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Community and Q&A

Bathroom/laundry vents through HRV?

jimblodgett | Posted in General Questions on

We are building a super insulated house. I’ve been a carpenter long enough and studied enough that I’m confidant the envelope will be tight.

The thing I am least comfortable designing is the ventilation system. I am planning a balanced, ducted system, with the HRV acting as the air handler. Simple enough to balance if the system includes only fresh and exhaust air, but what about bath and laundry area fans? Do you run those through the HRV as well?

And what about the kitchen range vent? If these DON’T run through the HRV, where does their make up air come from? Passive vents?

Thanks for any opinions – jim

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  1. Riversong | | #1

    You don't indicate your location or climate, but if you're building a truly superinsulated house I'll assume you're in a cold, heating-dominated climate.

    Never run a kitchen range hood through an HRV. The aerosolized grease will destroy it, and because there is often some amount of cross-over the combustion gasses can also be problematic.

    You may want to have an exhaust port in the kitchen, but it should be at least 10' from the range. If the range hood moves more than 250 cfm, you should have a make-up air source - particularly if there's any atmospherically-vented combustion appliances (which shouldn't be included in a super-tight home).

    Venting high-moisture areas like bathrooms through the HRV may contribute to frosting of the transfer plates and require additional electric consumption for the defrost cycle. If you vent the bathrooms through the HRV, make sure there's a local boost switch that increases the fan speed when the shower is being used. Even with that, you may find that because the HRV is exhausting the entire house it will not adequately remove the bathroom steam. Small, efficient, seperately-ducted bath exhaust fans will be far more effective in removing source moisture. If they are in the 80-100 cfm range, then the HRV (along with envelope leakage) will serve as the make-up air source by becoming temporarily unbalanced. A negative pressure imbalance is far preferable to a positive-pressure imbalance which will push moisture into the thermal envelope.

    In a cold climate, there will generally be a slight positive-pressure imbalance in an HRV with equal cfm fans, because the incoming air will expand upon warming.

    For this reason, and for simplicity, I prefer an exhaust-only ventilation system using efficient bath fans controlled by both local switches and 24-hour programmable timers, with passive make-up air inlets strategically-located. If the heat load of the house is sufficienty reduced by superinsulation and envelope tightness, then the incremental savings of heat recovery is not substantial.

    A 1995 study of several US cities found that typical annual cost of 80 CFM of continuous ventilation was calculated at $86 with heat recovery and $188 without heat recovery.

  2. GBA Editor
    Martin Holladay | | #2

    Every HRV requires exhaust air to balance incoming fresh air. HRV manufacturers recommend that exhaust air be pulled from bathrooms and laundry rooms. This is the standard way to design and install an HRV system. Most HRVs include controls that allow occupants to boost the fan speed from a switch in the bathroom when required to remove moisture.

    Homes with an HRV do not require separate exhaust fans for bathrooms. Just install the HRV as recommended by the manufacturer, with exhaust air ducted from the bathrooms. As Robert noted, a range hood should not be connected to an HRV. In a tight house, a range hood should be as small as possible -- low cfm ratings on a range hood are much better than high cfm ratings. As long as your combustion appliances are sealed-combustion appliances, I think that there should be no need for a makeup air source for a small range hood -- assuming you have installed an HRV with fresh air ducts.

  3. homedesign | | #3

    I agree with Martin about running the bath exhaust thru the HRV...
    That's the way the Germans do it.
    Why not remove moisture from the wet rooms at every chance.
    A 1995 study of typical or even Energy Star homes does not tell us how a true Low Energy Home will perform.
    The Germans have a track record and they believe in HRV's.
    And I still don't believe that "Ample" R-value only works in Cold Climates....
    Heat moves from more to less and R-value slows the movement.
    Once we hit a HERS 50 what else can we do...
    Use HRV's and increase R-value at all the weak elements.
    Yes...we need better HRV's.

  4. Riversong | | #4

    While it may be standard practice to exhaust bathrooms into the HRV system, be aware that the current ASHRAE 62.2 ventilation standard requires a minimum of 50 cfm on-demand or 20 cfm continuous spot ventilation in bathrooms.

    Also, if kitchen exhaust flow rate is less than 5 ACH, a vented range hood is required with a minimum flow of 100 cfm on-demand or 5 ACH continuous.

  5. homedesign | | #5

    You also mentioned laundry room exhaust.
    Is a laundry room really a "wet room".
    With Modern Front loading washers and a dryer acting as an exhaust fan...
    I am not so sure we need to be extracting additional air from a laundry room...
    Perhaps if you plan to hang wet clothes in the laundry room?

  6. homedesign | | #6

    Another thought...
    The Germans seem to extract the air from bathrooms AND Kitchens.
    Not from directly above the stove ...because of issues with grease as mentioned before...but from the "kitchen area" to expel odors and moisture.

    I assume they also use a venthood above the stove vented directly to the outside.

  7. jimblodgett | | #7

    I really like the idea of a constant low volume air change, with an on demand boost for bathrooms and kitchens.

    Maybe I'll use a remote single fan for all the bathrooms, then run the exhaust from that fan through the HRV...maybe even use motion sensor switches that boost the fan when someone enters a bathroom instead of relying on people to turn on the fan...

    Someone (I think it might have been John Brooks) asked about laundry room fans. I am a carpenter and I sweat. A lot. And my clothes don't get washed every night. More likely twice a week. I was just thinking of the laundry area fan as a sort of fart fan...just to evacuate odors and whatever moisture I carry in from the great outdoors half the year.

    Can anyone refer me to any reading regarding various ventilation techniques? Where can I read up on what those freakin' Germans are doing?

  8. homedesign | | #8

    Good point about odor...some people have dogs..cats and liter boxes too.
    Here is a directory for Passivhaus Germany
    This page concerns ventilation
    There is more discussion here at GBA

  9. jimblodgett | | #9

    Thanks for the link, John.

    I have been following that passivehaus for beginners conversation, thanks. I'd sure like to see Katrin speak in person, great video someone posted.

