AAV

An air admittance valve (AAV) is a mechanical check valve installed on a drainage line that opens when water flows past it, allowing air to enter the system. It closes when there's no flow or when positive pressure exists in the vent line, blocking sewer gas from escaping into the home. The valve sits inside a plastic body typically 1.5 to 2 inches in diameter and installs vertically above the flood rim of the fixture it serves.

In the field, AAVs let you vent a toilet, sink, or shower without running a vent stack through the roof—which saves money and work but with real limits. They work best on individual fixtures or small branches; they can't replace main vent stacks on multi-story buildings or handle heavy drainage loads. The valve's spring or float mechanism can stick or fail if lint, hair, or grease clogs it, so they need to be accessible for cleaning or replacement.

Code acceptance varies by jurisdiction. Many places allow AAVs for island sinks, wet bars, and secondary fixtures, but prohibit them on primary drains or anywhere the main vent would normally go. Check local plumbing code before spec'ing one—and never use an AAV as an excuse to skip proper venting on a main stack. When they're approved and installed right, they speed up the job and solve real vent routing problems on remodels.