GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Sizing exhaust only ventilation?

Ine | Posted in Mechanicals on

I am roughing in plumbing and electric and need to add ducts if we choose to use an HRV which is what I had originally planned. The house we are building has radiant floor heat in the basement and an outside air supplied sealed fireplace on the first floor so no ducts yet. Some other costs have exceeded estimates and I am thinking of using an exhaust only system to ventilate the house (Northern Michigan approx. 8000 HDD, R-40 walls, R-60 roof, hopefully fairly tight, about 2400 sf). My wife’s response to this was “so we spend all this time and money building a tight, insulated house and you want to poke holes in the wall to let in cold air?”

So my question is, how much make up air does one need, what’s the best way to let it in and how much does this effect the heating load? Or, if I do end up with the HRV, is it possible for me to install it and have any chance it will perform correctly?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    I'll answer your last question first, "If I do end up with the HRV, is it possible for me to install it and have any chance it will perform correctly?"

    The answer is maybe -- but considering the range of questions you're raising, probably not. It may be time to hire an experienced installer of ventilation equipment.

    The most common way of sizing ventilation systems and establishing a ventilation rate is to meet the requirements of ASHRAE 62.2.

    Exhaust-only ventilation systems do not necessarily require passive air inlets.

    Here is some more information on these topics:

  2. Ine | | #2


    Let me rephrase my last question. I've studied all the ventilation info on GBA as well as other sites on line and know what I want to achieve but I don't have a full compliment of test equipment. I have installed woodworking dust collection equipment and understand duct sizing, estimating cfm based on diameter, run length, bends, etc. When it is installed, if it collects most of the dust I know it works but, without test equipment, can one do a reasonable job on an HRV or does it need to be perfectly balanced to perform well? In the second link above, you gave this advice: "2. In your own home, experiment with lower ventilation rates by adjusting the programming of your ventilation equipment. If you have no complaints — no condensation on your windows during the winter, no mold, no stuffiness, and no smelly-sock odors — you're okay." so I guess you're saying that I could play it by trial and error?

    Regarding exhaust only systems, if one doesn't install make up air ducts, how effective is the system if one does achieve a really tight house, like in the 1 to 2 or so ACH range? Wouldn't I want to control where the air is coming from?

  3. GBA Editor
    Martin Holladay | | #3

    1. If it's your own house, you need to (a) meet code, and (b) install a ventilation system that meets your family's needs. Your liability (for a ventilation system that doesn't comply with ASHRAE 62.2, for example) is less than if you were a builder selling homes with bad ventilation equipment. So, go with trial and error, if you want. But the closer you are to ASHRAE 62.2, the better.

    2. If you want to control where the air is coming from, install an HRV.

    3. Concerning passive air ducts -- I'll cut and paste my answer to a previous questioner:
    "Let's consider a 2,550 square foot house with 2.3 ACH @ 50 pa. The house has an envelope surface area of 6,732 square feet. The Equivalent Leakage Area of the infiltration and exfiltration leaks is 128 square inches.

    So this "tight" house already has envelope leaks equivalent to 32 fresh air vents (each of which provides 4 square inches of ventilation). So are 3 or 4 of these devices going to make much of a difference? No -- they are just going to irritate the occupants.

    In other words, even this theoretically "tight" house already has plenty of random leaks — more than enough to provide makeup air for the exhaust fans."

  4. Riversong | | #4


    A house in the 1-2 ACH50 range will have an Effective Leakage Area (which is the accepted US standard at 4 Pa, as opposed to the Canadian Equivalent Leakage Area at 10 Pa) of about 1.25 in² per 100 sf of surface area. This is a house much too tight for natural ventilation and I would say also too tight to rely on leaks for fresh air intake.

    Martin is a non-believer in passive fresh air inlets, but I am a strong advocate of them in tight houses with exhaust-only ventilation (which is the only system I've ever used in my super-insulated homes).

