If a room has a volume of 3500 ft^3 and the target ACH is 4, what is the outdoor airflow in CFM?

If a room has a volume of 3500 ft^3 and the target ACH is 4, what is the outdoor airflow in CFM?

• A. 140 CFM
• B. 233 CFM
• C. 280 CFM
• D. 400 CFM

Answer: (D)

Air changes per hour tell you how many times the room’s air is replaced in one hour. To turn a target ACH into outdoor airflow in CFM, use the relationship CFM = (ACH × Volume) / 60. The 60 converts hours to minutes, since CFM is cubic feet per minute. Plugging in the numbers: ACH is 4 and the room volume is 3500 ft^3. CFM = (4 × 3500) / 60 = 14000 / 60 ≈ 233 CFM. So the outdoor airflow needed is about 233 CFM. A value like 400 CFM would give roughly 6.9 ACH for this room, higher than the target of 4.

Air changes per hour tell you how many times the room’s air is replaced in one hour. To turn a target ACH into outdoor airflow in CFM, use the relationship CFM = (ACH × Volume) / 60. The 60 converts hours to minutes, since CFM is cubic feet per minute.

Plugging in the numbers: ACH is 4 and the room volume is 3500 ft^3. CFM = (4 × 3500) / 60 = 14000 / 60 ≈ 233 CFM.

So the outdoor airflow needed is about 233 CFM. A value like 400 CFM would give roughly 6.9 ACH for this room, higher than the target of 4.