The Round Barometric Bypass Damper is used to limit air pressure in a zoning installation while closed zones would otherwise overly restrict the airflow, allowing pressure to build. Air is taken from the supply plenum to relieve pressure buildup. The reason for limiting pressure is only to limit air noise to a level acceptable to the homeowner. If noise is never objectionable, the damper will never need to open and could be eliminated. However, since many factors influence air noise, it is good practice to install a bypass unless the designer is certain that it is not needed.