    Three things are required of an effective ventilation system:
    1) exhaust air from moisture and pollution sources, including baths, kitchen and laundry
    2) provide fresh air to those places that most need it, including bedrooms, office and living areas
    3) move sufficient air to maintain good indoor air quality and keep relative humidity below 40%

    In a 2400 sf home, assuming 8' ceilings, you would need to move 80 cfm continuously or the equivalent intermittently to maintain 0.25 ACH. Two efficient bathroom exhaust fans can handle that load. I wire Pansonic WhisperLites in parallel with a short-term timer in the bathrooms and a 24-hour programmable timer preset for an average of 0.25 ACH. You may find you can get by with less, but I would start there.

    I prefer the American Aldes Airlet 100 passive air inlets. They're inconspicuous, have a mylar reed valve to slow incoming gusts and a dust/insect filter, and deflect incoming air to the ceiling where it mixes and is not noticed by occupants. You'll likely get only 5-10 cfm air flow per inlet, but you don't need to balance inlet capacity with exhaust flow, since air will move diffusely through envelope leaks and it's preferable to maintain a negative indoor pressure during the heating season as that prevents the exfiltration which invariably leads to condensation and moisture problems.

  5. Michael Chandler | | #5

    I do generally install a passive air intake behind the dryer but my flow hood testing of our houses that lack these inlets shows that air is getting in through distributed leakage to compensate for that flowing out through the bath fans so I feel that Martin is right when he advocates less reliance on passive intake ventilation.

    So, if we are to provide 7.5 cfm per occupant for each minute of the day then that means that we will need to provide 7.5 x 60 = 450 cubic feet per occupant per hour, the equivalent of a 100 CFM bath fan running for 4.5 minutes.

    If the average individual spends 12 hours per day at home and 12 away from home we need to run that bath fan 4.5 minutes times 12 hours or 54 minutes per day to provide adequate fresh air for that occupant. If the house has some occupants such as teenagers or adult children who only occupy the home intermittently there should be a mechanism by which the house can dynamically respond to the variation in occupancy levels.

    Time delay occupancy sensors on bath fans turn out to be an excellent way to allow the home to respond to varying occupancies. We set the master bath fan at a 30 minute time delay on the assumption that it will get triggered on average four times a day by two occupants, we put the kids bath fan on a thirty minute delay and the powder room on a 15 minute delay. This way if the kids are away at college there will be less additional ventilation since their bath will not see any use. If the occupants are on vacation or there is a social event in the home and the powder room bath sees additional activity the house will automatically adjust for that change in occupancy.

    We use Energy Star rated 120 CFM Panasonic bath fans but any Energy Star rated bath fan will do. It is essential to have your energy star certifier verify the output of the fan to confirm the efficacy of your timer calculations. Assume that all bath fans run at 70-80% of rated output and you will be pretty close, even with 6" hard duct to the outdoors.

  6. Doug McEvers | | #6

    I would only consider the use of an HRV in a 8,000 hdd climate, an exhaust only system will work in conjuction with the stack effect to keep the floors cold. If you get much beyond the 2 ach50 for the size house you describe you may not need any additional ventilation. Our house is 3,328 sf with a tested ach50 of 3, I have no provision for fresh air, the windows show no condensation and I have a hard time keeping humidity levels above 25%. Excess ventilation has a tendency to over-dry homes in a cold climate, this should be part of the equation when deciding on a system.

  7. Ine | | #7

    Thanks everyone for the answers. About 700 sf of floor is loft space open to the living room. From a couple of comments above, it sounds like this will change the dynamics of any of these venting options. There are doors on the 2 bedrooms and 2 baths on the main floor, otherwise everything is an open plan.

    Robert, how many of the Aldes Airlets would you put in a house this size?

    Michael, thanks for the example. That gives me some good ideas of how to do the math in the ASHRAE 62.2 standards for my particulars.

    Doug, with an R-60 roof of dense pack cellulose on top of 2" rigid foam that is sealed as tight as we could get, is there going to be much stack effect?

  8. Riversong | | #8


    I put one Airlet in each (modestly sized) bedroom and two in each living area, with at least 4 per 1000sf floor. I also supply combustion air directly to a woodstove and put a 6" supply inlet behind a clothes dryer which is ideally pneumatically isolated from the main living space with a weatherstripped door. With an exhaust-only system, be sure to leave a full 1" of clearance under each door, or use transfer grills or plenums in hallways to allow air to move freely.

