CSLB C-20 HVAC ContractorPlanning and LayoutMedium
During Manual D duct sizing, a technician selects a friction rate of 0.10 in. wc per 100 ft. Per the duct sizing table for this friction rate, a 6-inch round duct carries approximately 100 CFM and an 8-inch round duct carries approximately 200 CFM. A branch run must deliver 180 CFM to a bedroom register. Which duct size should the technician select?
- A6-inch round duct
- B8-inch round duct
- CReduce the branch design airflow to 100 CFM to match the 6-inch duct
- DTwo 6-inch round ducts run in parallel to the same register
Show answer & explanationAnswer & explanation
Correct answer: B. 8-inch round duct
The 6-inch duct only carries 100 CFM at the chosen friction rate, which is less than the required 180 CFM. The 8-inch duct carries approximately 200 CFM, meeting or exceeding the requirement without exceeding velocity or friction limits, so it is the correct size to select.
Why the other options are wrong
- A. A 6-inch duct is undersized and cannot deliver the required 180 CFM at this friction rate.
- C. Reducing design airflow to fit undersized ductwork compromises the load calculation and comfort.
- D. Running parallel ducts to one register is impractical and not standard Manual D practice.
Duct Sizing Table Rounding
When a required CFM falls between two duct sizes on a friction rate table, always round up to the next larger duct size that meets or exceeds the required airflow.
- Never round down below required CFM
- Rounding up prevents excess velocity and noise
- Duct sizing tables pair friction rate with CFM capacity per size
Memory trick: When in doubt, size it out — always round up on the duct chart.