UPDATED on June 29, 2017, with information on Aldes constant airflow regulators.
A balanced ventilation system — for example, a system with a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV) — exhausts stale air from some rooms in a building, while simultaneously introducing fresh outdoor air to other rooms. The best balanced ventilation systems use dedicated ventilation ductwork. Usually, these systems pull exhaust air from damp, smelly rooms — bathrooms and laundry rooms — and introduce fresh air to the rooms where people spend most of their time — bedrooms and the living room.
Some of these balanced ventilation systems operate at a low speed for 24 hours a day. Others have timers that operate the fans for a certain number of minutes — perhaps 20 or 40 minutes — per hour. These controls aim to ventilate the house at a pre-determined rate — for example, the rate recommended by the ASHRAE 62.2 standard. Depending on whether you use the old ASHRAE formula or the new ASHRAE formula, and depending on the size of the house and the number of occupants, a single-family house might require anywhere from 45 cfm to 120 cfm of ventilation air.
Many HRV manufacturers advise builders that the exhaust function of an HRV is adequate for removing moisture and odors from bathrooms. However, a few HRV manufacturers and some builders provide different advice; they advise that even when a bathroom has an exhaust grille connected to HRV ductwork, it’s still important for every such bathroom to have a separate bath exhaust fan.
Which approach makes the most sense?
Advice from Venmar
Venmar Ventilation is a manufacturer of HRVs with headquarters in Quebec. According to specialists at Venmar, it’s perfectly possible to use an HRV system as the only method of exhausting air from a bathroom.
John Pothier, a technical specialist at Venmar, told me that most Canadian homes take that approach. A Canadian rule of thumb calls for the HRV system to be commissioned so that, with the bathroom booster switch on “high,” the exhaust airflow from the most important bathroom in the house is at least 50 cfm, while the exhaust airflow from from secondary bathrooms is at least 30 cfm. According to Pothier, this approach satisfies most homeowners.
Advice from Zehnder
The advice from Zehnder, a Swiss manufacturer of HRVs, is similar to the advice from Venmar. Barry Stephens, the U.S. representative of Zehnder, advises builders that the exhaust side of their HRVs is adequate to handle the needs of bathrooms.
Zehnder HRVs have three speeds, and the manufacturer recommends that the systems be set up so that the medium speed meets the requirements of ASHRAE 62.2. When Stephens commissions a system, he aims for 20 to 24 cfm of continuous ventilation from each bathroom, 10 cfm from each half bath, and 35 cfm from the kitchen. With this type of ventilation system, it’s not unusual for a single-family house in the U.S. to have 4 or 5 exhaust grilles.
“We want continuous ventilation at medium speed to be at about 50% of the airflow capacity of the HRV,” Stephens told me. “I advise homeowners to use the boost switch when they take a shower. The Zehnder has a wireless bathroom boost switch with a timer. I tell people to punch the switch to high speed for 30 minutes. This usually provides about 30 or 35 cfm. It’s enough. Remember, we’re commissioning 100% of our systems, so I’m talking about verified airflow rates. We have about 1,000 systems installed, and we’ve had almost no issues at all with this approach.”
Stephens pointed out that when the timer that controls the boost function turns the fan back to medium speed, the system is still exhausting enough air to help remove remaining humidity from towels and bathmats. When I asked about separate bath exhaust fans, Stephens said, “It doesn’t make sense to install redundant systems. Do you really want two more 6-inch holes in your house, with cold air streaming in?”
Advice from Broan-Nutone
Broan-Nutone is a major manufacturer of bathroom exhaust fans, as well as a distributor of relabeled HRVs manufactured by Venmar. The company’s interest in selling bath exhaust fans may explain why the manufacturer’s advice differs from the advice given by Venmar and Zehnder.
According to Judi Weber, assistant manager of the technical support group at Broan-Nutone, “When you have an HRV, you size it for the whole home, not for the bathroom, shower stall and tub. If you are getting 20 cfm of exhaust when your HRV is operating, that is not going to effectively remove the humidity as fast as most people want it to be. Bathroom mirrors may get foggy on you. Our position, definitely, is that people will be happier if they have separate bathroom exhaust fans, and not to depend on the HRV to remove humidity.”
