Air changes per hour
Air changes per hour is a measure of how many times the air within a defined space (normally a room or house) is replaced.
The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH. In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.
ACH equation in Imperial units
- N = number of air changes per hour
- Q = Volumetric flow rate of air in cubic feet per minute (cfm)
- Vol = Space volume L × W × H, in cubic feet
Air change rate
|This table needs additional citations for verification. (October 2013)|
|Commercial kitchens & Toilets||15–30|
The Passive House standard requires a 0.6 ACH. Many if not most uses of ACH are actually referring to results of a standard blower door test in which 50 pascals of pressure are applied (ACH50), rather than the volume of air changed under normal conditions.
- Bearg, David W. (1993). Indoor Air Quality and HVAC Systems. CRC Press. p. 64. ISBN 0-87371-574-8.
- "International Passive House Association - Guidelines". Retrieved 23 March 2013.
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