  10. Coyo | | #10

    Please don't even consider trying to build a super insulated home without installing a mechanical ventilation system. Anybody who claims a modern HRV system cost you more money running it then what it saves you in "recovered heat" has no idea what they are talking about. If you try to build an efficient home you need to build airtight - and you will need to ventilate it in a controlled manner - which you can only do by means of a mechanical ventilation system. The key is a well designed system which is installed and balanced properly - something you might want to let a professional handle who knows what he is doing. This is not just running some pipe and tying it together and keep in mind that this is your fresh air supply you need for a healthy home and living environment. All your bathrooms should tie into the HRV system - only a big Jacuzzi tub will require an additional exhaust vent - which I would avoid at all costs if you can in a super insulated house. You are stuck with the kitchen exhaust which needs to exhaust to the outside - you absolutely cannot tie it into the HRV system. Martins advise on low CFMs is very sound for your range hood. If you have a attached garage space in your plans make sure you seal it of very well and swing your sealed door into the garage-you want to pull the door to the seal once your range hood kicks in...otherwise you suck in all those nice Benzenes and other goodies from your garage space into your living space.

    We custom build what I call a "drying closet" which we seal of and provide a separate heating and ventilation loop for replacement of a conventional dryer - which IMO has no real place in a "Super insulated" home as well. Something to think about.

    There is much more then stuffing insulation into a wall to build a truly efficient "Passiv" home - and good ventilation is a big part of it. Happy Building - TC

    PS: John B. - I think I need to start teaching you German-you have no idea how much good research papers your missing out on :-)

  11. GBA Editor
    Martin Holladay | | #11

    Thanks for providing good advice. My only quibble concerns your statement that "Anybody who claims a modern HRV system costs you more money running it than what it saves you in 'recovered heat' has no idea what they are talking about."

    Well, I'm one of those people. I certainly agree that it's important for every tight home to have a mechanical ventilation system. But the occupants of the home should be told the truth: operating the ventilation system incurs an energy penalty and costs money. Any HRV requires electricity to run, and every HRV has a thermal efficiency that is less than 100%. During the winter, heat escapes the house through an HRV's exhaust. That's an energy penalty -- and so is the penalty incurred to run the HRV fan or fans.

    So, indeed, it costs the homeowners more money to run the HRV than it saves in recovered heat. But that doesn't mean that they shouldn't run the HRV.

  12. jimblodgett | | #12

    Thorsten - About running your bath exhausts through your HRV - do you include a booster fan that is switched from each bathroom? Or is the low volume the HRV constantly runs at enough to clear the steam in a modern bathroom?

    Would you mind explaining a little about the construction details of your "drying closet"?

    I'd be surprised to read any professional opinion that said a tight house doesn't need controlled mechanical ventilation. Some folks use an HRV, other folks use other systems. I'm game, but I have to believe in what I'm doing, so I'll leave my options (and my mind) open until something is clear.

  13. homedesign | | #13

    You are correct..I am frustrated that I can not read German.
    I look at the pictures and try to guess what is going on.
    I understand the dedicated exhaust for the cooktop..but it looks like "They" are also extracting air from the kitchen and passing thru the that correct?
    I want to hear about your drying closet myself.

  14. homedesign | | #14

    I know that just looking at conductive heat transfer and air changes does not tell the whole story...
    But Conduction happens and so do air changes (by design and by accident).
    Just look at this simple calculator....
    Assume that you build a very tight house....0.6 ACH50 (almost zero infiltration)
    Now you choose ventilate at 0.5 ACH
    The example home shows 23millon BTU/yr for "air change"
    Don't you think that it is cheaper to capture 70% to 90% of those BTU's?
    Our USA companies could build HRV's with EC-motors if there was a demand.
    Some people import HRV's from Europe TOO.

  15. homedesign | | #15

    I hope that more people will go with a detached garage or NO garage...
    But if you have to have one...That idea of Thorsten's(opening the house door into the garage) makes a lot of sense.

  16. GBA Editor
    Martin Holladay | | #16

    HRVs with ECMs are manufactured in North America -- for example, the Lifebreath 155 Max, the vanEE HRV 90H-V ECM, and the Venmar AVS HRV EKO 1.5.

  17. homedesign | | #17

    Martin ..thanks for the source,
    Now how can something like that use more operational energy than the BTU's saved?
    Don't forget have to ventilate anyway!

  18. GBA Editor
    Martin Holladay | | #18

    Of all ventilation options, a properly designed HRV system has the lowest operating cost. However, it will still have an operating cost.

    Factors that raise the total cost of an HRV (compared to a simpler system like an exhaust-only ventilation system) include the high capital cost of the equipment, the high installation cost of the equipment, and the ongoing costs of filter replacement and cleaning.

  19. homedesign | | #19

    I only quibble with your choice of words in post #8:

    "So, indeed, it costs the homeowners more money to run the HRV than it saves in recovered heat"
    You said "more money to run" which I take as operational cost.

  20. homedesign | | #20

    oh darn...these post numbers change as you go

  21. GBA Editor
    Martin Holladay | | #21

    I think we agree, but we are using different ways to describe the energy inputs and energy recovery associated with ventilation.

    You're right that an HRV has a lower operating cost than other types of mechanical ventilation. But the amount of heat recovered by an HRV is always less than the heat that is lost. That's why the thermal efficiency of the unit is always less than 100%. In other words, ventilation results in lost energy; that energy has to be made up by an input of electricity (to run the fan) and an input of electricity or heat (to condition the fresh air).

  22. Jesse Thompson | | #22

    It is common to use recirculating range hoods in combination with HRV / ERV in Passivehouses. Being German, they use higher quality hoods than the typical cheap American rental unit recirculating hood, but the idea is to use your hood to trap grease and filter the air, but not incur any extra envelope air leakage or unintentional pressure differentials.

  23. jimblodgett | | #23

    Hey! A recirculating range hood. THERE'S something to consider. Thanks, Jesse.