A longstanding debate
When HRV systems were first installed in the 1980s, the system designers surely intended the HRV’s exhaust function to fulfill homeowners’ expectations for bathroom exhaust. Yet this time-honored approach is often questioned.
The current debate touches on several issues:
- Code requirements;
- The length of time takes to remove excess moisture from a bathroom;
- Simplicity vs. complexity; and
In the 2009 International Residential Code, bathroom fan requirements can be found in section R303.3 and section M1507.3. As long as a bathroom or toilet room is equipped with a window that has at least 3 square feet of glazing, and as long as half of the window can be opened, most building codes do not require the installation of a bathroom exhaust fan.
If the bathroom or toilet room has no window, however, it must have an exhaust fan with a minimum ventilation rate of 50 cfm if it is operated intermittently or 20 cfm if it is operated continuously.
Let’s imagine an example: a typical HRV might be adjusted to operate at 70 cfm to meet ASHRAE 62.2 requirements. Typically, such systems include a booster switch in each bathroom to allow users to bump up the air flow rate of the HRV from low speed (in this case, 70 cfm) to high speed (say, 150 cfm).
If an HRV operating at 70 cfm (continuous) is installed in a house with four bathrooms, each bathroom might end up with 17.5 cfm of continuous exhaust or 37.5 cfm of intermittent exhaust. These flow rates aren’t enough to meet minimum code requirements for windowless bathrooms. (Needless to say, actual air flow rates are likely to be significantly less than the numbers given in this example, because of static pressure drop associated with the duct system.)
Some builders have wondered whether it’s possible to install motorized dampers in exhaust ducts, with controls that close the exhaust ducts of all bathrooms except for the bathroom where an occupant has activated the fan speed boost switch, so that the entire exhaust air flow of the HRV is pulled from one bathroom. According to John Pothier, technical specialist at Venmar Ventilation, such a system wouldn’t work and to his knowledge has never been attempted. The main technical problem with the proposal is that individual exhaust ducts aren’t sized to handle to full air flow of the HRV.
Although it’s quite possible that some installed HRV systems don’t meet minimum code requirements in the U.S., few building inspectors are likely to attempt to verify the exhaust airflow rates of these systems. After all, a code inspector doesn’t usually show up at a job site with a flow hood. Moreover, it’s probable that most homeowners with fully ducted HRV systems will be completely satisfied with the performance of the system, even if the system doesn’t quite meet minimum code requirements.
In Canada, where HRV systems are far more common than they are in the U.S., the building code differs. In Quebec, for example, the above-code energy efficiency program NovoClimat (a program that resembles the Energy Star Homes program) requires that all homes include an HRV. When I discussed the NovoClimat program with Jean Pothier, he told me, “If you follow NovoClimat, no bath fans are allowed in these homes. You need an HRV to each bathroom. This is how they do it.”
However, Pothier later amended his explanation of NovoClimat requirements. The latest version of NovoClimat requires that the two most-used bathrooms in a house must be exhausted by the HRV system and must have a minimum exhaust airflow of 40 cfm each. If the house has more than two bathrooms, the remaining bathrooms can either be tied into the HRV system (as long as the minimum exhaust rate is at least 40 cfm) or can be served by independent exhaust fans.
Are HRV exhaust rates effective?
If a house has an HRV that continuously exhausts air from the bathrooms at a rate of 20 cfm per bathroom, are the owners likely to be satisfied? There is no clear answer to this question.
Posting on GBA, Doug McEvers, a builder from Eden Prairie, Minnesota, wrote, “I have used this system [an HRV for bathroom exhaust] for 25 years and never had a complaint.”
On the same GBA thread, Mark Klein, a builder from Amherst, Wisconsin, commented, “We have been using HRVs for 25 years and have experimented with a few different approaches. Early on we tried using the HRV as the only exhaust in baths and our clients felt that they did not have sufficient exhaust.”
Another GBA reader, T.J. Elder, noted, “I’d suggest it makes more sense to omit the exhaust fan when installing an HRV, and understand the difference in airflow. It will not perform as well at immediately clearing the air as a dedicated exhaust fan, because it’s designed to operate continuously at low speed.”
