CSLB C-20 HVAC ContractorPlanning and LayoutHard
A Manual D duct design must deliver a total system airflow of 1,500 CFM to match the equipment's rated output. A specific bedroom has a calculated heat load of 3,000 Btu/hr out of a whole-house total load of 45,000 Btu/hr. Using the proportional load-based method for branch duct airflow allocation, how much airflow should be routed to this bedroom's branch duct?
- A75 CFM
- B200 CFM
- C100 CFM
- D150 CFM
Show answer & explanationAnswer & explanation
Correct answer: C. 100 CFM
Room CFM = (Room Load ÷ Total Load) × Total System CFM = (3,000 ÷ 45,000) × 1,500 = 0.0667 × 1,500 = 100 CFM.
Why the other options are wrong
- A. Understates the proportional share by using an incorrect ratio.
- B. Double the correct value; likely from doubling the load ratio in error.
- D. Overstates airflow, roughly 50% higher than the correct proportional value.
Proportional Branch CFM Allocation
In Manual D duct design, each room's branch airflow is allocated proportionally to its share of the whole-house heat load relative to total system airflow.
- Room CFM = (Room Load / Total Load) × Total System CFM
- Ensures each space receives conditioning matched to its load
- Prevents over- or under-conditioning individual rooms
Memory trick: Give each room its fair share of the total air pie.