Air changes per hour
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Air changes per hour(ACH)
A metric of the air-tightness of a structure, often expressed as ACH50, which is the air changes per hour when the house is depressurized to–50 pascals during a blower door test. The term ACHn or NACH refers to “natural” air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. ACHn or NACH is used by many in the residential HVAC industry for system sizing calculations.