    The problem with using envelope leakage as your air supply is the same reason we use mechanical ventilation in the first place: to both insure sufficient hourly or daily air exchange and to control the location of exhaust and inlet air. To rely on leakage of unkown amount and unknown location offers no guaranty that fresh air will be provided where you need it most and that much of it won't simply short-circuit from the nearest leaks directly to the exhaust fan(s).

    According to Lawrence Berkeley's Building Tightness Limits formula, a 2-story 2400sf house in Michigan with 5 occupants would require 3.5 ACH50 in order to be loose enough for natural ventilation to meet ASHRAE minimum requirements. MN and WS, for instance, require a minimum of 1200 and 1500, respectively, CFM50 for natural fresh air exchange (divide those numbers by house volume and multiply by 60 to get required ACH50).

    I don't believe occupant-sensing bathroom fan delays would meet ASHRAE 62.2 requirements, since it requires an assumed bathroom usage for air exchange rather than insuring a minimum daily air flow with bathroom boost..

    A better way to alter air flow with occupancy is to use Airlet 500 humidity-sensing fresh air inlets. Either increased occupancy or other humidity sources will increase the air flow from the normal minimum flow rate. With the exhaust fans on a timer, you insure a minimum flow.

    The Panasonic fans I tested on my last house, with about 6' of 4" hard duct and two elbows, were moving 86% and 92% of the rated flow. And I always duct exhaust fans down at least 3' before going out a sidewall (never through the roof or a soffit), so that cold air can't passively back-siphon and any condensation can drip out.

  9. GBA Editor
    Martin Holladay | | #9

    I'm sure you know this, but readers may not: when you wrote, "According to Lawrence Berkeley's Building Tightness Limits formula, a 2-story 2400sf house in Michigan with 5 occupants would require 3.5 ACH50 in order to be loose enough for natural ventilation to meet ASHRAE minimum requirements," you failed to make it clear that this formula applies to houses without an exhaust ventilation system. Since we are talking about houses with an exhaust ventilation system, the formula is irrelevant.

    We've had this disagreement before, but — since you're raising this issues again — I'll have to respond to two more of your points. You wrote, "The problem with using envelope leakage as your air supply is the same reason we use mechanical ventilation in the first place: to both insure sufficient hourly or daily air exchange and to control the location of exhaust and inlet air."

    Concerning the first point: it is the fan, not the passive air inlets, that ensures sufficiently hourly or daily air exchange. As long as you have the fan, properly commissioned, you have the air exchange.

    And if you think that by installing passive air inlets, you have managed to control the location of inlet air, you are dreaming. Passive air inlets are easily overwhelmed by the stack effect in wind. These inlets — especially those installed in 2nd-floor bedrooms — are just as likely to act as places where air can escape from a house as air inlets.

    I maintain that you have no idea where the fresh air in your house is entering. Air doesn't follow "smart arrows." It still comes in the cracks, whether you want it to or not.

  10. Doug McEvers | | #10


    You have done a great job with the thermal details in your house, the point I was making is this:
    Mechanical ventilation without heat recovery (bathroom fan system) pulls air through the walls and ceilings and this air is cold and will have a tendency to settle near the floors. Now there is a dynamic according to ASHRAE called "infiltration heat recovery" that may be valid and will slightly warm the air as it is pulled through the building envelope from the outdoors.

    I much prefer the HRV/ERV system for a cold climate with the energy recovery and fresh air tempering that comes with it. I think you also get a measure of control with a HRV/ERV that you do not get with the exhaust only system. You can bring in only the amount of fresh air you need based on indoor humidity levels or your own personal comfort. ASHRAE 62.2 is a lot of fresh air and I have asked others far wiser than I if they felt it to be excessive and they said yes. I am not advising you to disregard 62.2, but remember this is the top end of the scale, anything more is energy wasted.

  11. Riversong | | #11


    My comment about the ACH50 minimum for natural ventilation was in response to Doug's statement that "If you get much beyond the 2 ach50 for the size house you describe you may not need any additional ventilation." 2ACH50 is much too tight to not require mechanical ventilation. That was my point.

    Of course it's the fan which "insures sufficient hourly or daily air exchange", but the significant part of the formula in my statement was "and to control the location of exhaust and inlet air."

