An air admittance valve (AAV) is a one-way mechanical valve installed locally at a plumbing fixture’s site, allowing proper venting to occur without a connection to a larger venting system and stack vent. Designed to take the place of a fixture or branch vent, AAVs are an excellent alternative if you cannot connect to an existing venting system due to complex pipe-routing issues or obstructions.
AAVs allow air to enter a plumbing drainage system when negative pressure develops in the piping system (because of the flow of water). When pressure in the system is equalized (indicating that no water is flowing),
