I suggest the ventilation designer either 1) estimate the post-weatherization CFM50 based on her experience, erring in the direction of tight, or, even more extreme, 2) check “No” for “Use infiltration credit” in the first section of the RED Calc tool. The objective here is to install an OVERSIZED dwelling-unit ventilation fan which the final job inspector can “tune in” after measuring the actual post-weatherization CFM50.

What do I mean by “tune in”? This is adjusting the dwelling-unit ventilation rate using the actual post-weatherization CFM50; no estimation is needed now. There are two ways to do this with the RED Calc ASHRAE 62.2 tool; one is by adjusting the fan continuous airflow rate with a variable-speed fan control and the other is by setting the fan-run time with a programmable intermittent timer. To illustrate these two options, assume the post-weatherization CFM50 = 1400 and the “Required mechanical ventilation rate, Qfan” = 46 cfm (23 L/s). Please refer to the screenshot of the RED 62.2-2016 tool for Denver, Colorado.

Tune in the DUV Fan with a Variable-Speed Control

As noted just above, the required continuous airflow rate for this dwelling is 46 cfm (23 L/s). As explained above, the fan should be oversized when installed during weatherization; so, let’s assume a fan was installed with an 80-cfm (40 L/s) capacity. After the final inspector conducts the post-weatherization blower door test (1400 CFM50) and enters the data into the RED Calc ASHRAE 62.2 tool, arriving at the result of 46 cfm (23 L/s), the inspector then tunes in the fan with a variable-speed control to this rate (please see green highlight in screenshot below). After you inspect the ventilation system components and confirm all fan airflow rates with measurements, the job is done.

Tune in the DUV fan with a Programmable Intermittent Fan Run-Time Control

Again, it is important to oversize the dwelling-unit ventilation fan during installation. As with continuous ventilation, you can lower the effective ventilation rate, but you cannot increase it to more than the fan capacity. For this final adjustment of the effective ventilation rate, it is useful to use the “Dwelling-Unit Ventilation Run-Time Solver” near the bottom of the ASHRAE 62.2 RED Calc tool. Assuming an 80-cfm (40 L/s) capacity fan was installed during weatherization – the “fan capacity” – the “Fan run-time per hour is 34 minutes. Please see the yellow highlighted lines in the screenshot.

Tune in the installed programmable automatic timer to operate the fan for at least 34 minutes each hour. After you inspect the ventilation system components and confirm all fan airflow rates with measurements, the job is done.

RED Calc Fee ASHRAE 62.2-2016 screenshot for example


[1] If the alternative compliance path to ASHRAE 62.2 is used, a recirculating range hood may be left in place with no alterations. In this case, the flow rate for this recirculating hood counts as zero cfm.

[2] The ASHRAE 62.2 RED Calc Free or Pro tools determine the dwelling-unit ventilation rate, which is a complicated calculation.

Rick Karg

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