  24. Coyo | | #24

    Martin...I don't think you are one of these people. This is a sore topic for me as I spend endless hours debating it for too many years. I agree that there is a simple truth to it that running an HRV comes with costs and energy penalties. But my question is - what is your baseline you are measuring this by? Not providing any ventilation? If that is the case - yes I agree.

    If we are agreeing on that providing fresh air is a good idea then it becomes a different story all together. In a cold climate a modern HRV system (even with the rather inefficient units available in the US) will outperform any other means of providing the required ventilation if you look at your dollar for dollar spent. There have been a ton of problems with the early HRV/ERVs and I was buying into these arguments ten years ago. A study from 1995 with higher actual costs in operation has absolutely no meaning about the state of the industry today anymore. The early days were grim and riddled with more problems than results. But we are past that stage and especially right now we are getting more attention from all our manufactures about higher efficient units than ever before. This is a supply and demand question. Venmar released their new EKO this Spring with 2.53 CFM/watt, FanTech is cooking up something good and we hopefully get to install a test unit this summer. If you look at the technology leaps in Germany on ventilation over the last 5 years it is mind boggling. They were able to achieve this because the industry finally decided to drop the argument that mechanical ventilation costs money and rather focused on developing high efficient units which run on considerable less energy. This is what upsets me about our state of the industry - we are still arguing about something pointless. We need to provide ventilation into our homes, there is no way around this - as we all need to breathe. Lots of fresh air, at all times. Once we stop complaining about energy costs related to do so and rather demand more efficient units our manufactures will follow suite and build them.
    It is my believe that one of the main reasons the idea of super insulated homes failed for us in the 70's is because we did not understand and realize that air tightness and good controlled ventilation is an essential part of the overall home system. These homes were simply uncomfortable to live in - and developed lots of moisture and mold issues. Let not make the same mistake again 30 years later. The Passiv Haus Institute picked up these early pioneer ideas and improved on them and by now have a very good and defined building standard developed to build super insulated right. And mechanical ventilation is a key requirement of this standard.

    In this whole argument about energy efficiency we should never forget that in the end however the main question is – how much is a healthy house environment worth? It is priceless. TC

  25. Coyo | | #25

    Jim - yes every bathroom gets a booster switch and we have no issues with the HRV not keeping up with the humidity level in a bathroom. A modern HRV and proper installation works for us now even in 50 below temperatures as well - which was certainly not the case 10 years ago.
    A recirculating range hood is a very good energy saver but a code violation in many areas in new construction. I would make sure to check with you local regulations. You might check on your code provisions on air admittance valves for eliminating your plumbing stacks as well at the same time...another big saving point.
    Drying closet details...uh, in a nutshell a "closet" in the laundry which is sealed, insulated, has a floor drain, moisture protection, an air intake coming in high and a exhaust going out low, a timer loop and lots of cloth line and a rod. I will write a detailed report about this sometime this winter (when things are slowed down) including construction details and performance data.

  26. Coyo | | #26

    Last comment to mechanical ventilation - we will install 3 test units this summer with a pre-heat exchanger on the air intake which runs of a small geothermal loop run by a high efficient pump and preheat incoming air to ground temperature. In the summer reversed it will provide cooling. I believe this to be our missing link to make mechanical ventilation really effective and see a huge gain for little extra costs. Very much looking forward to collect data on these during the winter - and think that we will make this a standard installation next year on all our projects. Ok-enough now, back to work I go. TC

  27. homedesign | | #27

    I am convinced that Thorsten is headed in the right direction.
    I have not seen a good HRV installation locally or on web.
    We need to know which are the best HRV's available and we need guidelines for how to install and commision them properly.
    Does anyone have links?
    maybe I am just not looking in the right place.
    Maybe...FHB or JLC or EDU could do an article?

  28. Riversong | | #28

    I believe that Thorsten and PassivHaus are going in exactly the wrong direction. This is a purely technological response to the crises created by techonological responses. And it's based on the assumption that we require a perfectly-controlled indoor environment in order to be happy and healthy. How did homo sapiens manage to survive to this point without such technological marvels?

    While I'll admit to building superinsulated, well-sealed houses for nearly 30 years, I have come to appreciate that humans were not meant to live in hermetically-sealed containers with artificial life-support systems. Isn't that the nightmare scenario of much science fiction?

    We need shelter which functions like a third skin (after our biological skin and our clothing), which needs to breathe on its own and naturally maintain a healthy even if variable environment - which includes such things as a good negative ion balance, undermined by the use of any plastic materials.

    If we build with natural (viz. non-toxic as well as ecologically-benign) materials, they offer an organic humidity-buffering capacity which significantly reduces the need for air-exchange. Authentic "green" building is shelter that can be composted in the garden at the end of its useful life-cycle, rather than built for "durability", which is a eupehmism for permanence much like the 100 million tons of plastic forming a new continent in the north Pacific.

  29. GBA Editor
    Martin Holladay | | #29

    Here's a link to an article on installing HRVs:

    It's an article I co-wrote with David Hansen when I was an editor at the Journal of Light Construction. David Hansen has been installing HRVs in Vermont for over 20 years, and doing an excellent job.

  30. Jesse Thompson | | #30


    Logical next questions:

    At what number does "well-sealed" become "hermetically sealed"? How was that number determined?

    Why aren't the continuously operating bath fans in the houses you describe considered "artificial life-support systems"? Don't bad things happen to the inhabitants and the structure if they aren't operational and powered?

  31. Coyo | | #31

    Robert - I absolutely share your vision of designing and building truly "sustainable shelters". Homes should be living, breathing and renewable shelters for us to live in safe and healthy. As of now we however have not been able to develop such dynamic, natural and functional building systems from renewable resources which function as a 2nd skin and are a natural extension to our human bodies. This is my "Science Fiction" I envision for building homes in the future.
    For now I believe it is very beneficial for anyone serious about building homes to study up on the Passiv Haus standard. It is not the answer to all our problems but a good road to travel on...which will give us a wealth of knowledge to build on top of and improve upon to push the envelope further and develop materials and systems which will change the way we build today. If we stay stuck in our ways and egos we might be getting back into real "sustainable shelters" in a different form then we envisioned.

