Air Exchanges Per Hour: Natural Infiltration and Exfiltration Rates
A blower door test can go a long way towards identify how “leaky” a house is, but since it runs at a standardized 50 Pa, it’s more about providing a number for comparison than showing how much air infiltration/exfiltration is actually happening at any given time. That is, if the house is at 3 ACH50, then one might assume that all the air in the house is exchanged three times every hour — but that would only be true if the house had a constant 20mph wind blowing on it (give or take). If there is no wind, then it’s almost certainly not exchanging air at anywhere near that rate.
Is there, then a “natural” ACH for a house, either independent of what’s happening outside the house or, barring that, is tied to the average pressure differential in that specific area? And if there is such a natural number, how is it determined?
FWIW, I got to thinking about this when reading up on Conservation Technology’s exhaust ventilation solution: http://conservationtechnology.com/building_ventilation.html. They claim that if a house’s “natural infiltration/exfiltration rate” is 80cfm, then the math shows that their exhaust fan is MORE energy efficient than a corresponding HRV/ERV. How would you calculate/determine that 80cfm in the first place, though?
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