Air Changes per Hour (ACH): What It Means

What an air change per hour actually means#

One air change per hour means the unit processes a volume of air equal to the room's volume in an hour. Five air changes per hour means it does that five times. It is a rate of turnover for a specific room.

The catch is in the word "specific". ACH is meaningless until a room volume is fixed, the same machine is four ACH in one room and two ACH in a room twice the size.

Why the metric is already inside CADR#

CADR, clean air delivery rate in cubic feet per minute, is the delivery half of the same idea. Given a room size, delivery and air changes are two readings of one number.

That is exactly how EPA's published table works: a minimum CADR for each room area, calculated on an eight-foot ceiling, already folds the air-change logic into the figure. You do not need ACH to use the table; you need the table.

Does it matter?#

It is a background metric, useful for understanding, poor for shopping. ACH quoted in a listing without a delivery rate is arithmetic dressed as a specification: pick a flattering room size and the multiple improves.

Do not buy on ACH claims. Ask for the CADR instead, set it against the published minimum for your room, and the ACH will take care of itself.

What to check on the box#

• Whether an ACH claim comes with the CADR figure it was derived from.
• Which room size the claim assumes. Without one, the number is decoration.
• A published CADR you can set against EPA's minimum for your actual room.
• A certain suspicion of any multiplier that gets more impressive as the room gets smaller.