  32. Riversong | | #32

    Thorsten, vernacular building systems all over the world ARE those living, breathing and renewable shelters. Here in New England, there is a revival of straw-bale, earth-plastered, and cob homes that are both comfortable and healthy - for both the occupants and the environment.

    The reason our culture has led us into the current global multiple crises is our insistence that, with science and "advanced" technology, we can improve on Mother Nature and millennia of indigenous architecture.

    Why, then, do we continue to believe that more of the same science and technology will get us out of the hole that science and technology dug us into?

    And, of course, coinciding with "advances" in scientific living has been an alienation from the natural world and from community interactions. The single-family suburban house - and its requisite personal automobile(s) - have created islands of isolation. Now, the "green" building movement is trying to put us into hermetically-sealed boxes to bring the isolation and alienation to the highest level, at which we won't even notice the rapidly deteriorating global climate because we live in a completely artificial climate indoors.

  33. Riversong | | #33


    You ask "At what number does "well-sealed" become "hermetically sealed"? How was that number determined?"

    You missed the fact that I was making a "mea culpa" for contributing to the movement towards sealed homes. But I've avoided using plastic sheeting and foam insulation and other extreme materials and methods that bring air-tighteness to the level of air-sealed, like the PassivHaus movement which is attracting far too much attention. And the distinction is far more qualitative than quantitative.

    You ask, "Why aren't the continuously operating bath fans in the houses you describe considered "artificial life-support systems"? Don't bad things happen to the inhabitants and the structure if they aren't operational and powered?"

    First, my exhaust-only systems are not continuously-operating. They are programmed to meet minimum code or (Energy Star) program standards, and I limit the mechanical ventilation to 0.25 ACH rather than the more common (ASHRAE) 0.35 ACH.

    Second, while my houses are very tight compared to conventional houses, there is still some natural leakage.

    Third, with passive make-up air inlets there will be some stack effect air exchange without the fans running.

    Fourth, most of my homes have wood stoves which also create air exchange during the heating season that doesn't rely on outside power, even though they have closely-coupled combustion air supplies. In fact, in two of those homes, the woodstove was the primary "exhaust fan".

    Fifth, I use only cellulose insulation systems, with low-perm but not impermeable walls which create a moisture buffer that helps moderate indoor RH without excessive mechanical ventilation.

    But, perhaps most importantly, I avoid the use of toxic materials in the indoor environment and generally build for people with relatively non-toxic lifestyles so that pollution sources are limited and minimal natural and occupant-forced air exchange can likely maintain a healthy indoor environment.

    And, even with all that, I now believe I've gone too far to the "dark side" and am leaning strongly to natural building systems such as straw bale, earth plasters and cob. Not only are those materials far more environmentally benign (particularly if locally-sourced) and non-toxic, clay is both an excellent moisture buffer (second only to end-grain wood, such as cordwood masonry) and a source of the negative ions which are so essential to both physical health and psychological well-being.

  34. homedesign | | #34

    I think that this discussion has morphed from how to configure an HRV to...
    Should we have HRV's? to...Should we learn from the Europeans?
    But that's OK
    Air tightness has nothing to do with having an abundant supply of fresh air.
    We can have both.
    We can always open our windows if we want to... or we can use automated ventilation.
    The Germans are not suffocating in their homes.
    If you do not like foam and can still build passivhaus.
    Airtight enclosures with fresh air supplies are not going to kill us or undermine our society.
    Airtight does not have to be vapor tight.
    Some German homes have a simple 3 position ventilation switch that the homeowner can use...
    How complicated is that?

  35. jimblodgett | | #35

    For me, it's pretty clear. I am trying to build a house that requires as little energy as possible. So even though we have a moderate climate here in Western Washington, the initial investment required to super insulate will benefit the homeowners for the next 100 years.

    I'm just trying to ventilate it the best way I can figure out.

    There's plenty to be learned from the Passivehaus folks and the natural house folks and the build whatever sells folks and anybody else who shares an opinion. No one has all the answers.

    I appreciate everyone's involvement in this discussion. Thanks.

    Hey. What about recapturing some of the heat from the clothes drier? Anyone have any experience or thoughts about that?

  36. Coyo | | #36

    Robert-I spend plenty of times in "vernacular" shelters anywhere from Africa, Siberia, Alaska and South America to have a pretty good understanding of "sustainable shelters". But in my humble opinion we have to be realistic about our challenges we face and the environments, amount of people and all the other constrains coming with all that when we look into building systems and the way we build. Straw-bale, earth-plastered, and cob homes are a good answer for some areas, climate zones and a limited amount of people. In many case I have seen them turn into very problematic structure after you put the 800 pound gorilla aka homeowner in there who does not control his environment as he should and creates all sorts of problems.

    That all said, and no more, I need to apologies to Jim: sorry we turned your simple question about ventilation into a personal vendettas. You are absolutely right that no one has all the answers - but we all have our opinions and egos. Good luck building. TC

  37. Jay Walsh | | #37

    In my current work as an Energy Star rater as well as a rater for the USGBC LEED for Homes green building program I have been involved in the installation and testing of many HRV/ERV’s from various manufacturers. Most of the HRV and ERV units are very similar in construction and function. Here are some of the highlights of what I have seen, tested and feedback from others.

    The comments below refer only to independently ducted systems and not those installed as part of the homes HVAC ducted system.

    1. While the ECM motors are very efficient and use MUCH less electricity (these are my preference) they are noisier and can produce a hum similar to a refrigerator. Locate these and duct them in a way to avoid the transmission of this noise.

    2. When choosing the exterior supply and return locations avoid these locations: Asphalt or composite type roofs, or south or west facing walls if vinyl sided, when heated by the sun they off-gas and this will be drawn into the home. Also avoid locations where cars maybe parked and idling. Don’t laugh, I’ll seen all these things done.

    3. Do not locate the HRV in a cold attic or similar location, and in very cold climates be sure to provide for a connection of the HRV to the homes drain system. This will be necessary for when the system needs to defrost (which it will need if the outside temp falls below 10 F).

