CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A technician is sizing a return air grille for a residential system to avoid customer noise complaints. Per common industry guidance (ACCA Manual D), what is the generally recommended maximum face velocity through a return air grille?
- A800 FPM
- B1,200 FPM
- C500 FPM
- D300 FPM
Show answer & explanationAnswer & explanation
Correct answer: C. 500 FPM
Return grilles are typically sized to limit face velocity to about 500 FPM to keep noise levels acceptable in occupied spaces; higher velocities create noticeable air noise.
Why the other options are wrong
- A. Too high; will generate noticeable whistling/noise.
- B. Far exceeds acceptable noise limits for residential returns.
- D. Overly conservative; would require an unnecessarily large grille.
Return Grille Face Velocity
The recommended maximum face velocity for residential return grilles is about 500 FPM to control noise.
- Higher velocity = more noise at the grille.
- Grille free area, not duct area, determines face velocity.
- Undersized grilles are a common cause of noise complaints.
Memory trick: Five hundred keeps it quiet.