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Musings of an Energy Nerd

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

An HRV provides balanced ventilation. If the HRV is properly adjusted, stale air is exhausted from the building at the same rate that fresh air is being introduced. Ventilation always exacts an energy penalty, though, so it's important not to overventilate. Builders who are leery of overventilating homes sometimes wonder whether the recommended air exchange rates for whole-house ventilation are adequate to clear condensation from bathroom mirrors.
Image Credit: Riverdale Net Zero

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…

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  1. kyeser | | #1

    Steamed Mirrors
    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.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Kye Ford
    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.

  3. BobConnor | | #3

    Why so expensive?
    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?

  4. kimcrow | | #4

    Booster fans / Commisioning
    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!

  5. GBA Editor
    Martin Holladay | | #5

    Response to Robert Connor
    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.

    1. qofmiwok | | #42

      My fully ducted Zehnder is costing $27k. These other numbers don't make sense to me. Just the equipment is $14k let alone installation.

      1. GBA Editor
        Martin Holladay | | #43

        My comment with suggested prices is 7 years old. These days, contractors are quoting very high numbers for all kinds of work -- due to huge increases in materials costs and a booming construction economy that allows contractors to raise their bids.

        You win the prize for the most expensive Zehnder installation ever mentioned on GBA. It's a dubious prize, for sure.

  6. GBA Editor
    Martin Holladay | | #6

    Response to Kim Kornylo Walton
    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.

  7. jinmtvt | | #7

    a few points ..
    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 .

  8. Marc Rosenbaum | | #8

    HRVs and fans
    - 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.

  9. DavidButler | | #9

    competing objectives, and more on ERV's
    @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.

    1. Gregor72 | | #41


      RE: "Panasonic specifically prohibits WhisperComfort (ERV) to be installed in baths and other high moisture areas."

      Does this apply to Panasonic HRV's as well?

      To all,

      I am currently building a tight 550SF ADU in Portland, OR. We are struggling to find a good spot to place the HRV unit. (I am an owner, first time designer/builder) It seems the best option is the Panasonic FV-04VE1 WhisperComfort™ Spot ERV ceiling/wall mount system. But this is an ERV, and it seems HRV is typically specified for our dry summer, cool/humid winter climate. Is there any drawback to using an ERV in the PNW?

      Also concerned about the effectiveness of this "Spot" ventilation device. Though our floor plan is open upstairs (300SF), the downstairs is broken up by bath(75sf) and bedroom(130sf) and small hallway(25sf). If the ERV/HRV and it's 2-port in/out is located in the ceiling above the stair landing downstairs, how can it drive fresh air throughout the entire 2 story dwelling effectively, especially if doors are not left open?

      In addition, these ventilators being balanced systems, how do we account for make-up air when running a 400cfm range hood and ~100cfm bath exhaust fan?

      Am I overthinking this?


  10. GBA Editor
    Martin Holladay | | #10

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

  11. vt_ph | | #11

    Bath Fan HRV
    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!.

  12. charles3 | | #12

    mirror fogging
    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.

  13. AndyKosick | | #13

    on the lighter side
    Everybody's thinking it, I'm just saying it... heated mirrors.

  14. user-874928 | | #14

    Not all HRVs and ERVs are created equal.
    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.

  15. kim_shanahan | | #15

    Kitchens and jumper ducts

    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?

  16. GBA Editor
    Martin Holladay | | #16

    Response to Kim Shanahan
    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).

  17. JasonMM | | #17

    ERV Bath Fan - UltimateAir
    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.

  18. DavidButler | | #18

    Heated mirrors?
    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.

  19. badgerboilerMN | | #19

    Thank you Barry. There is no
    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.

  20. kyeser | | #20

    How to mitigate losses of bathfans
    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?

  21. GBA Editor
    Martin Holladay | | #21

    Response to Kye Ford
    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.

  22. davidtwlam | | #22

    Alternative method to fixing foggy mirrors
    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)

  23. DavidButler | | #23

    understandg potential risks is requisite to good design practice
    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.

  24. erica99 | | #24

    separate toilet "closet"
    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.

  25. GBA Editor
    Martin Holladay | | #25

    Response to Erica Downs
    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.

  26. user-5967000 | | #26

    Comfort+ROI > EnergyEfficiency
    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!

  27. GBA Editor
    Martin Holladay | | #27

    Response to Nathaniel Hieter
    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.

  28. pacificstart | | #28

    Two HRV's in a single house?
    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?

  29. GBA Editor
    Martin Holladay | | #29

    Response to Steven P
    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.

  30. user_8675309 | | #30

    A question about a bath fan and hrv register
    I have had a Panasonic bath fan in my downstairs bathroom for a number of years(and plan to leave in and use if needed) and am now in the process of installing duct for my hrv which I plan on running 24/7 at 55cfm for the house. Should I worry about the bathroom fan pulling air away from the exhaust register of the hrv? What type of distance should there be between the two?