    4. Locate the HRV in an accessible place so you can remove and clean (or replace) the filters and inspect the condensate trays.

    5. The filters in these devices are only designed to keep out large particles from contamination of the exchanger. There are better filters that fit these devices and even MERV filters. Should you plan to use another type of filter for the system it will add airflow restriction and will affect the air exchange rate of the device. Consider the size of the HRV if you plan to add
    additional filtration.

    6. Flow restriction from ducts. Consider carefully the location of the HRV to provide the shortest run of ductwork. The longer the ducts, the greater the number of bends, and the type of duct used will all play into the performance of the system. Keep the runs short as possible, use rigid ducts over flex ducts wherever you can. If you have very uneven lengths of runs on the exhaust side, consider some method of balancing damper in the system to adjust
    for equal exhaust from bathrooms.

    7. Have the system installed by a professional and tested to insure a balanced supply and exhaust so you do not create a positive or negative pressure in the house. Also, many of the systems I have tested have issues with proper airflow of supply and exhaust registers and they often fail to meet the proper volume (CFM) of air to the rooms they are serving. Improper duct sizing and the lack of inline dampers to adjust flow seem to be the issue here. Often the registers closest to the HRV get the greatest airflow while the further registers have little or no flow. This means the system will not perform the function you desire, pulling moisture from kitchen and bath or provide the prescribed amount (CFM) fresh air to bedrooms etc.

    8. Sizing formula for continuous home ventilation from ASHARE Standard 62.2.
    (Number of bedrooms +1) x 7.5 + (Homes Conditioned Floor Area / 100).
    Example (3 BR +1) x 7.5 + (2400 CFA/100) = 54 CFM of continuous ventilation.

    Final comments: Having tested over 50 of these installed systems of various configurations (here in the Northeast) I have concluded that you should not rely on these systems to manage the bulk moisture removal from high moisture areas such as bathrooms. Most systems when switched into boost mode do not increase the exhaust rate in the bathroom sufficiently. This is simply due to the fact that when placed in boost mode the increased cfm is divided across all the exhaust ports connected to the HRV. I would leave the HRV to do its primary intended purpose of exhausting stale and contaminated air and replacing fresh air to the home, and in addition to the HRV, use an efficient bath fan to exhaust bulk moisture from the bathrooms.

  38. GBA Editor
    Martin Holladay | | #38

    Concerning your conclusion that "you should not rely on these systems to manage the bulk moisture removal from high moisture areas such as bathrooms": are you basing this conclusion on observed moisture problems (moldy drywall, rotting window stools) or simply on airflow measurements made at the bathroom exhaust vent? I contend that even if the measured airflow when the HRV is in "boost" mode seems low, the fact that the house has a ducted whole-house ventilation system makes moisture problems highly unlikely. Remember, overventilation carries an energy penalty.

  39. homedesign | | #39

    In addition to your other tips ...
    how about making sure that there is a path for the air to get into the bathroom.
    When the bathroom door is closed...sometimes under the door may be the only path...and it is no wonder that "boosting" the fan does not increase air change much.
    Are the installations that you see providing transfer grilles or jump ducts all along "the path"?

  40. Jay Walsh | | #40

    John –
    Thanks for the suggestion to add to my HRV Recommendations the undercutting of the bath door to increase airflow and fan performance. And with regards to the installation of “transfer grills along the path” – No I have not see this being done. The only application of this I have seen is for proper Return Air for forces air systems as an option to providing Return Air ducts from each room.

    Martin –
    You refer to”… whole-house ventilation..” and I view what is being spoken about in this thread as two separate issues. One being Indoor Air Quality for the occupants and the second Spot Ventilation for moisture control. Unless a single system can effectively meet the specific requirements of both these issues, the systems to control each should remain separate.

    A few other comments on the subject
    ASHRAE Standard 62.2 calls for baths to attain 50 cfm intermittent or 20 cfm continuous. The continuous aspect of this standard comes with two issues in my viewpoint. One is that you need to know you are actually achieving 20 cfm or more, and I have yet to find a builder measure this, and only two HVAC contractors or HRV installers actually test the installed systems to know for sure. Issue two is time and energy, the additional time necessary for a low volume continuous ventilation to exhaust the high humidity in baths which can penetrate the envelope of the bath and condense increasing the risk of mold growth, and the electrical consumption of most HRV’s. Unless the HRV has an ECM motor it runs at between 120 and 180 watts vs. an efficient bath fan at 24-34 watts. With the bath fan you do not get the heat recovery, but you do get rid of the potentially damaging high humidity quickly. Also, do you really want to flood your HRV with large quantities of moisture? During cold weather like here in the northeast you can create conditions where it may be necessary for the HRV to run in defrost mode (lowering its efficiency) as well as you are introducing moisture into the HRV that needs to be drained off. This is your fresh air supply, you want to limit the contaminants introduced into this system. Also due to the long runs of ductwork associated with most HRV installations, you may develop pooling of water in the ducts from condensing.

    Well-known building scientist Terry Brennan in a recent presentation on whole house ventilation displayed some very detailed analysis on moisture development and mitigation specifically in the bath area. His recommendation goes a bit beyond the ASHRAE standard and suggests 20 cfm continuous with an 80 cfm boost.

    A major contributing factor to the effectiveness of the HRV to exhaust and supply sufficient amounts of air has to do with the lengths of the duct runs, number of duct runs and the choice of duct materials. Consider that a 100 cfm bath fan when connected to any type of duct (under 10 ft) will loose 10-15% of its rated cfm. This I have found consistently of every fan I have tested. Well except for one I came across the other day, but then I found that there was no duct connected to it at all. Now consider an HRV rated at say 200 cfm (supply 200 and exhaust 200 cfm) with three exhaust ports connected to lengths of flexible ducts totaling 50 feet in length. You are likely to loose in the area of 50% or more of the rated cfm of that system. At least that is what I found in a typical system I tested just last week.

