Air changes per hour. ACH equals airflow in cubic feet per hour divided by room volume, or cfm times 60 divided by volume. It is used two ways: for supply or exhaust rates in spaces such as hospital rooms and labs, and for building tightness, where ACH50 is the leakage measured with a blower door at 50 pascals of pressure difference. Natural ACH estimates leakage under normal conditions.
Isolation room needs 12 ACH and negative pressure, we're only getting 9, the exhaust fan belt is slipping.
Know which ACH someone means: ACH50 from a blower door is roughly 15 to 20 times the natural rate in a typical house.
Also heard as
- air changes per hour
- air change rate
Related terms
Infiltration
Outdoor air that sneaks into a building through gaps in its shell, pushed by wind, stack effect and fans rather than by design.
Ventilation rate
The amount of outdoor air deliberately brought into a space to dilute indoor contaminants, usually in cfm per person and per square foot.
Building pressurization
Controlling the air pressure inside a building relative to outdoors, usually by bringing in more outdoor air than is exhausted.
CFM
Cubic feet per minute: the standard unit of airflow through equipment, ducts and outlets.