A zoned residential system uses motorized zone dampers with a barometric bypass duct to relieve excess static pressure when only the smallest zone is calling. A technician notices that with only that small zone active, the supply air feels unusually cold and there is noticeable turbulent airflow noise near the bypass duct connection. What is the most likely explanation?
- AThe bypass damper is oversized, recirculating too much cooled air back to the return and reducing airflow across the coil
- BThe evaporator coil is severely restricted by dirt, reducing heat transfer
- CThe bypass damper is undersized, causing excessive static pressure buildup
- DA zone damper is stuck in the closed position, blocking all airflow
Show answer & explanationAnswer & explanation
Correct answer: A. The bypass damper is oversized, recirculating too much cooled air back to the return and reducing airflow across the coil
When only a small zone is active, most system airflow is diverted through the bypass duct back to the return. If the bypass is oversized relative to the active zone's airflow demand, it recirculates excessive conditioned air, reducing net airflow across the coil. Reduced airflow across an active coil lowers the leaving air temperature (colder discharge) and creates turbulent noise where the recirculated air re-enters the return, matching the symptoms described.
Why the other options are wrong
- B. A restricted coil would reduce airflow and could cause icing, but would not specifically explain bypass turbulence noise as the primary symptom described.
- C. An undersized bypass would cause high static pressure and possible coil freezing from insufficient relief, not the noise pattern and cold air described here.
- D. A fully closed zone damper would stop airflow to that zone entirely, not produce cold air and bypass noise.
Zone Damper Bypass Airflow Issues
Bypass dampers relieve excess static pressure in zoned systems when small zones are active, but an oversized bypass recirculates too much air, starving coil airflow and causing overly cold supply air.
- Bypass duct routes excess supply air back to return
- Oversized bypass reduces net coil airflow
- Symptoms: cold discharge air, turbulent noise, possible coil icing risk
Memory trick: 'Too much bypass steals air from the coil.'