    Lastly, at what point do you decide that the installed system is inadequate, when mold starts to develop? If it is growing inside the walls, when do you think you’ll notice it? And even if after the first year you start to notice some slight surface mold, or that liquid water seems to constantly be dripping of the toilet tank or metal plumbing, what will it take to install a bath fan at this point? There is no need to expose families to potential health hazards or costly remediation. Design and install systems you feel will meet these minimum standards and do not skip the critical component of testing the systems performance.

  41. GBA Editor
    Martin Holladay | | #41

    I certainly agree with you completely that:
    1. Ventilation system installers should be commissioning their systems, and that commissioning must include air flow testing.
    2. Duct runs should be short, as airtight as possible, well designed, and competently installed.
    3. On average, the watt draw of U.S. and Canadian HRVs is too high and should be improved.

    All good points. However, I think you are overestimating the potential problems that could arise in homes with HRV systems. Obviously, an HRV system (like any building component) needs to be competently installed according to manufacturers' recommendations. Here in Vermont, we have a long history of such installations; I'm most familiar with the work of David Hansen from Memphremagog Heat Exchangers. Competent installers can also be found in Canada. Perhaps there are fewer such installers in your state of Massachusetts.

    HRV manufacturers have decades of experience with HRV ventilation systems. If installed according to manufacturers' recommendations, such systems will not lead to the moisture problems you mention.

  42. Beatrice Dohrn | | #42

    Hello Jim. I faced similar questions recently and opted for :

    Now I am in a mild climate -- Pacific NW -- but what put me over was leanring that once I put in a balanced system opening windows and doors was taboo.... Just not how we live here.

  43. homedesign | | #43

    Hello Beatrice.
    I am curiuos .. what happens when a person opens a door or a window in a house with a balanced system?
    Why is it taboo?
    What happens when you open a door or window in your house?

  44. jimblodgett | | #44

    Thanks for that link, Beatrice.

    Where in the Great Northwet are you? We are just outside of Yelm, WA, up in the hills.

  45. Coyo | | #45

    What happens when a person opens a door or a window?

    Really simple - fresh air (hopefully) comes into the building. End of story. A balanced ventilation system has nothing to do with not being able to open windows or doors - one of the big "Aniti-Passiv Haus" claims for years in Germany was (and still is) that you couldn't open any windows in a Passiv Haus. Total bogus, of course you can. Why not? TC

  46. Beatrice Dohrn | | #46

    Perhaps I have mixed issues -- balanced system vs. heat exch/transfer. I am a participant on the greenbuilding listserv and people I deemed very knowledgeable about this topic basically said that if you spend all the effort to get a system all tuned to treat and distribute air throughout your house with heat exch etc, you should not "defeat" that by regulating your environment with open doors and windows. Fact is, we live half outside a good part of the year... so.

    In any case, I opted for the "exhaust only" with a few independent intakes (because the house I am building is very tight, I think). I am in the hills outside Eugene Oregon. I came here from NY, thus have family far away and have been blogging quite extensively about the experience of designing and building here (myself with the help of one laborer mostly). Feel free to take a look at www.

  47. Riversong | | #47

    What happens when a person opens a door or a window?

    It eliminates the purported value of an obsessively-tight house and the energy-efficiency that can entail.

    When I was building superinsulated homes down in the hollows of Tennessee years ago, one local showed me how he regulated the temperature of his house in the winter: get the pot-bellied coal stove glowing red and then decide how much to open the front door.

    To build to PassivHaus standards and then open windows during either heating or cooling seasons is entirely self-defeating.

  48. Riversong | | #48


    I think you were wise to use the somewhat simplified occupant-sensing, moisture-sensing exhaust-only ventilation system - though I do wonder whether the marginal advantages of such a self-regulating system will offset the increased costs compared to a simple bath-fan exhaust system with completely passive make-up air inlets and whether it will adeqately evacuate bathroom steam from a hot shower.

  49. Riversong | | #49


    I appreciate both your experience with HRVs and your list of caveats, as well as your admonition to use independent source control fans for bathrooms. But, if independent bath fans are used then I wonder whether the incremental (theoretical) savings from a centrally-ducted HRV for background air exchange is worth the initial and operting and maintenance costs - particularly given the many pitfalls of improper design, installation and maintenance.

    What Terry Brennan's presentation illustrated (which I also teach in my Hygro-Thermal Engineering class) is that a "balanced" ventilation system does nothing to alter natural air exchange, while an exhaust-only system reduces natural air exchange - particularly the potentially-destructive exfiltration of moist air in a cold climate.

    I might add that the obsession with a carefully-balanced HRV system is misplaced, since cold-climate homes would benefit from negative pressure and hot/wet climate homes from positive pressure.

  50. David | | #50

    Interesting topic. Thanks to all who shared. We built a new home in 1990, above the super good cents standards of the time. We installed our Air-to-air heat exchanger (HRV). Push buttons in the bathrooms for high speed. Flow tested by our PUD (and we were able to balance). It worked great. Constant fresh air. Open doors and windows when you want to. The loss of the design only occurs when the outside temp differs from what you want in the house.

    We're now planning a new home. And a new HRV for Scappoose, OR. We do plan on directing the exhaust to the exterior coil for the heat pump. I would prefer getting all the therms I can out of any conditioned air I can. We plan on using a heat exchanger for waste water also.

    What did you end up installing? We plan on installing our own HRV. The duct work will go through some open web trusses. Moving about 120CFM, so plenty to balance 25 CFM through the bathrooms and kitchen with 20CFM for the laundry room. We also looked at the flow from supply to exhaust. Where does the air come from that exhaust from each bathroom? The Master bedroom/bath combo took to most attention.

    Hope it all works.

  51. jimblodgett | | #51

    "What did you end up installing?"

    Still deciding on the heat system but we plan to exhaust the bathrooms seperately from the HRV.

    I'm not sure where Scappoose is, but we are in Western WA at about 500 feet elevation, so we don't get very cold and we don't get very hot. Still considering a ducted heat pump system but leaning more towards spot heating room individually as a back up to wood stove heat. Lots of pros and cons, but we're investing significantly in a tight, well insulated shell (360 degrees) that I think the heat demands will be very, very low.

