Which statement about ACH and room volume is true?

• A. ACH does not depend on room volume
• B. ACH is calculated as (CFM × 60) / room volume
• C. ACH increases when room volume increases with fixed CFM
• D. ACH is the same as CFM

Answer: (B)

Air changes per hour tells you how many times the entire volume of air in a room is replaced each hour. To get that value, you link how much air is moved each minute (CFM) to how big the room is (volume in cubic feet). Multiply CFM by 60 to convert to cubic feet per hour, then divide by the room volume. The result is ACH.

Why this fits best: you can see clearly how room size and airflow interact. If you push the same amount of air into a larger room, it takes longer to refresh the space, so ACH goes down. If the room is smaller with the same airflow, ACH goes up. The formula (CFM × 60) / room volume captures this directly. For example, a 10,000 ft³ room with 500 CFM yields 3 ACH; doubling the room to 20,000 ft³ with the same 500 CFM gives 1.5 ACH.

Why the other statements aren’t correct: ACH does depend on room volume, so saying it doesn’t is not true. With fixed CFM, increasing room volume actually reduces ACH, not increases. And ACH is not the same as CFM—the former is air changes per hour, the latter is volume of air moved per minute.