  31. GBA Editor
    Martin Holladay | | #31

    Response to Jon Kunesh
    Most HRVs have more than one grille from which the HRV pulls stale air, and I presume that your HRV system will be drawing stale air from two or more bathrooms, and perhaps from a laundry room or kitchen. If one of your bathrooms has an exhaust fan that is occasionally turned on, the HRV system will continue to work. (What is likely to happen is that more stale air will be removed by the HRV from other locations when the bath fan is running, and less stale air will be removed from that bathroom by the HRV.)

    I don't think you have to worry about a minimum distance between these two grilles.

    As the article on this page points out, however, your bathroom exhaust fan may be unnecessary once you install a good HRV system.

  32. user_8675309 | | #32

    Re: A question about a bath fan and hrv register
    Thanks Martin! There are 3 exhaust grilles total so I should be fine. I'll find out if the bath fans are needed in a week or so when everything gets hooked up. 100 feet of rigid duct up so far and I am so good at spreading duct sealant!

  33. Mdhowes | | #33

    I have a simple question. What is the point of an HRV Bathroom exhaust timer? Is the unit not constantly exchanging the air anyway? Should I not be running my HRV continuously? I have both an exhaust fan and HRV exhaust duct/with Timer switch in my master bathroom. Thanks in advance

  34. GBA Editor
    Martin Holladay | | #34

    M.D. Howes,
    Unless you give us more information, it's hard to know what you have. It sounds like you are describing an HRV with a user-controlled booster switch in the bathroom -- a switch that boosts the fan speed when the bathroom is used. This type of booster function is sometimes controlled by a timer (allowing the user to set the high fan speed to run, for example, for 5 minutes after the person leaves the bathroom).

    If that's what you have, the function is user-controlled. Use it if you want -- or don't. If your HRV is running continuously at low speed, you may not need the booster function.

  35. Mdhowes | | #35

    Thanks Martin. That sounds exactly what I have. I was just wondering why I had this timer in the bathroom if the unit was in continuous operation anyway. A "booster" makes perfect sense. Will hit the timer and listen for an increase in fan noise at the HRV. Thanks again.

  36. ian700 | | #36

    I dont know why the venmar rep is saying you cant have a HRV or ERV with a power grill, because ive installed a system like that about a dozen times now. The bathrooms are all connected the same way as a bathroom fan in 6” flex or 6” hard pipe (depending if its an attic or not) and connected to a 6” primex terminator sleeve to a main trunk line that goes to the HRV/ERV. All duct work is duct sealed so its air tight.

    In the wall we install a gabg box and a posh timer which opens the grill from 10% to 100% and activates a relay in the mechanical room to turn the HRV to high speed. It works great and your not blowing all that air you spent money on heating outside.

    the HRV is sized with a fan big enough to accomodate 2 bathrooms at once when on high speed with two grills open, the likeleyhood of all bathrooms being used at the same time is unlikely. When not in use the system is on low speed drawing a small amount of air from all the grills that are open 10%

    The system we use is made by Eneready, they sell the timer switches, grills relay and low voltage power adapter.

    Ive also installed this system with a regular fan, customer had a big shower, so if they were using it for a long time they could turn on the fan but most of the time the HRV/power grill system would be enough.

  37. Cold_country | | #37

    I've reviewed the guidelines for ventilation rates (62.2 2010, 62.2 2013) but am still uncertain about the size of ventilator I should purchase. To confound the issue, I hope to reduce radon gas levels while adding humidity and clean air to the house. Currently, I've drawn out plans for my 2,600 sq. ft. house with 6 exhaust points, 3 (1 bathroom, 1-18'x26' room, & one in the hallway) in the basement for radon gas mediation, and 3 (2 bathrooms and 1 laundry room) upstairs; plus 3 fresh air vents, 1 in the master bedroom, 1 in the living room, and 1 in the dining room. Using the high end of the chart, I'd need an ERV that produces 150 cfm, but if I just vented the basement and kept the existing bathroom fans, I could possibly get by with 100 cfm or less. Am I calculating this right? Another complicating factor is I'd like an ERV with ECM motor, but the price jumps significantly from 100 cfm to 150 cfm ventilator. Any comments on cfm minimum for my situation would be much appreciated!

  38. charlie_sullivan | | #38

    There no single correct number that is the right amount of ventilation. Furthermore, you are getting some ventilation from air leakage now. Do you know how tight your house is? Have you had a blower door test done?

    But 62.2 2013 comes out to 100 CFM if you have 3 bedrooms, so 100 should be plenty even without taking credit for envelope leakage.

    In any case, while ventilation may help the radon situation, it's not a proper radon mitigation strategy and you should consider doing a proper radon mitigation system as a first priority.

  39. Cold_country | | #39

    Charlie Sullivan. Thanks for your comments and questions....good points all. No, I haven't had a blower door test done, but with all of the bathroom vents, I likely do have considerable leakage. The radon gas issue is what prompted me to investigate an HRV/ERV. I had a certified contractor install a radon gas mitigation system in the basement (where the highest readings of radon exist), but it didn't bring the gas levels down to the 4 pCi/l standard. I thought an ERV would remedy the remaining radon gas levels and help with humidity and allergy issues. There are 3 bedrooms, but that's just the upstairs. Downstairs there are 2 unfinished bedrooms, a large storage room, and a bathroom. I intend to exhaust at three points the basement and, optionally, the upstairs bathrooms (2) and laundry room, and bring fresh air to the upstairs living room, master bedroom, and dining room So, I'm afraid it's going to take a ventilator that produces more than 100 cfm. Thanks.

  40. charlie_sullivan | | #40

    A sliver lining on getting a 150 cfm unit, if you in fact only need 100 cfm, is that a 150 cfm ecm unit, run at 100 cfm, will be more efficient than the 100 cfm unit. So if the cost isn't too prohibitive, you can feel good about that choice.

  41. Expert Member
    ARMANDO COBO | | #44

    FYI - This is from the Broan website... Fresh Air Systems FAQ

    "Must I also install bath fans if I choose to install a Fresh Air System?
    A Fresh Air System cannot be as effective if the high levels of humidity generated by today's shower and tub systems is allowed to dissipate throughout the entire house. It should be removed at the source by running a bathroom exhaust fan for 20-30 minutes after each shower. Your bathrooms should be properly measured and fans for each room must be chosen with proper CFM (Cubic Feet of air per Minute). Then the house's general ventilating system can control the moisture."

    "Can I ventilate my bath fans through a Fresh Air System's ductwork?
    It is not recommended that you do this. When a Fresh Air System (FAS) has been properly sized and balanced, any vent fan that operates during operation of the FAS will unbalance that system, rendering it ineffective. The high humidity from the bathroom, along with any odors, may be circulated throughout the rest of the house."

  42. Buildinginthemountains | | #45

    I am in the process of roughing our HVAC into a new home build in NW Montana (it is fairly dry, humidity issues pointed out are not as much of a concern here as they are other places). We have a heated concrete slab and plan to insulate the walls and underside of the roof with spray foam. Long story short we are hoping to have a very airtight home because of that. That is yet TBD once we are able to do a blower door test (framing has just been completed). We have heated floors, but we still want A/C and a whole home ventilation system with air cleaning capabilities, so still need to run ducting. Given the cost and limitations with space, we can only run one set of ducting (having HRV and ventilation run on its own ducting is not an option). Because of that we are planning to have use a variable speed heat pump, variable speed air handler, American Standard Accu Clean Whole House Air Cleaner and plan to integrate a Broan HRV into the same ducting. The plan is to integrate the HRV with our Air Handler following Bruce Manclark and Dan Wildenhaus steps found in this post:

    Control option 3 is what i'd like to do which is: Interlock the HRV controls with the air handler blower so that the air handler cannot run without the HRV. The controls must be set properly to operate both systems so that the HRV runs during calls for heating or cooling, as well as calls for the air handler to run whenever the system calls for ventilation. This option maximizes distribution with every call for ventilation, while ensuring that all heating and cooling runs integrate ventilation.

    In addition to that system - as of right now our HVAC contractor is planning to vent each of these fans directly outside rather than integrate them with our ducting/HRV: 5 bath fans, dryer, range.

    I'm not sure if that is really the best way to go though. Most of the arguments in this thread for and against needing bath fans in addition to an HRV make sense. It makes the most sense to me that limiting penetrations should be a priority. If ducting the bath fans into our main return trunk line is an option then we would not have worry about trying to control unwanted infiltration from the penetrations. We also would not have to figure out how to balance our ventilation system while also taking all those additional fans into account. On top of the more functional design we can shave some cost by not buying/installing the bath fans. Because of those reasons it seems like just ducting our bath fans to our return trunk line is the best option for our system in my opinion.

    This comment summarizes one of my main concerns best: "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.”"

    However, if our HRV manufacture (Broan) is explicitly saying that is not recommended to venilate bath fans through the fresh air systems ductork, then what options does that leave us?
    (as Armando Cobo's post pointed out "Can I ventilate my bath fans through a Fresh Air System's ductwork?
    Broan FAQ anwer: It is not recommended that you do this. When a Fresh Air System (FAS) has been properly sized and balanced, any vent fan that operates during operation of the FAS will unbalance that system, rendering it ineffective. The high humidity from the bathroom, along with any odors, may be circulated throughout the rest of the house."

    I know nothing about Broan as a company or if they've even considered/tested the approach in question. If they are saying it is not recommended i would assume that warranty issues may come into play though. Can anyone comment on that?

    Has anyone out there actually done what i am proposing? (Interlocking a Broan HRV with an Air Handler and also using the Broan HRV for bathroom ventilation, instead of using bath fans)

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