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Community and Q&A

Seeking ductless fresh air supply for balanced ventilation

vivo | Posted in Mechanicals on

Can anyone recommend how to balance ventilation for fresh air supply into open concept space up/down duplex of 700/1100 ft2 respectively where there are only interior walls are for bathrooms and bedrooms and where ceiling fans and inline fans ensure air movement and recirc? we are trying to avoid ductwork and we have sufficient exhaust with fans in bathrooms and kitchen, but since house will have tight envelope, i’m a little concerned about fresh air supply. house is design for good passive ventilation, but in event house is shut up in winter or occupants are vacationing, i’m concerned about house breathing sufficiently. building inspector wants me to cut a hole in the wall behind the fridge to ensure i get the 2x airchanges i require which seems counterintuitive to build a tight envelope with minimarl air leakage for a controlled environment and then go punch a hole in wall of 6-8 inches in diameter for untempered fresh air supply.
what about air exchanger or something with a damper activated mechnically or by sensor re humidity, ok maybe losing out on heat recovery, but would exhaust and supply air and save on installation of ducting?
please help with any and all suggestions to achieve balanced and controllable ventilation and fresh air supply that is cost effective and practical for small, open concept up/down duplex in the pacific northwest;Victoria, BC aver winter temp 41 degrees, heating dominant, no cooling required in summer.

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  1. GBA Editor
    Martin Holladay | | #1

    There is nothing in the code to support your building inspector's suggestion to cut a hole in the wall behind your refrigerator. You should respectfully ask your inspector to cite the code provision supporting the suggestion.

    The only "ductless fresh air supply" method that I am aware of uses passive air inlets (for example, the Fresh 80 from or the ASV-90 from

    However, research has shown that these passive air inlets are unnecessary and counterproductive. To read more about passive air inlets, visit

    You wrote, "we have sufficient exhaust with fans in bathrooms and kitchen." Assuming that these exhaust fans are equipped with appropriate controls — for example, timers that operate the fans frequently enough to assure that the house is ventilated at the rate recommended by ASHRAE 62.2 — what you have is an exhaust-only ventilation system. Such a ventilation system is a perfectly adequate method for ventilating a house.

    Fear not — fresh air will enter your house to replace the air that leaves your house when the fans are operating. This rule has no exceptions: air in = air out. The air will enter through the random cracks that exist in all homes, even those built with an attention to airtightness.

    If you are concerned about the distribution of fresh air and you want to upgrade your system, you should install a heat-recovery ventilator (HRV) with dedicated ventilation ductwork. The ductwork can be boxed in so that its presence is less obtrusive.

  2. Riversong | | #2


    You don't need (or want) 2 ACH - current standards require about 0.25-0.35 ACH (depending on occupancy load). Martin is correct that you need a programmable timer to control your background ventilation rate, but he's wrong that passive air inlets are "unnecessary and counterproductive".

    If you read the "Musings" article he linked to, you'll note that the primary criticism of exhaust-only ventilation systems is the lack of distribution of fresh air to the spaces where it's needed: bedrooms and living rooms.

    Relying on natural leakage does not solve that problem. But installing passive make-up air inlets (which can be fully passive or humidity-controlled or occupant-sensing), directs the fresh air to where it's needed.

    The study that Martin continues to point to (read my comment below the article) is a highly-flawed study that looked only at leaky, poorly-constructed houses. If your house is built tightly, then it would certainly benefit from passive air inlets (I use the Aldes Airlet 100).

  3. Michael Chandler | | #3

    I use the cape back-draft dampers that I get from R E Williams supply with a six inch exterior hood with a filter cut from an expanded metal range hood filter. I put the inlet behind the dryer rather than the fridge but either is a good alternative.

    Hood testing of these make up air dampers done during my Energy Star / Builders Challenge reviews show that 50% of the make up air from my bathfans is coming in this way and the balance is distributed from other points.

    I have a short piece on how we do this at my website which includes sources of supply. I no longer use the Cooper 6109 time delay motion sensors for my bath fans though. They don't seem to hold up over time so I'm trying out some other units. WattStopper Legrand is getting good reviews so far.

  4. David W. Bearg, PE, CIH | | #4

    While a previous poster has stated that: fresh air will enter your house to replace the air that leaves your house when the fans are operating. This rule has no exceptions: air in = air out. The air will enter through the random cracks that exist in all homes, even those built with an attention to airtightness,
    I feel it is important to mention an important characteristic of the exhaust fans: The greater the pressure drop the fan is working against, the less air that actually gets moved. I therefore recommend that there be a dedicated pathway, with minimal resistance, for the replacement air to be drawn into the building when the exhaust fans are on. One option is to have a duct that penetrates to the outdoors and yet terminates facing downward in a bucket. In this approach the bucket fills with cold air but doesn't allow air to come in, unless it is drawn in by the action of an exhaust fan.

