CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A mechanical ventilation system delivers a continuous 90 CFM to a bedroom with a total volume of 4,500 cubic feet. What is the air changes per hour (ACH) for this room?
- A2.4 ACH
- B6.0 ACH
- C1.2 ACH
- D0.5 ACH
Show answer & explanationAnswer & explanation
Correct answer: C. 1.2 ACH
ACH = (CFM × 60) ÷ room volume = (90 × 60) ÷ 4,500 = 5,400 ÷ 4,500 = 1.2 ACH.
Why the other options are wrong
- A. Doubles the correct value by an arithmetic error.
- B. Overestimates significantly; not consistent with the given CFM and volume.
- D. Underestimates; does not correctly convert CFM to hourly volume.
Air Changes Per Hour (ACH)
ACH measures how many times the total air volume of a space is replaced in one hour, calculated as (CFM × 60) ÷ room volume.
- Higher ACH means more frequent air replacement.
- Used to evaluate ventilation adequacy and IAQ.
- Formula: ACH = (CFM × 60) / Volume (cu ft).
Memory trick: Sixty minutes times CFM, divided by the room's whole scene.