I asked John Pothier from Venmar whether the HRV-only approach ever resulted in homeowner complaints. Pothier answered, “Sure. Some homeowners don’t want a foggy mirror. Once I had a case — well, this bathroom had a big hot tub. The homeowner also had a rain shower system. His ventilation system was working. It was exhausting more than 50 cfm. There were only two exhaust locations in this house. But he said, ‘My mirror is fogging up, and I don’t like it.’ So I told him, ‘Put in supplementary ventilation. If you think that the HRV is not powerful enough for your activities, it is your prerogative to put in a bath fan.’ ”
Simplicity versus complexity
Builders who prefer simple systems to complex systems are likely to balk at the idea of a fully ducted HRV system plus separate exhaust fans. When I asked Joseph Lstiburek about this approach, he answered, “I do not like combining systems. It is the old engineer in me showing through. I do not like complexity. Controls become more complicated. Programing them correctly and operating them correctly adds to the complexity.”
I told Lstiburek that, in my experience, most owners of homes with fully ducted HRVs and no independent bath fans are happy with the performance of their systems. Lstiburek answered, “My experience is the same as yours. … [But] HRVs are not as reliable as exhaust fans. It results in a bigger and therefore more expensive HRV. In cold climates it increases defrost problems. Having said that, I can live with it, and many times do. The setup is far better than no HRV. So if I can get an HRV in the structure by having to economize by eliminating the additional cost of extra stand-alone exhaust fans, I will, if the alternative is just exhaust fans with no HRV.”
Homeowners who are willing to pay for a balanced ventilation system probably don’t want the system to include components that put the system out of balance. According to GBA reader Matt Fletcher, “As a Passive House Consultant-in-Training and a design/build contractor I recall there was significant discussion over this particular subject during my consultant training courses. Both sides of the argument were explored. Adding an exhaust fan to the building envelope that utilized an HRV to perform the ventilation would unbalance the interior air pressure. … If you exhaust air separately out of an airtight balanced system then it defeats the whole principle of the balanced system.”
The cost question
HRV systems are expensive. If homeowners pay $5,000 to $8,000 for a fully ducted HRV system, they probably don’t want to hear that the system won’t be able to exhaust air from their bathrooms. It’s hard to imagine that these homeowners want to pay extra to install redundant systems.
For homeowners who are worried that an HRV won’t clear steam from their bathroom quickly enough, it might make more sense to skip the expensive HRV and just install separate exhaust fans in each bathroom. If one of these exhaust fans is controlled by a 24-hour timer, the homeowners might be completely satisfied with this simple exhaust-only ventilation system.
What about ERVs?
A final note of caution comes from Max Sherman, a senior scientist at Lawrence Berkeley National Laboratory and former chairman of the ASHRAE 62.2 committee.
“From the [ASHRAE] 62.2 compliance point of view, 20 cfm continuous extract complies, which should be easy for a ducted HRV to meet,” Sherman pointed out. “So for an HRV, I think there are plenty of reasonable designs without having a separate exhaust. The situation for an ERV, though, needs a bit more thought. Since an ERV recovers moisture (and maybe formaldehyde), you are not really exhausting moisture from the bathroom, you are redistributing it. There may be times of the year where that is just fine, but there will be times when you really just want to exhaust it. So I have more sympathy for adding the extra exhaust (e.g. instead of a booster fan) when the system is an ERV.”
Commissioning is essential
Remember, if you don’t commission your ventilation system, you really have no idea what your exhaust airflows are.
If the performance of your HRV system is disappointing, check the airflow rates at each exhaust grille. You can’t conclude that your HRV system is wimpy unless you have first verified that the system is properly balanced and providing the exhaust airflows specified by the system designer.
Since this article was written, Aldes (a manufacturer of HRVs) has come up with a system of components that regulates exhaust air flow from several bathrooms while still providing heat recovery. The system uses individual constant airflow regulators (CARs) in each bathroom grille to achieve the required zoning.
For more information, see VentZone Zoned IAQ with Heat Recovery Kits.
Martin Holladay’s previous blog: “All About Radiant Floors.”