Bypass Damper Size Calculator
Bypass duct size for a zoned HVAC system.
Enter your values
ⓘ Live results
Results
- Bypass airflow
- 1,200 CFM
- Bypass duct diameter
- 19.1 in
- Note
- Excess bypass air raises supply temperature; use with caution or prefer variable-speed blower
Quick answer
A bypass duct is sized to handle the difference between full system CFM and the smallest zone's CFM, at a moderate design velocity.
Bypass CFM = total CFM - smallest active zone CFM
How to use the Bypass Damper Size Calculator
- 1Enter the system's total blower CFM and the smallest single zone's CFM.
- 2The bypass must handle the difference when only that small zone is open.
- 3Consider a variable-speed blower with zone control instead of a bypass to avoid coil freeze-up risk.
Worked example
Using the values the calculator loads with:
Inputs
- Total system CFM1600 CFM
- Smallest zone CFM400 CFM
- Bypass velocity600 FPM
Results
- Bypass airflow1,200 CFM
- Bypass duct diameter19.1 in
- NoteExcess bypass air raises supply temperature; use with caution or prefer variable-speed blower
Frequently asked questions
Is a bypass duct always needed?
Not with modern variable-speed equipment that can reduce airflow to match a small zone without a bypass.
What's the risk of an oversized bypass?
Excess air recirculating back to the return can overcool or overheat the return air and reduce dehumidification.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.