CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A residential air handler supplies 1,200 CFM. To avoid excessive noise and static pressure, a sheet-metal return trunk duct should be sized so that air velocity does not exceed 500 FPM. What minimum cross-sectional area should the return trunk duct have?
- A3.6 sq ft
- B0.5 sq ft
- C1.2 sq ft
- D2.4 sq ft
Show answer & explanationAnswer & explanation
Correct answer: D. 2.4 sq ft
Duct area is found by dividing airflow by allowable velocity: Area = CFM ÷ FPM = 1,200 ÷ 500 = 2.4 square feet. This ensures air moves slowly enough through the return trunk to limit noise and static pressure buildup.
Why the other options are wrong
- A. This oversized duct would result in unnecessarily low velocity (333 FPM), wasting material and space.
- B. This area would produce a velocity far above 500 FPM (1,200 ÷ 0.5 = 2,400 FPM), creating excess noise.
- C. This area still results in 1,000 FPM velocity, double the design limit.
Return Trunk Duct Sizing
Trunk ducts are sized so airflow (CFM) divided by cross-sectional area (sq ft) does not exceed a target velocity (FPM) to control noise and pressure loss.
- Area (sq ft) = CFM ÷ Velocity (FPM)
- Typical trunk duct design velocity: 500–700 FPM
- Undersized ducts increase velocity, noise, and static pressure
Memory trick: 'Divide the flow by the speed limit to get the size.'