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Air flows (cfm) – Superinsulated / Super-tight home

T_Barker | Posted in Energy Efficiency and Durability on

After some review it seems that typical heating/cooling air flows in a super insulated / super tight (0.5 ACH50) home can be in the range of 20 cfm/100 sq.ft. (e.g. 400 cfm for a 2,000 sq. ft. home). For the same home/occupancy, ERV/HRV air flow calculations are in the 100-200 cfm range. So no longer a 10x difference. So why all the talk that these two systems still need to be separate?

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  1. GBA Editor
    Martin Holladay | | #1

    T. Barker,
    There's a lot to say on this issue, but one point is that the cfm for forced-air heating may be much less than the cfm for forced-air AC. I quoted Gavin Healey on this topic in one of my articles ("Finally, a Right-Sized Furnace"). Healy said,

    "Say you have the smallest [Dettson] furnace [the smallest furnace now on the market], and heating air flow that allows you to be at 500 or 700 cfm of air. If you wanted a 2-ton cooling load, you might want 1,200 cfm for cooling in California. So to get that air flow, you might be forced to choose the next larger sized furnace in terms of Btu/h, just to get the desired air flow for cooling performance.”

    From my experience, 200 cfm of ventilation air is quite high for a 2,000 sf home -- 75 cfm to 100 cfm is much more likely.

  2. T_Barker | | #2

    Yes, good point about the cooling cfm in some locations.

  3. Expert Member
    Dana Dorsett | | #3

    ASHRAE calls out 0.03 cfm per square foot + 7.5 cfm per occupant. Occupancy is defined as 1+ the number of bedrooms, ergo a 2 bedroom house is defined as a 3 occupant house.

    So 0.03 x 2000' is 60 cfm. 3 x 7.5 = 22.5 cfm, for a total of 82.5 cfm.

    A 4BR house would add only 15 cfm, bringing it up to 97.5 cfm.

    Building Science Corportation considers that excessive, overkill, and has proposed dropping that to 0.01 cfm per square foot + 7.5 cfm per occupant. That knocks 40cfm off the total, so even a 4 BR 2000' house would only need 57.5 cfm.

    A 2 BR would only need 42.5 cfm of continuous ventilation (per BSC).

    That's an order of magnitude lower in range than the heating/cooling cfm range.

    From a ventilation design point of view it's worth considering installing the minimum that meets ASHRAE, but from an operational point of view try running it at the BSC levels, using the REAL occupancy number rather than defaulting to # bedrooms +1, and only adjust upward if/when it seems needed.

  4. this_page_left_blank | | #4

    My limited experience leads me to question BSC's numbers. My house is 2400 sq ft, three occupants (one of whom is a toddler). I've tried running our HRV at sub 50cfm rates, and the CO2 concentration very rapidly rises. I've never let it get much over 800ppm before changing the setting, but I have little doubt that it would get well over 1000ppm within a day or less. I could not imagine running it at anything near this level on a regular basis.

    1. Expert Member
      Dana Dorsett | | #5

      Trevor: I'm curious, do you cook or heat water with with gas? Are there any other known sources of CO2 other than the human occupants (dogs, candles, a home beer fermenting crock, perhaps)?

      Are bath fans & kitchen exhaust always used, supplementing the HRV's daily air exchange?

      Also, where do you typically set the HRV's cfm?

      For 3 occupants the BSC minimum for a 2400' three occupant house is just barely below 50cfm, at 46.5cfm, not a lot below. ASHRAE would call for 94.5 cfm. I suspect your average is somewhere between those numbers, but the real number would be a good data point.

      1. this_page_left_blank | | #9

        The house is all electric, no pets or brewing (at least during the periods I am describing).

        We have no bath or kitchen fans. I wasn't aware that the CFM prescriptions were counting on this supplementation. We do run the HRV on boost (~180CFM) periodically for humidity and odour exhaustion, but it's pretty minimal.

        I don't have a very accurate way of measuring the CFM. My estimates are based on the HRV specs, power draw, duct design and the CMHC "garbage" bag test, which fairly well agree with each other. I think the standard setting is about 100CFM. I may be way off, but I'd be pretty surprised if I'm off by half.

  5. Jon_R | | #6

    Whether the CFM ratio is 12:1 or 4:1 doesn't make a big difference. Both are technically possible but not common.

    A furnace with a fan range that is efficiently "HRV compatible" sounds like a good selling point to me. HRVs could overcome furnace duct pressures and auto balance (ie, duct sharing without balance/flow problems is possible).

  6. T_Barker | | #7

    I just ran across this in one of the GBA articles from a few years ago:

    "Dr. Feist in December 2007, used the same definition for a Passivhaus that is enshrined on Passipedia: 'As long as you build a house in a way that you can use the heat-recovery ventilation system — a system that you need anyway for indoor air requirements — to provide the heat and cooling, it can be considered a Passivhaus.' "

    So I'm not crazy after all!

    1. Expert Member
      Dana Dorsett | | #11

      Sure you are! ;-)

      It's actually KINDA crazy to be discussing the heating solution and it's cfm requirements before even so much as calculating the heat load of the arbitrary "... super insulated / super tight..." house.

      In houses that can be heated with ~100 cfm air flows a ducted & cooling heating solution isn't necessarily the go-to approach. Houses with loads that low would tend to be good candidates for point source heating, and tying it into the ventilation system just makes it more complicated & expensive.

      There are many ways where a Feist-ian approach (and not just on HVAC considerations) isn't well applied to North American climates.

  7. GBA Editor
    Martin Holladay | | #8

    T. Barker,
    The problems with Dr. Feist's definition of Passivhaus are the following:

    1. Using ventilation air for heating barely works in Germany, and simply won't work in most of North America, especially if you follow Feist's insistence that 100% of the air coming out of your supply registers should be outdoor air, with no air recirculation. (What's wrong with recirculation? Ask Feist -- he cringes when anyone suggests it.) The colder the climate, the harder it is to condition a house using only ventilation air.

    2. One result of Feist's arbitrary rule is to encourage overventilation, a "solution" with all kinds of ancillary problems -- especially when outdoor air is humid.

  8. Yupster | | #10

    T. Barker, you might be interested in this product from Lifebreath, a combined hrv and hydronic air handler. https://www.lifebreath.com/product/clean-air-furnace-8/
    Not quite what you are talking about but the appliances are combined anyway!

  9. Jon_R | | #12

    Nice proof that it's possible, although it appears that HRV CFM changes quite a bit with heating fan CFM (they should be independent). A design with automated dampers could fix this.

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