CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A technician installs a return filter grille with a net free filter opening of 16 inches by 20 inches to handle 1,000 CFM of return air. What is the approximate filter face velocity, and does it fall within the commonly recommended maximum of about 300 fpm for extended filter life and low pressure drop?
- A225 fpm; well below the recommended maximum
- B450 fpm; within the recommended range
- C450 fpm; exceeds the recommended maximum, so a larger filter area should be installed
- D300 fpm; exactly at the recommended maximum
Show answer & explanationAnswer & explanation
Correct answer: C. 450 fpm; exceeds the recommended maximum, so a larger filter area should be installed
Net filter area = (16 x 20)/144 = 2.22 sq ft. Face velocity = 1,000 CFM / 2.22 sq ft ≈ 450 fpm, well above the commonly recommended maximum of about 300 fpm. Exceeding this velocity increases pressure drop, reduces filtration efficiency, and shortens filter life, so a larger filter face area is needed.
Why the other options are wrong
- A. 225 fpm understates the actual calculated velocity.
- B. 450 fpm is correct but it does NOT fall within the recommended range.
- D. 300 fpm is the guideline limit, not the calculated result here.
Filter Face Velocity
The air velocity passing through a filter's net free area, calculated as CFM divided by filter area in square feet; excessive velocity increases pressure drop and reduces filter effectiveness.
- Face velocity = CFM / filter area (sq ft)
- Recommended max is generally around 300 fpm
- Undersized filters cause high static pressure and reduced airflow
Memory trick: Small filter, big rush — velocity crushes the airflow.