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Community and Q&A

Does ACH relate directly to heating/cooling load?

dsmcn | Posted in General Questions on

If 62.2 requires .35 ACH, does that mean that the entire house would have to be re-heated or re-cooled every three hours or so?

In a video seminar, Katrin Klingenberg said that during a power outage, adequate fresh air could be obtained by opening the windows in a passive house, and there would not be very much heat loss in consequence. She did not specify how many windows, nor how far open.

So assuming mechanical supply (or exhaust) ventilation, but no HRV or ERV, how much penalty is there per air change?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, not all of the heat in the interior of your house is held in the air. There is also heat in your furniture, walls, books, and floors. So even if you ventilate at 0.35 ach, you aren't removing all of the indoor heat every three hours.

    Here are some calculations gleaned from another GBA thread:

    Air has a specific heat of 0.0182 Btu/cf/°F. So each cubic foot of air would require 70 x 0.0182 = 1.274 Btus to warm from zero to 70°F. A 2,000 SF house with 8' ceilings exchanging 0.35 ACH would be losing 7,134 Btu/hr or 1.71 therms per day for ventilation.

  2. Expert Member
    Dana Dorsett | | #2

    The the real heat loss from incidental infiltration turns out to be a bit lower than the simple-model calculation. This is because much of the heat of the exfiltration is given up to the insulation & other materials along the exfiltration path, lowering the local conducted heat load. This heat-exchanger effect isn't super-efficient, but it's still a substantial fraction of the heat loss implied by the simple-math model- not super-easy to measure, but possible.

    For an open window or a mechanical exhaust venting the heat-exchanger effect is a lot lower than say, air moving through fiberglass insulation, and the numbers are much closer to the simple-math model. But with supply-only venting the heat exchanger effect is likely to be a significant fraction of the total, but depends on the actual exfiltration path.

    The thermal mass of the entire volume of air is miniscule compare to even wallboard in the house, so even with the heat loss from ventilation it takes quite awhile for the house to cool substantially, if you're keeping the ventilation rates down to the sub-1 ACH range. How big you have to crack the windows and how many to achieve 0.35ACH depends on things like wind speed, height of house, etc, so there is no single universal number to throw out. It's not exactly a throw-open-the-windows kind of deal though.

  3. GBA Editor
    Martin Holladay | | #3

    For more information on why the energy penalty associated with infiltration and exfiltration is less than might be expected from energy models (or from the calculation method which I provided), see Air Leakage Degrades the Thermal Performance of Walls and Energy Modeling Isn’t Very Accurate.

  4. heidner | | #4

    When Katrin suggested opening a window - that was for a short term event -- a power interruption. Leaving a window open all the time and not using an HRV (or ERV) for balance ventillation is a different story entirely. If you live in Alaska (or northern MN) and its -50F outside... the little window opening can provide a lot of cool air. Open a window the same amount during the winter time in Seattle Washington and the heat loss may be small. Open a similar window during the winter time in Texas, AZ -- without side humidty and temperature above 75F and that little crack may result in an uncomfortable sticky room.

    In all cases the loss(gain) of energy depends on both sensible and latent heat AND the delta's between the inside and outside. Then you can use the HRV/ERV efficiencies.

    For a power outage that might last 100 hours out of 8000 hours in a year. The window ajar is minor compared to the other problems you might be having -- no well water, etc.

    Try to use the manual -- open windows -- just a crack for several years instead of using a HRV/ERV to CONSTANTLY adjust just for the "right amount" of air exchange -- and your life slowly revolves around managing your home environment -- instead the home environment supporting your living experience. A little extreme - but it also shows the value of using a balanced ventilation system and not worrying about short term power outages.

  5. dsmcn | | #5

    Thank you for all the explanations.

    Is the effect identical during cooling periods, when using the AC instead of the furnace? Does drywall, etc. temper the loss?

    I suspect in hot/humid or mixed/humid climates, increased humidity may be more of an issue than the loss of cooled air?

  6. Expert Member
    Dana Dorsett | | #6

    The phyisics of thermal-mass math works the same in either cooling or heating mode, from a sensible-cooling (temperature) point of view.

    In humid climates the latent cooling load (humidity) is always an issue no matter HOW you're ventilating the place (even with an ERV to temper the rate of humidity transfer).

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