Ventilation overkill
I am building a home to energy star and LEED standards. We have an ICF basement and crawl space with air and vapor barriers to ground. We have SIP walls and a conventional framed roof. We have a heat pump HVAC and all our combustion units are direct vent and all materials are VOC. We are using an ERV.
The house has a conditioned space of 21,000 cuft and our n factor is 24.5. Ashrae requires 0.35 ACHn, which is over 7000 cft/h. There is only 2 people living in the house and at 15cfm per person is1800 cfh.
Question: in a tight house with minimal pollution injection why ventilate with so much outside air. This seems contrary to energy efficiency. How did they come up with this high ventilation rate, which I assume is a worst case? What would be the best case based on these conditions. My feeling is the 1800cfh (0.85ACHn) should be adequate if achievable.
Any ideas?
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Replies
Paul: You are right; ventilation has nothing to do w/ saving energy. It has a lot to do w/ your health, though, and the health of your house. Humidity needs to go; stale air (dog farts, sweat, whatever you do have that is not great, like rug give-offs, CO2, etc) needs to go. Robert Riversong posted some numbers on the heat capacity of air, and that was very helpful in my getting over the worry of "heat going out the door". It ain't worth worrying about, and fresh air is. A few folks have posted here with a feeling that maybe the ASHRAE cfm is a tad high, so maybe tweaking it a little would be OK. Not knowing for sure, and knowing how little heat goes out with the air, I'm going to stick pretty close to what is suggested, except perhaps when it hits 40 below. Then I may turn the cfm knob down; maybe. Breath well, live long. You likely won't die broke.
The amount of air the house needs is in ASHRAE 62.2, summarized in this reference:
http://www.mainegreenbuilding.com/files/file/Panasonic/ASHRAE%2062_2%20doc.pdf
Your 21,000 sqft, assuming 8 ft ceilings, gives about 2600 sqft of living area. Also assuming three bedrooms, the formula gives 2600/100 +(3+1)*7.5 or 56 cfm.
Thank you guys very helpful.