Air Admittance Valve (AAV)
A one-way valve that lets air into a drain when needed but stays sealed otherwise, providing venting for a fixture without running a pipe to the roof.
An air admittance valve, sometimes called a cheater vent or by the brand name Studor, is a small one-way valve that screws onto a drain line, often under a sink. When water draining downstream creates suction, the valve opens to let air in and break the siphon, then snaps shut to keep sewer gas from escaping. It solves the same problem as a vent stack but without the cost and disruption of running a new pipe up through the roof.
AAVs are common where extending a traditional vent is impractical: an island sink with no wall behind it, a basement bathroom addition, or a remodel that added a fixture far from the existing stack. Their convenience comes with limits. They are mechanical devices with a spring or gravity-sealed diaphragm that wears out, and a failed AAV that sticks open becomes a direct path for sewer odor into the room, the very smell it was installed to prevent.
Local code governs where an AAV is allowed, since they cannot vent an entire system on their own and must remain accessible for replacement. A hidden or buried AAV, or one used where code requires a proper vent, is a common inspection flag. When a sink develops a sewer smell that traces to an AAV, replacing the inexpensive valve usually solves it, provided it was installed in a legal, reachable spot.
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- S-Trap : An older S-shaped trap configuration, now banned by code, that can siphon its own water seal dry and let sewer gas into the room.
- Cleanout : A capped access point on a drain or sewer line that lets a plumber insert a snake or camera directly into the pipe to clear clogs or inspect it.
- Branch Drain : A horizontal drain line that carries waste from one or more fixtures over to the main vertical stack, sitting between the fixture trap and the soil stack.
- Drum Trap : An older cylindrical trap, common under bathtubs in homes built before the 1970s, now outdated because it clogs easily and is hard to clean.