CSLB C-20 HVAC ContractorPlanning and LayoutHard
A return air grille measuring 20 inches by 20 inches has a manufacturer-rated free area of 75%. To keep the return air face velocity at or below the recommended maximum of 500 feet per minute for noise control, what is the maximum CFM this grille should handle?
- A1,042 CFM
- B2,000 CFM
- C694 CFM
- D1,389 CFM
Show answer & explanationAnswer & explanation
Correct answer: A. 1,042 CFM
Gross grille area = 20 in × 20 in = 400 sq in = 2.78 sq ft. Free area = 2.78 × 0.75 = 2.083 sq ft. Maximum CFM = Velocity × Free Area = 500 fpm × 2.083 sq ft ≈ 1,042 CFM.
Why the other options are wrong
- B. This overstates airflow well beyond the grille's rated free area capacity.
- C. This value is roughly half the correct answer, from a doubling error.
- D. This value results from using gross area without applying the 75% free area factor.
Return Grille CFM Sizing
Return grille capacity is found by multiplying the recommended face velocity by the grille's effective free area (gross area × free area percentage), not gross area alone.
- Free Area = Gross Area × Free Area %
- CFM = Velocity × Free Area
- Typical return grille max velocity ~500 fpm for noise control
Memory trick: Only the open holes carry the air — apply the free area factor first.