A face and bypass damper is an airflow control arrangement in which two parallel damper sections modulate the proportion of conditioned air passing directly through a heating or cooling coil (the “face”) versus the proportion routed around it (the “bypass”). As the face damper closes, the bypass damper opens, and vice versa, allowing precise temperature control of the mixed air delivered downstream. This opposed-action configuration provides continuous capacity modulation without cycling the coil’s heating or cooling source on and off.
Technical Details and Operation
Face and bypass damper systems typically use parallel blade dampers for both the face and bypass sections. The two damper sections are mechanically or electronically linked so they operate in opposition: when the face damper is fully open, the bypass damper is fully closed, directing 100% of the airflow across the coil. A controller modulates the damper positions based on a discharge air temperature sensor or space temperature signal, commonly using a 0-10 VDC electric actuator or a 3-15 PSI pneumatic actuator.
Key design considerations include:
- Face velocity: Typical coil face velocities range from 400 to 550 feet per minute (FPM). Exceeding recommended velocities can cause moisture carryover from cooling coils and reduce heat transfer efficiency.
- Damper leakage: Leakage performance is rated per AMCA Standard 511, with Class I dampers (leakage at or below 4 CFM per square foot at 1 inch water gauge) preferred for tighter control.
- Pressure drop: The bypass path must be designed with a pressure drop similar to the coil section to ensure predictable airflow distribution across all damper positions.
Common Applications
Face and bypass dampers are widely used in constant volume air handling systems where the supply fan delivers a fixed airflow rate and temperature control must be achieved without varying fan speed. Specific applications include:
- Dehumidification control: The cooling coil operates at full capacity to remove moisture, while the bypass damper reintroduces warm, unconditioned air to prevent overcooling the space.
- Preheat systems: Hot water or steam coils in outdoor air handling units use face and bypass arrangements to prevent coil freezing while controlling mixed air temperature.
- Zone temperature control: Individual zones in multizone air handling units can be served by dedicated face and bypass sections for independent temperature regulation.
Related Standards and Codes
Damper construction and performance testing fall under AMCA Standard 500-D for rating damper performance, including leakage and dynamic pressure loss. ASHRAE Handbook chapters on air handling and distribution provide guidance on coil face velocity limits and system design. UL 555 applies where fire-rated damper assemblies are required in ductwork penetrations.
Practical Significance
Face and bypass damper systems reduce mechanical wear on compressors, boilers, and other heating or cooling equipment by allowing continuous operation rather than frequent cycling. They offer a relatively simple and cost-effective method of capacity control, particularly in retrofit situations where variable frequency drives or variable air volume conversions may not be practical. Proper balancing of the face and bypass airflow paths during commissioning is essential to achieving accurate temperature control and avoiding hot or cold spots in the supply air stream.