  5. GBA Editor
    Martin Holladay | | #5

    What makes you think that if "a duct penetrates to the outdoors and yet terminates facing downward in a bucket" that such a system "doesn't allow air to come in unless it is drawn in by the action of an exhaust fan"?

    I can assure you, several moving forces will draw air into the house through such a hole in the wall -- not just "the action of an exhaust fan." All are common. Here are three:
    1. The stack effect.
    2. Wind.
    3. An unbalanced furnace or air conditioner with supply duct leaks that leak to the home's exterior.

    Any time you make a hole in your house, you will increase the air leakage rate. By putting a hole leaking to a bucket in your basement or first floor, you are making it easier for air to leave your ceiling via the stack effect.

  6. Allison A. Bailes III | | #6

    Using negative pressure ventilation without dedicated makeup air is not a good idea. Yes, air will come in through the "random cracks that exist in all homes," but you don't want it coming in through random leaks. You want to know that what's coming in is really 'fresh' air; what comes from a garage, unconditioned basement, or attic is probably not the kind of air you want to bring in.

    Actually, there is an exception to air in = air out rule, and David Bearg alluded to it. In a tight house, when the fan first comes on, it will exhaust some of the house air without pulling in an equal amount from outside. (If this exception didn't exist, there would be no such thing as a vacuum chamber.) This creates a pressure difference that the fan has to fight against, and when doing so, it won't move as much air, as David says. So, makeup air is essential for that reason as well.

    Since you're in a one-way heating climate, Allison, negative pressure ventilation is an option for you. For those reading this who are in hot or mixed humid climates, however, you should go with balanced or positive pressure ventilation because you don't want to suck in untempered/unconditioned air with all its humidity.

    If you want a balanced system with minimal ducting, I suggest the Panasonic Whisper Comfort. This is an inexpensive ERV that requires only two ducts (both to the outside), rather than the usual four (two to outside, two to inside). You can run them at 10, 20, or 40 cfm, and one on each floor of your duplex should easily meet your ASHRAE 62.2 required ventilation rate.

  7. GBA Editor
    Martin Holladay | | #7

    Anyone who wants to be sure they don't get fresh air through random leaks — which is evidently one of your concerns — should install an HRV or ERV with dedicated ductwork.

    If you are installing an exhaust-only ventilation system, then peppering your house with holes — I'm sorry, with "passive makeup air vents" — is no assurance that the holes will be used by the incoming air. Researchers have confirmed that air will continue to enter the house through random cracks whether you like it or not; moreover, the holes you have installed are just as likely to act as air outlets as they are to act as air inlets — especially if you put your holes on the 2nd floor of a two-story house, where many bedrooms are located.

  8. Allison A. Bailes III | | #8

    You're absolutely right, Martin. Air will always take the path of least resistance, which is not necessarily the intended path. (See the black insulation beneath the baffles in vaulted ceilings for proof.) Living in and working mainly with hot and mixed humid climates, I never recommend negative pressure ventilation. This discussion now makes me realize that I probably wouldn't recommend it in one-way heating climates either. Thanks!

  9. David W. Bearg, PE, CIH | | #9


    I'm confused, on hand the house in question has had attention to airtightness, but the examples of driving forces all denote a leaky situation:

    1. The stack effect. For stack effect there needs to be a pathway for the buoyant air to leak out.
    2. Wind. If the windows are open, what's so bad about air entering the duct.
    3. An unbalanced furnace or air conditioner with supply duct leaks that leak to the home's exterior. Another leak ! ! !

    My recommendation is to reduce UNINTENTIONAL air movement across the building envelope, while making the INTENTIONAL air movement, to dilute and remove air contaminants, to be operating at the highest CFM per watt rating of the fans.


  10. GBA Editor
    Martin Holladay | | #10

    You're right -- a well built house will have a lower rate of air leakage than a poorly built house. But all houses leak. That's why we test them with blower doors -- to measure the leakage rate.

    When it's cold outside, the stack effect occurs in all houses. Wind affects every house as well; when the wind is blowing -- even if all windows are closed -- one side of the house is under positive pressure, while the opposite (leeward) side of the house is under negative pressure. The negative pressure sucks conditioned air through the walls on the leeward side; while the positive pressure forces exterior through the wall on the side exposed to the wind.

    Of the three problems I mentioned, the problem of leaky ducts is easiest to prevent.

    However, once you introduce an open outdoor air duct into your home -- basically, a hole in the wall -- there is a very easy path for outdoor air to enter. Of course, that hole makes it easier for air to exit from the high-pressure areas of your home. Those high pressure areas can be found near the ceiling (due to the stack effect) and on the leeward side of the home during a windy day.

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