    Biggest concern for me remains effective ventillation. A little leary about mechanical devices for that, but willing to give it a try. Cost/payback in energy savings period is not very important to me - more interested in getting effective ventillation without wasting whatever energy it took to heat the air we'll be exhausting, so although an HRV system really doesn't pencil out here in our climate, I still think it makes sense from an energy conservation standpoint.

    Funny, the farther you go down this energy efficiency road, the more ways you find to recapture those already used btu's. At some point it certainly becomes more of an exercise than actual benefit to the homeowner's wallet. I've probably crossed that threshold already, but still feel responsible to my grandkids, and their grandkids, and their grandkids...

  52. Interested Onlooker | | #52

    Concerning ACH :

    If the ventilation system delivers 0.5 ACH to the living spaces and bedrooms which then exhaust to the HRV unit through much smaller spaces (e.g. bathrooms and kitchens) does the ACH in the smaller spaces not increase? Perhaps to the code-required 5ACH?

  53. jimblodgett | | #53

    Um...well...maybe, but I don't think in terms of the air changes/hour in each room so much. I think about the entire house. I am counting on my HVAC contractor to balance that exchange so that most, if not all of the stale air is changed. At least I THINK that's the value a professional adds to the process. I sure hope so.

    But you know what? I read your post early this morning and was hoping someone far more learned would come along with an answer to your question. I'm just a carpenter, and I have enough trouble building things plumb square and level.

  54. Riversong | | #54

    Yes you want more air exchange in the polluted areas. But where did you get this "code-required 5ACH"? That would be a requirement for an extremely leaky and insanely inefficient house.

  55. Interested Onlooker | | #55

    "But where did you get this "code-required 5ACH"? "

    No idea - must have mis-read something earlier in this thread... oops

  56. Interested Onlooker | | #56

    For a modest bathroom of 50 sq ft floor area, the figures quoted earlier of 20cfm continuous and 50 (or 80) cfm boost equate to

    3 ACH continuous

    7.5 (or 12) ACH boost

    So the apparently plucked-at-random figure of 5 ACH isn't too far off...

  57. Riversong | | #57

    Interested Onlooker,

    You're confusing the air flow required to evacuate large volumes of moisture with the whole-house air exchange necesary to maintain good indoor air quality.

    In a house built with mostly non-toxic materials for relatively non-toxic occupants, 0.25 ACH is more than enough to manage indoor humidity levels and maintain good air quality (assuming that the outside air is of good quality), as long as spot ventilation in kitchens, baths and laundry areas is used to evacuate point sources of pollution.

  58. Interested Onlooker | | #58

    "You're confusing the air flow required to evacuate large volumes of moisture with the whole-house air exchange necesary to maintain good indoor air quality."

    I'm not confusing them at all - I only wondered whether intelligent routeing of the whole-house air exchange via areas of high humidity or odors could succesfully be used to deal with those problems.

  59. jimblodgett | | #59

    "I only wondered whether intelligent routeing of the whole-house air exchange via areas of high humidity or odors could succesfully be used to deal with those problems."

    Are you saying you might not need exhaust fans (independent of the HRV) in the wet areas?

  60. Interested Onlooker | | #60

    "Are you saying you might not need exhaust fans (independent of the HRV) in the wet areas?"

    Hi Jim,

    It's certainly what I was wondering. If a relatively high intermittent airflow is acceptable when the bathroom is in use then it should be OK when no one is in there. If it can be achieved as I have suggested then it appears that it might be possible to dispense with light- or moisture-triggered boost fans with their attendant cost (capital and running), energy use and complexity.

    I look forward to a response from someone who has tried it and can give an informed yea or nay.

  61. GBA Editor
    Martin Holladay | | #61

    And, after two pages of discussion, we are right back where we started. I'll say it again:

    1. Read the installation manual from the HRV manufacturer.

    2. If you install an HRV or ERV with dedicated ductwork, the exhaust air is usually pulled from the bathrooms and laundry room, and sometimes from the kitchen (but never the kitchen range hood).

    3. Bathroom exhaust air is ducted through the HRV. That's what they are designed to do! This is a ventilation system, for crying out loud.

    4. The HRV manufacturers provides or recommends a bathroom wall switch which allows the bathroom user to boost the speed of the HRV fan when the bathroom is in use, if desired. Otherwise the HRV fan operates as programmed -- either 24 hours a day or intermittently, as the homeowner prefers.

    5. You don't need a separate exhaust fan!

    6. If you feel (due to you facts arrived at through research or superstition) that your bathroom is inadequately ventilated by your HRV, then increase the fan speed or the number of minutes per day that your ventilation system operates. In other words, change the program.

    7. The whole point of an HRV is that it includes both exhaust ventilation and supply ventilation. It makes no sense to buy an exhaust fan for the bathroom in a house with an HRV.

    8. If you don't believe me, call the tech help hot line of your HRV manufacturer -- or read the manual.

  62. Garth Sproule | | #62


    Here is a quote from John Straube describing a perfect HVAC for a small residential system.

    To provide ventilation, a perfect system would provide separate ducted supply and return air systems. To reduce the energy used to heat or cool this ventilation air, a Heat Recovery Ventilator (HRV) can be used to transfer heat between the supply and exhaust streams as well as to provide the fans needed to move the air. In climates with significant outdoor heat and humidity, an Energy Recovery Ventilator (ERV) would be preferred to pre-cool and pre-dry incoming air using the cool and dry exhaust air.4 Of course point sources of pollutants, such as the kitchen stove and bathroom would have dedicated exhaust fans to remove these pollutants.

  63. Riversong | | #63

    I agree with John Straube on this. A whole-house HRV programmed to move the minimum required ACH for indoor air quality, and exhausting 150-200 cfm from multiple rooms simultaneously, is not going to be able to adequately evacuate the water vapor from a shower, certainly not as well as a 100 cfm dedicated bath exhaust fan.

