An air volume of 420 m3 room with an air supply rate of 840 m3/h and no losses: what is the air changes per hour (ACH), and does it meet a target of 4–6 ACH?

• A. ACH = 2; does not meet the 4–6 ACH target.
• B. ACH = 2; meets the 4–6 ACH target.
• C. ACH = 0.5; does not meet the target.
• D. ACH = 4; meets the target.

Answer: (A)

Air changes per hour measures how many times the room’s air is refreshed in one hour. It is found by dividing the ventilation rate by the room volume. With a supply rate of 840 m3/h and a room volume of 420 m3, and assuming there are no losses, the ACH is 840 ÷ 420 = 2 changes per hour. The target is 4–6 ACH, so an ACH of 2 is below that range and does not meet the target. If there were losses or recirculation, the actual ACH could differ, but under the given conditions the calculation shows 2 ACH and the target is not satisfied.