CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A furnace supplies 1,200 CFM to a rectangular supply plenum. To keep air velocity within the recommended maximum of 700 feet per minute, what is the minimum required cross-sectional area of the plenum?
- A1.71 square feet (about 247 square inches)
- B2.40 square feet
- C3.50 square feet
- D0.58 square feet
Show answer & explanationAnswer & explanation
Correct answer: A. 1.71 square feet (about 247 square inches)
Area = CFM ÷ Velocity = 1,200 CFM ÷ 700 fpm = 1.714 square feet, which equals about 247 square inches. Sizing the plenum with at least this cross-sectional area keeps velocity from exceeding the recommended maximum.
Why the other options are wrong
- B. Too large; would result in velocity below 500 fpm, oversized plenum.
- C. Far oversized relative to the airflow requirement.
- D. This would result from dividing velocity by CFM incorrectly.
Plenum Sizing by Velocity
Supply plenum cross-sectional area is calculated by dividing system airflow (CFM) by the maximum recommended velocity to prevent noise and pressure loss.
- Area (sq ft) = CFM ÷ Velocity (fpm)
- Typical max plenum velocity ~700-900 fpm
- Undersized plenums increase static pressure and noise
Memory trick: Divide the flow by the speed to find the space you need.