    Far from "dreaming" about controlling intake location, I have proven it in the homes I've built. The study you've cited in the past about faulty operation of passive air inlets was of leaky and poorly designed homes. In a very tight, well-designed home passive air inlets work. Even if there were a slight stack effect reversal when the exhaust fan was not running (which will maintain some passive air exchange during the off-cycle), when the fan(s) is/are operating a significant portion of the incoming air comes in at the designed locations with envelope leakage making up the rest.

    Most envelope leakage is at sills and wall plates. It's far preferable to have most incoming air at the ceiling rather than at the floor (or at unknown locations). While air may not follow "smart arrows", it flows - like all fluids - in the path of least resistance, proportionally to the relative resistance of each opening. Passive make-up air inlets, while creating sufficient restriction to slow passive air movement (and, with the Airlets, using reed valves to dampen incoming wind), provide sufficiently low resistance to allow a specified volume of air at any particular negative pressure. They have an air-flow curve just like a fan.

    In a 2-storey house at 70° inside and 30° outside, the stack effect pressure would be less than 5 pascals. Any exhaust fan that can provide more than 5 pascals of negative pressure will overcome the stack effect and draw air in through passive inlets.

  12. Riversong | | #12


    Incoming cold air will "settle near the floors" only if it comes in near the floors. If it comes in near the ceiling (and is deflected to the ceiling, as the Airlets are designed to do), then the small volume of air will mix with the warmest air and will neither be felt as a draft or settle to the floor.

    That's why I use and advocate passive air inlets and particularly the Airlets. They place incoming air where it's most needed and least noticed without any of the liabilities of uncontrolled leakage.

  13. Riversong | | #13

    And Doug,

    There is widespread disagreement about whether ASHRAE 62.2 provides too much or too little fresh air. But it was never intended to be a "best practices" standard. Originally it was meant to eliminate body odor in a "typical" house, whatever that is, and to dilute other pollutants and provide makeup air for combustion appliances and clothes dryers.

    In a toxic house with toxic occupants, 62.2 is almost certainly inadequate. In a non-toxic house with non-toxic occupants, it's likely too much air exchange.

    In the houses I've built for the "organic" clients I've built for, I've always aimed for 0.25 ACH mechanical.

    But whatever you might believe about the adequacy or excess of 62.2, that - like all prescriptive codes - is a minimum standard.

  14. Ine | | #14


    You noted in a previous post that you route exhaust fan ducts down at least 3' before venting out a sidewall. Do you do that or was it a typo and you drop them 3" ? I understand you'd want condensation to flow to the outside.

    I saw in an article - I think one in the JLC archives on installing an HRV - where the author used PVC sewer and drain pipe for some venting runs. It seems like this would make for easier air tight pipes but it doesn't seem to be commonly done. Are there reasons most don't use it?

  15. Riversong | | #15


    No typo. With a 3 foot drop before exiting, absent a wind or a negative pressure from another source and a completely defective vent flap, cold air cannot rise back up the duct.

    I don't see any reason why not to use thin-wall plastic pipe, as long as the diameter is the same as the fan outlet, except that galvanized steel ducting is also easy to seal with REAL duct tape (foil mastic) and has thinner walls and can be squeezed into non-conforming spaces.

  16. Doug McEvers | | #16

    In MN I believe the ventilation requirement is 15 cfm per bedroom + 15, so a 3 bedroom house would require 60 cfm continuous. Or house with it's 3 ach50 or .176 nac and 28,288 cubic foot volume would have a ventilation rate of 83 cfm continuous or 23 cfm beyond code.

    One must figure in the infiltration rate for a home and add only enough mechanical ventilation to meet the code, this is why control is critical. In a new house with an HRV we normally run 20 minutes on, 40 minutes off on low speed unless overridden by the humidistat or a remote timer in a bathroom. If the humidity level is above the setpoint the HRV will run 20 minutes per hour on high speed until the setpoint is reached.

  17. Riversong | | #17

    Current ASHRAE standard is 1 cfm/100 sf plus 7.5 cfm/person, which includes a default credit of 2 cfm/100 ft² for infiltration (which is the equivalent of 0.15 ACHnat with 8' ceilings).

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |