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Does a Home with an HRV Also Need Bath Fans?

Most homeowners find that an HRV with dedicated ductwork moves enough air to clear condensation from bathroom mirrors

Posted on Apr 25 2014 by Martin Holladay

A balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). system — for example, a system with a heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ) or an energy-recovery ventilator (ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the 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 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. standard. Depending on whether you use the old ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 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 commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. 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
  • Affordability.

Code requirements

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 glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., 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 HomesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. 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 HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. 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 envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. 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 ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. 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 formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen."), 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.

Martin Holladay’s previous blog: “All About Radiant Floors.”

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  1. Riverdale Net Zero

Apr 25, 2014 7:17 PM ET

Steamed Mirrors
by Kye Ford

I find it interesting that often the criteria for a successfull level of ventilation in a bathroom is wether or not the mirrors fog. When I take a shower I sometimes forget to turn on our Panasonic fan/light units and our vanity mirrors hardly fog at all...
When my wife showers even with the fan on and door wide open there is literally condensation dripping down the mirrors.

What is the difference? Occupant behavior. My wife's shower is so much hotter it would scald me.

We have lived in several different houses and yes the dedicated bath fans all worked fine and were sized according to bathroom volume but there is no getting around differences in the amount of steam created.

The dew on the mirrors clears quickly but should dew on mirrors be the defector "rule of thumb" by which we judge success?

I can live with a little dew if it means eliminating an air leaking bathfan in favor of an HRV.

Apr 26, 2014 6:29 AM ET

Edited Apr 29, 2014 3:24 PM ET.

Response to Kye Ford
by Martin Holladay

I agree with you. I understand that the amount of condensation on a bathroom mirror has more to do with occupant behavior than the effectiveness of the exhaust system.

If you have a high-flow showerhead, and you like long, very hot showers, you're likely to get condensation on the bathroom mirror -- especially if your bathroom is small.

I also understand the energy penalty associated with overventilation. From an energy perspective, I don't like high-cfm exhaust fans that run longer than necessary. I much prefer the approach suggested by most HRV installers: providing continuous ventilation at a low rate, without separate exhaust fans. That's the energy nerd approach. It works. If you're a Buddhist, you get it. Be a little patient; your bathmat is going to dry. Breathe in, and then breathe out. Everything is OK.

But the final arbiter -- to person who needs to be satisfied -- is the homeowner, not me. If the homeowner wants a big honking fan that will dry the bathroom mirror in 60 seconds, then sometimes a builder has to install the exhaust fan.

Apr 26, 2014 1:42 PM ET

Why so expensive?
by Robert Connor

For a machine that is sheet metal with 2 fans and a heat exchanger, why are one of the devices so expensive? More so than a furnace? Are the manufacturers gouging us?

Apr 26, 2014 1:59 PM ET

Booster fans / Commisioning
by Kim Kornylo Walton

We have recognized for a long time that (E) HRV's are clumsy in the exhaust department. Boosting whole house ventilation for one person showering seems like a funny thing to do. The inability of (E)HRV's to spot exhaust is a shortcoming.
We have experimented with the installation of inline (in the duct) fans dedicated to each exhaust duct (very near the recovery unit). We started with 150 cfm fans connected to a timer for each exhaust location (bathrooms, laundry and kitchen)

The results were:
- initially there was cross duct exhaust, some of the fans redistributed the exhaust through the interconnected ductwork to another room! inline Butterfly dampers fixed this problem by preventing back flow.
- The kitchen needed more umph so the inline fan was switched out to a 350 cfm with better results.
- The laundry room exhaust is never used, would not install a booster fan from this location in the future.
Seems to be a decent and not terribly expensive solution without energy or envelope penalties. The issues mentioned above (different habits for different people) affect the effectiveness of this or any exhaust system. The booster fan solution does the job that a dedicated exhaust for each room would do.

We had this system commissioned, which is not normal procedure for a residence. For $250 the system was balanced and the performance perceptibly improved. When investing thousands into a mechanical system it is a no brainer to have proper commissioning done so that the system actually does what it is supposed to do!

Apr 27, 2014 8:53 AM ET

Response to Robert Connor
by Martin Holladay

You can buy a Panasonic one-room ERV for $320 online. Ducting (only two ducts are required rather than the usual 4) is extra. In a cold climate, this device will only work from April to November -- you can't use it in December, January, February, or March.

A fully ducted Zehnder (manufactured in the Netherlands for Zehnder, a Swiss company), installed, is likely to cost you $8,000.

If a good Chinese company enters the market, prices may eventually drop.

Apr 27, 2014 8:56 AM ET

Response to Kim Kornylo Walton
by Martin Holladay

You are the kind of innovator who sometimes discovers a new way of installing equipment. Your suggestion may work, or may cause unexpected problems.

In the meantime, it's safe to say that installing an inline booster fan in the exhaust duct connected to an HRV or ERV is contrary to manufacturer's installation instructions and may void the warranty.

Apr 28, 2014 12:57 PM ET

a few points ..
by Jin Kazama

First off, simple electronic devices come with user manual, cars come with an owner manual...
houses should ... that is 1 reason why we get so many user errors

On the pricing issue ...
Most of the "good to ok " models of HRV cost in the 700-1200$ range . All all those are strong enough to be used as bathroom exhaust using boost function.

Paying more than 3000$ for the installed item is just getting ripped off.
As far as i know, it doe not take much more than 6-8 hours to install a simple HRV in a house
with a few ducts and grills as the only additional expenses.

Mirror fogging : if the consumer reason to complaint it is ... i don't see much hope on having them figure out what's right and wrong about energy efficiency.
Instructing homeowners about usage and their attemps toward HRV/bathroom exhaust
could be a priority of the isntaller/builder then ?

This winter i hardly ever operated the panasonic exhaust fan on the small bathroom we are using
( the main one is not operational yet ) or only 2-3 minutes at the end of my long and steamy hot showers to change the bathroom air somewhat ..
might be much more necessary in the future when i'll have finish sealing up the house,
which is the case with low ACH number houses.

And although i plan on intalling the HRV soon, i might keep the panasonic fan just to get a little 5-20 minute boost during summer time, where humidity is high and the "lost energy" is lower .

I have the exhaust fan installed with a 5-10-20-30 timed switch with is simple and works really well during high humidity season ( mainly through summer )
and was inexpensive system

There is probably not much penalty occured in using a high flow exhaust fan when the temperatures are within 10c to 30c .

Lastly, i am pretty sure that there is a spot on the market right now for lower
cost , high effiency low cfm HRV for small high effiency houses. 5000-8000$ cost for an efficient system in a small efficient but budgeted house is too far of a stretch to be justified.

Easy to push high end stuff in a 400K-700K$ passive house,
but it is also easy to include Alcatara dashboard finish on a 350 000$ Ferrari .

Apr 28, 2014 2:11 PM ET

HRVs and fans
by Marc Rosenbaum

- Often will add bath fans if the HRV size doesn't allow a boost rate of at least 50 CFM per bathroom. Put the bath fan in the 1/2 bath first, then the bath likely to get used the least. Don't need a 200 CFM HRV in most cases even for large houses, in terms of normal ventilation rates (62.2-2010, not 62.2-2013).
- Placement of the exhaust location in the bath has an effect on mirror fogging (as does shower temperature and duration). In our "new" DER, we balanced to 25 CFM continuous from each bathroom, and haven't needed the boost. The exhaust grilles are high and close to the shower.

Apr 29, 2014 3:09 PM ET

competing objectives, and more on ERV's
by David Butler

@Martin, thank you for an excellent and much needed article. Many aspects of building science involve competing or even incompatible objectives. Your response to Kyl (comment #2) is spot on. Assessing/managing homeowner expectations is paramount. Sometimes a conversation about pros and cons of a particular design element is necessary to guide homeowner expectations. I find this to be an essential step when properly sizing AC capacity. Ditto with ventilation.

The bigger problem is when the designer or specifier doesn't understand the trade-offs. That's why peer discussions like these are essential in our industry. You won't find this in a book, nor sadly, in many classrooms.

Regarding ERV's... I'm glad to see you touch on that but I think it deserves its own article. We're seeing a lot more ERV's go into high performance homes and we're also seeing more winter moisture problems as envelopes get tighter. Contrary to common practice (and manufacturer installation guidance), I do not recommend an ERV as primary exhaust for high-use baths. Too much moisture is recycled back into the house. Whether or not this causes problems in a given home depends on many factors, but it's not a risk worth taking.

To wit, Panasonic specifically prohibits WhisperComfort (ERV) to be installed in baths and other high moisture areas. Good for them! Other manufacturers should follow their lead or at least highlight this risk in their application and installation guides.

As for those super high efficiency (and high dollar) ERV's... I can't help but wonder how much more energy might be saved if a standard model were installed, with remaining $$ diverted to RE or more productive EE enhancements! To me, an $8k ventilator is green-washing at its worst. Ok, maybe not the worst.

Apr 29, 2014 3:22 PM ET

Edited Apr 29, 2014 3:23 PM ET.

Response to Marc Rosenbaum and David Butler
by Martin Holladay

Marc and David,
Thanks to both of you for your helpful comments.

Apr 30, 2014 4:23 PM ET

Bath Fan HRV
by Steven Landau

I put in occupancy sensors in each bathroom, with a 15 min off delay. The sensors connect to the lights, and the Zehnder HRV Boost input. I never have fogging, or a smelly bathroom due to someone forgetting to hit the switch.

Works great!.

Apr 30, 2014 5:52 PM ET

Edited Apr 30, 2014 5:58 PM ET.

mirror fogging
by charles CAMPBELL

I take short, not-so-hot showers in a small bathroom with a low-flow showerhead and 50 cfm bath fan running. The mirror fogs anyway. Guessing it has to do with the temperature of the condensing surface - it's on a thin, exterior wall with no outsulation. But I'm in Georgia, where you would think outdoor temperatures would lead to less condensation, not more.

Apr 30, 2014 9:47 PM ET

on the lighter side
by Andy Kosick

Everybody's thinking it, I'm just saying it... heated mirrors.

May 1, 2014 12:35 AM ET

Not all HRVs and ERVs are created equal.
by Barry Stephens

There are some considerations to be taken in to account with this discussion.

Robert, there are significant differences in products. Efficiency can translate in to a significant difference in comfort with regards to temperature of incoming air. For example, with outside air at 30F and inside air at 70F, a ASE of 75% will result in 60F incoming air temperature, while a ASE of 90% results in incoming air at 66F. Quite a difference in comfort. Similar differences in sound levels are also an important consideration, and that is reflected in the size and insulation of the box.

With regards to Max Sherman's comments on ERVs, I am hoping that LBNL will one day realize or acknowledge that ERVs vary widely, and that their assumptions are in some cases misguided. For example, ERVs with enthalpy wheels, notorious for cross-flow leakage, should not be compared to well designed and manufactured cross-counter-flow ERVs with dPoint membrane based heat and moisture exchangers. Wheels are known (and certified) to leak at 10-50%, while dPoint units are less than 3%. This is third-party verified by both HVI and PHI. So pontificating with broad-strokes declarations is misguided and incorrect. And the formaldehyde theory. Data please. I thought we had put that one to bed, pending some actual data. And again, enthalpy wheel, or otherwise?

And with regards to the theory that ERVs will retain too much moisture if used for bath exhaust, can we also recognize that ERV SYSTEMS are not all created equal either? Take a typical whole house system. There would be perhaps four to five exhaust points (bathrooms, kitchen, possibly basement or mud room or laundry) and an approximately equal number of supply points. So assume a 50% RH in the home, and 90% in the bathroom with the shower going on. The bathroom with the shower going on represents 20-25% of the total exhaust flow, so 75-80% of the exhaust is at 50%, and 20-25% is at 90%. Do the math. Not as significant as presented. And that bath with shower is only intermittent, and the remainder of the time, the bathroom is at close to ambient. If the incoming air is at low humidity, you transfer some of the moisture to the incoming air. If the outside air is at high humidity, you still transfer a large percentage of incoming humidity to the outgoing air.

I still don't think it makes sense to punch more leaky holes in the walls and stick bath fans in them if you have a properly designed, installed and commissioned HRV OR ERV system.

May 1, 2014 10:17 AM ET

Kitchens and jumper ducts
by kim shanahan


Great article on a subject we will all be learning more about in coming years. The Zehnder rep, and others in comments, mention exhaust grills in kitchen areas. I was told explicitly by the makers of the HRV we used that kitchens exhausts should not be included in the ducted system, presumably because greasy air could foul the heat exchangers. Is that not true with some manufacturers?

Another key consideration for us was the concern that if laundry rooms and bathrooms were the only exhaust ports for whole-house ventilation, then how would the home perform with occupants who kept all bath and bedroom doors closed all the time? Because we couldn't guarantee how the occupants behaved, we used stud wall cavities with high and low grills to create passive jumper ducts for all baths, laundry and bedrooms. I'm surprised neither you nor any posters have mentioned this as a precaution when using baths-only for whole-house air exchanges. Were we just being overly cautious? Or should this be a consideration?

May 1, 2014 10:37 AM ET

Edited May 1, 2014 10:38 AM ET.

Response to Kim Shanahan
by Martin Holladay

Q. "I was told explicitly by the makers of the HRV we used that kitchens exhausts should not be included in the ducted system, presumably because greasy air could foul the heat exchangers. Is that not true with some manufacturers?"

A. You are correct. All HRV manufacturers agree that an HRV should never pull exhaust air from a range hood. However, many HRV manufacturers permit -- and Zehnder encourages -- the installation of an exhaust grille in the kitchen ceiling, as far from the stove as possible. This method is often preferred by Passivhaus builders, who sometimes (if local code officials permit) also install a recirculating range hood fan equipped with a charcoal filter rather than a range hood fan that exhausts outdoors.

For more information on this method, see Makeup Air for Range Hoods and Kitchen vent fan options to control cooking odors.

Concerning the question of whether bathrooms need jumper ducts: bathroom exhaust airflows are relatively low, in the range of 20 cfm to 50 cfm, as the article points out. Door undercuts are usually adequate for these low air flows, although you are free to install a jumper duct if you want (with some loss of acoustical privacy).

May 1, 2014 1:42 PM ET

ERV Bath Fan - UltimateAir
by Jason Morosko

I own, built, designed, and live in a passive house. Plus designer / engineer heat/energy recovery ventilation. Bathroom exhaust, as noted above, can vary greatly. Foggy mirror... please. Define 'unacceptable' ventilation exhaust rate from a bathroom in terms of measurement of bathroom indoor air quality / comfort. The science is still being determined. I have had no negative results of venting the two bathrooms in my passive house with our rotary enthalpy wheel heat exchanger over the past two years (this personal reflection). Keep in mind I actively monitor humidity, CO2, and radon. In the bathroom- the humidity goes from 40% up to 65%.. and then back to 40% within... an hour. 25 CFM being exhausted. No noticeable change in the overall house humidity level. To add- we have been making this ERV since 1992 and have not seen a problem concerning topics relevant to this discussion.
To Max Sherman- please stop with the formaldehyde thing until you have data. The way the information is presented- those that read it might think that no ventilation is better than ventilation that might recirculate formaldehyde... and there is no data even to support re-circulation of formaldehyde. Also note- the majority of commercial ventilation that use heat exchangers are enthalpy and heat wheels.

May 1, 2014 1:49 PM ET

Edited May 1, 2014 1:52 PM ET.