    A boost switch helps, but reprogramming to increase the run time and whole house air exchange beyond what is necessary for baseline levels is counterproductive to the goal of saving energy.

    And relying on the tech line or the manual of the HRV manufacturer is like relying on the spray foam salesman to give you the straight dope on the "greenness" or effectiveness of petrochemical foam.

  64. John D | | #64


    I'm trying to figure out whether or not the HRV system in my mom's new townhouse condo is workng properly. There's a bathroom on level 4 and level 5 of the home and they each have a vent with a boost button. If the boost is activated should a piece of tissue paper be held in place by this extra suction?


  65. Riversong | | #65

    If the boost is activated should a piece of tissue paper be held in place by this extra suction?

    It depends on whether it's one-ply or two-ply.

  66. GBA Editor
    Martin Holladay | | #66


  67. John D | | #67

    Thanks Martin. The contractor who installed it maintains the HRV system works fine but we have our doubts.

  68. Mike S. | | #68

    Hi there,
    I recently purchased a spec home installed with an HRV. I'm finding that the bathrooms vent moisture horribly. Should I constantly have my Van EE system running on max so that when I turn on the "boost" switch in the bathroom it will vent better? The home builder suggested only having it on when the house is full with people, this way it isn't constantly running. This is what I'm confused about, should they have installed seperate exhaust only fans in the washrooms? I think I would've preferred this. I have regular inspections every 3 months for the first year and am thinking of bringing this up next time. Thanks in advance for any advice.

  69. GBA Editor
    Martin Holladay | | #69

    It's hard to diagnose the problem without knowing the cfm of your bath exhaust. Was the system commissioned? Ideally, a technician needs to measure the airflow of your bath exhaust in normal mode and "boost" mode.

    Several factors could affect the airflow, including a problem with a damper, a problem with the ductwork, or a problem with the way the system was balanced.

  70. mRM6vGQVCS | | #70

    Hi Martin, unfortunately I'm not sure if the system was commissioned or what the cfm of the washrooms is. I'll have to bring this up with the home builder at our next inspection and ask then. Thanks!

  71. Riversong | | #71


    An HRV is designed to run 24/7 on low speed to maintain the code-minimum indoor air exchange necessary both for occupant health and for controlling humidity and mold. The bathroom boost switch is designed to give additional exhaust flow when showering, though it's rarely as effective as a stand-along bath exhaust fan.

  72. user-917907 | | #72

    ...Drying closet details...uh, in a nutshell a "closet" in the laundry which is sealed, insulated, has a floor drain, moisture protection, an air intake coming in high and a exhaust going out low, a timer loop and lots of cloth line and a rod. I will write a detailed report about this sometime this winter (when things are slowed down) including construction details and performance data.
    Answered by Thorsten Chlupp
    Posted Wed, 07/08/2009

    Does anyone know if Thorsten Chlupp ever posted the promised report about construction details of drying closets? If not, does anyone know how to contact him?

  73. Kopper37 | | #73

    Jack - I'm not aware of anything more from Thorsten on the drying closet, but you can find more discussion on this technique right here:

  74. user-871583 | | #74

    Robert's comment that a HRV is ineffective at exhausting moisture from baths is inaccurate. I have talked to many people with properly commissioned systems that have no problems. A properly designed system will entail dampers that shut down the rest of the house when a specific bathroom(s) is on boost to concentrate all of the HRV's suction to that/those bathroom(s). If Robert does not like this added complexity then I sure hope he is driving a horse and buggy as opposed to a modern automobile so as to not appear hypocritical. Or how about ice chest over a modern efficient refrigerator. Technology is not the enemy.

  75. user-5975040 | | #75

    For @Sean - you said: "A properly designed system will entail dampers that shut down the rest of the house when a specific bathroom(s) is on boost to concentrate all of the HRV's suction to that/those bathroom(s)".

    Is this common in HRV systems? To use dampers to focus on a specific room during the boost?

  76. GBA Editor
    Martin Holladay | | #76

    Your question is addressed in my article, Does a Home with an HRV Also Need Bath Fans?

    In that article, I wrote, "Some builders have wondered whether it’s possible to install motorized dampers in exhaust ducts [of an HRV system], 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."

  77. charlie_sullivan | | #77

    Pothier essentially says that it wouldn't work well if it was done poorly. But there's no reason one couldn't do it right, with the ducts sized sized large enough for it to work well. It would be a tricky design, because of the complication of making sure it's reasonably well balanced in all the different modes. By the time you pay for the engineering of that as well as the larger ducts, the dampers, and their wiring, it will be an expensive system, so I'm not recommending it, just clarifying that it could be made to work if you really wanted to do it.

  78. Chaubenee | | #78

    Robert Riversing, as I understand or recollect, you are advocate of a simple passive air inlet? There is another thread where the gent is asking about make up air for bath fans, kitchen ducted hood and bath fans. It seems like everyone is discouraging him entirely from coming up with a make up air solution. I mentioned one from Honeywell where there is a barometric switch that senses pressure change and responded by opening damper to make up air outside duct (which leads to furnace return, or not.)

  79. Expert Member
    MALCOLM TAYLOR | | #79

    Robert probably can't answer you. I think he was banned from GBA about five years ago.

  80. jimblodgett | | #80

    Strange to see this discussion come back to life after all these years. Pretty good discussion. We never did build the house for our daughter that brought this question up in the first place. We did build a super insulated house for our son recently and the bathroom exhausts are in fact part of the HRV system. They plan to move into the house before winter; I'll try to remember to come back and post how well the system works.

    On a separate note, one question I asked somewhere back in the middle of this conversation was about make up air for the kitchen exhaust fan if that couldn't be part of the HRV system. In our son's house we provided fresh air intake in the cabinet toe space adjacent to the stove - same diameter as the stove exhaust fan duct. That should do it.

  81. BillDietze | | #81

    Banned? I thought he left in a state of high dudgeon (he also had a great vocabulary - so that's my tribute to him I guess). Riversong's passion, and the responses to same, got me hooked in on this GBA site. Rats.

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