CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A branch duct supplies air to a bedroom register in an occupied living space. To avoid objectionable air noise, Manual D recommends the branch duct velocity not exceed approximately which of the following?
- A1,200 feet per minute
- B900 feet per minute
- C600 feet per minute
- D300 feet per minute
Show answer & explanationAnswer & explanation
Correct answer: C. 600 feet per minute
Manual D recommends limiting branch duct velocities in occupied spaces to around 600 fpm to prevent noticeable air noise at registers, while trunk ducts in occupied spaces can run slightly higher (700-900 fpm) since they are farther from occupants.
Why the other options are wrong
- A. Too high; this velocity would create excessive register noise.
- B. This is closer to the acceptable trunk duct velocity, not branch duct.
- D. Unnecessarily conservative and would require oversized ductwork.
Duct Velocity Noise Limits
Manual D recommends maximum air velocities in occupied spaces to prevent noise: about 600 fpm for branch ducts and 700-900 fpm for trunk ducts.
- Branch ducts near occupants need lower velocity
- Trunk ducts can run faster since farther from occupants
- Excessive velocity causes register whistling/noise
Memory trick: Slow it down near the bed, speed's fine in the trunk instead.