Heated mirrors?
by David Butler

Wouldn't that be like shooting the canary in the coal mine?

Regarding jump ducts in baths, etc... Martin is correct, door undercuts are generally adequate for the flow rates we're talking about. But it needs to be designed, not left to chance. The Manual D guideline for undercuts is to allow 2 CFM per square inch of gap. Pocket doors are generally leaky enough to ignore.

May 1, 2014 8:04 PM ET

Thank you Barry. There is no
by Morgan Audetat

Thank you Barry. There is no substitute for knowledge and experience.

We use ERVs exclusively and try to ignore competitor arguments like; too much moisture is reintroduced into the house with an ERV...Seriously?

We use a lot of Renewaire and have not used a flatulence fan for years. Timed fans with push-button point of use 20 minute timers work perfectly in my home and dozens of others we have remodeled or designed for.

If anyone can show me how to install a whole-house ERV or HRV in 6-8 hours I will pay to learn or better yet have them install all of our units from now on!

Kudos to Jason as well. I have more, rather than less, interest in the manufacturers perspective so often discounted if not mocked in some circles.

May 1, 2014 8:17 PM ET

How to mitigate losses of bathfans
by Kye Ford

In reading the above comments, there seems to be a hung jury as to wether or not a stand alone HRV will suffice for ventilation in a bath, good points and concerns both sides. So why not just include an exhaust vent and give ourselves the option of additional ventilation if needed? To me in the planning stages of a to be built "tight home" I am concerned about the unregulated air movement, heat loss out these exhaust fans. Wether in use or not. Ever stick your hand below an bath fan in the winter, cold air flows right through. I've tried various exterior dampners but I can still feel cold air flowing in.I even order Battic Doors in line exhaust sock which was supposed to collapse under pressure differences from outside to inside but that didn't seem to do much as the dampner built into the exhaust vent stills rattles whenever the wind blows.

I like the option of including the bath fan with the HRV but I liked to tighten up the conventional exhaust unit.

Any ideas?

May 2, 2014 8:45 AM ET

Response to Kye Ford
by Martin Holladay

Concerning this dilemma, my own conclusions parallel those of Barry Stephens, who said, "Do you really want two more 6-inch holes in your house, with cold air streaming in?”

I don't think that there is any way to prevent infiltration and exfiltration through the ducts and termination vents of bath exhaust fans. If I had an HRV system, I'm pretty sure that I would be satisfied with the way that the HRV handled bathroom exhaust. I think that the imagined advantages of separate bath exhaust fans are overshadowed by drawbacks.

May 2, 2014 11:38 AM ET

Edited May 2, 2014 11:49 AM ET.

Alternative method to fixing foggy mirrors
by David Lam

Here's a product that should solve the problem:

Takes a little bit of maintenance and money to buy the product, but I bet it's cheaper than to oversize the exhaust fans (capital cost) and over ventilate (operating cost)

May 8, 2014 4:06 PM ET

Edited May 8, 2014 4:07 PM ET.

understandg potential risks is requisite to good design practice
by David Butler

Barry wrote: "with regards to the theory that ERVs will retain too much moisture if used for bath exhaust... a typical whole house system... (has) perhaps four to five exhaust points... (this issue is) not as significant as presented."

Perhaps, or perhaps not. I see plenty of homes with one or two exhaust ports. In fact, there's nothing wrong with having a single exhaust inlet with multiple supplies if the objective is primarily ventilation as opposed to heat recovery from every exhaust. Not many homes are built to PH standards.

"that bath with shower is only intermittent, and the remainder of the time, the bathroom is at close to ambient."

I agree that continued operation of the ERV will typically keep moisture levels under control. But what about those periods of cool wet weather followed by a deep cold snap, as often happens? A home reacts much quicker to changes in temperature than to changes in moisture. And what about homeowners who decide to only run the ERV during showers (I've seen this on more than one occasion)?

