In zoned HVAC systems, a bypass duct relieves excess static pressure when some zone dampers close. A barometric damper inside the bypass opens to divert supply air back to the return side, preventing the blower motor from straining and the open vents from whistling. While effective for pressure relief, it can cause the evaporator coil to freeze if too much cold air cycles back to the return.
We need to add a bypass duct because the static pressure spikes to an inch when the upstairs zone closes.
Do not mix it up with
- Plenum: A plenum collects or distributes air for the whole system, while a bypass duct specifically shunts air from supply back to return.
Related terms
Static pressure
The pressure air exerts outward on duct walls, the resistance a fan works against, measured in inches of water column.
Damper
A blade or set of blades in a duct or opening that controls, balances or shuts off airflow.
VAV
Variable air volume: a system that holds supply air temperature steady and varies the amount of air to each zone.