CSLB C-20 HVAC ContractorPlanning and LayoutHard
A Manual D duct design specifies a total system airflow of 1,200 CFM distributed among five supply branches. Four branches are measured after installation and deliver 220, 180, 260, and 240 CFM respectively. Assuming no duct leakage, what CFM must the fifth branch deliver to match the design total?
- A275 CFM
- B320 CFM
- C300 CFM
- D240 CFM
Show answer & explanationAnswer & explanation
Correct answer: C. 300 CFM
Sum the four known branches: 220 + 180 + 260 + 240 = 900 CFM. Subtract from the total system design airflow: 1,200 − 900 = 300 CFM. This confirms the fifth branch must deliver 300 CFM for the system to balance to the design total.
Why the other options are wrong
- A. 275 CFM does not correctly account for the sum of known branches.
- B. 320 CFM overestimates the remaining required airflow.
- D. 240 CFM matches an existing branch, not the remaining balance.
Room CFM Balancing
The sum of all branch CFM values in a duct system must equal the total system design airflow; any unmeasured branch can be found by subtraction.
- Total CFM = sum of all branch CFMs
- Missing branch CFM = Total − sum of known branches
- Balancing dampers fine-tune actual field measurements to match design
Memory trick: What's left over is what the last branch must deliver.