I'm not saying ERV's should never be used for bath exhaust. What I *am* saying is that designers / installers / specifiers better know what they're doing. Playing down the risks doesn't help anyone. As we tighten homes to unprecedented levels, it's incumbent upon industry practitioners to fully understand the potential problems and unintended consequences, and apply appropriate design strategies as well as homeowner education regarding moisture management. Manufacturers also have an important role to play and, I believe, an obligation to at least describe potential risks in their installations guides.

May 23, 2014 9:22 PM ET

separate toilet "closet"
by Erica Downs

How would you treat a separate toilet "closet" within a main bathroom? The door would presumably be shut while in use, then open the rest of the time.

May 26, 2014 6:28 AM ET

Response to Erica Downs
by Martin Holladay

The usual choices apply: if the house has an ERV or HRV system, the room can either be equipped with an exhaust grille and a booster switch, or it can be equipped with a dedicated exhaust fan. The advantages and disadvantages of the two approaches are discussed in the article.

Apr 15, 2016 2:26 PM ET

Comfort+ROI > EnergyEfficiency
by Nathaniel Hieter

I am rebuilding our home after a fire (nobody was hurt) and have the rare opportunity for a construction "Do-Over". As we design the new home I find myself constantly fighting to remind myself that Comfort and ROI is more important than pure Energy Efficiency.

As it is, I've probably already blown ROI on several key systems...... and fear it will happen again for ventilation. I'd love to settle for an exhaust-only system. My family makes good use of bathroom fans. The downstairs floorplan is very open and I imagine I will install a central vac again to clean up after my three rugrats.

Upstairs is very chopped up: three bed rooms each with walk-in closet, two bathrooms, laundry room. To be honest, I'm less concerned with the energy loss than I am with getting fresh air to the bed rooms.

I had planned to simply put timers on the exhaust fans and route a passive feed to the bedrooms. However, after reading that report on passive air inlets I'm at a loss. I could slave a simple intake fan to the bathroom exhaust fans....... but good grief..... I don't want to end up just cobbling together a system I could buy complete in a box!

Apr 15, 2016 2:40 PM ET

Response to Nathaniel Hieter
by Martin Holladay

Your house needs to meet the needs of your family, and be affordable for your budget.

There is no simple answer to this question. But if you want a dependable source of fresh air in your bedrooms, and if your budget is big enough to afford an HRV, then by all means, install an HRV.

Jul 29, 2016 5:03 PM ET

Edited Jul 29, 2016 5:05 PM ET.

Two HRV's in a single house?
by Steven P

I'm planning for a new ICF build in the PNW. The structure should end up being very air tight and I will have hambro floors which will allow a ~20" gap between the ceiling structure and the ceiling drywall.

I am thinking to place the HRV in that cavity and since there will be two floors I'm also thinking of having an HRV for each floor instead of one single HRV.

I believe this approach may have some advantages such as:
- Shorter and simpler ducts between the unit and the intakes/outlets
- No need to pass ducts from one floor to another.
- ability to run at times one of the units at a different speed/cfm than the other
- redundancy - if one unit fails there is still one left

As for the cons:
- obviously more units to set up, maintain and subject failure
- balancing issue?

Has anybody done this?
Does it make sense to try?

Jul 29, 2016 5:30 PM ET

Response to Steven P
by Martin Holladay

The first step is to determine your ventilation airflow requirements according to ASHRAE 62.2. For more information on the ASHRAE 62.2 formula, see Designing a Good Ventilation System.

Many homes need only 90 cfm, 100 cfm, or 120 cfm of ventilation airflow, and those amounts are easily handled by a single HRV. I seriously doubt that you need two HRVs to meet your ventilation air flow requirements.

Don't make your house any more complicated than necessary. A single HRV is preferable from a maintenance and balancing perspective to two HRVs.

If you plan to install an HRV in a joist bay, remember that you need good access to the unit for servicing the core and changing the filters. At a minimum, you'll need an access panel. Most HRVs can't be stuffed between joists.

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