CSLB C-20 HVAC ContractorPlanning and LayoutHard
A zoned duct system has a total design airflow of 1,600 CFM. When only the smallest zone is calling for cooling, its zone damper allows only 600 CFM to the occupied space. To protect the blower and prevent excessive static pressure, a bypass duct routes excess air back to the return plenum. What CFM must the bypass duct be sized to handle under this condition?
- A1,000 CFM
- B2,200 CFM
- C1,600 CFM
- D600 CFM
Show answer & explanationAnswer & explanation
Correct answer: A. 1,000 CFM
The bypass duct must handle the difference between total system airflow and the airflow actually delivered to the open zone: 1,600 CFM − 600 CFM = 1,000 CFM. Sizing the bypass smaller would cause excessive static pressure buildup and blower strain; oversizing wastes capacity and can cause short-cycling of return air.
Why the other options are wrong
- B. 2,200 CFM incorrectly adds rather than subtracts the zone airflow.
- C. 1,600 CFM is the full system airflow, not just the excess.
- D. 600 CFM is the zone airflow itself, not the bypass requirement.
Zoning Bypass Duct Sizing
In zoned systems, a bypass duct is sized to handle the difference between total blower airflow and the airflow delivered when only partial zones are open, preventing excessive static pressure.
- Bypass CFM = Total system CFM − Open zone CFM
- Prevents blower overload when zones close
- Oversized bypass can cause temperature control issues
Memory trick: Bypass carries what the closed zones refuse.