CSLB C-20 HVAC ContractorVentilation and Air DistributionEasy
A rectangular supply duct measures 8 inches by 14 inches and carries 800 CFM. What is the approximate air velocity inside the duct?
- A1,300 FPM
- B900 FPM
- C700 FPM
- D1,030 FPM
Show answer & explanationAnswer & explanation
Correct answer: D. 1,030 FPM
Duct area = (8 × 14) / 144 = 0.778 sq ft. Velocity = CFM ÷ Area = 800 ÷ 0.778 ≈ 1,030 FPM.
Why the other options are wrong
- A. Too high; would require a smaller duct area than actual.
- B. Close but not the correct division result.
- C. Too low; underestimates the area conversion.
Duct Velocity Calculation
Air velocity in a duct equals CFM divided by the cross-sectional area of the duct in square feet.
- Convert duct dimensions from inches to square feet before dividing.
- Velocity = CFM ÷ Area (sq ft).
- Excess velocity causes noise; too little causes poor mixing and low delivery.
Memory trick: CFM over Area gives you the Air-speed Affair.