An air admittance valve, also called an AAV or Studor vent, is a one-way mechanical vent. It opens to let air into the drain system when suction develops, protecting the trap seals, then closes by gravity to keep sewer gas out. It is used where a normal vent through the roof is impractical.
An air admittance valve (AAV) is a one-way vent for a drain system. Plumbers also call it a Studor vent, after a common brand. It lets air into the pipes when needed, then seals shut. It stands in for a vent pipe run up through the roof where that run is hard to build.
How an AAV works
Every drain trap holds a small pool of water called a trap seal. That water blocks sewer gas from rising into the room. When waste rushes down a pipe, it creates suction that can pull a trap seal dry. An AAV fixes this. When negative pressure builds, the valve opens and lets in air to balance the pressure. That protects the trap seal. When flow stops, the valve closes by gravity and seals the vent so no sewer gas escapes. Oatey states its valve seals shut at zero pressure and under positive pressure.
Standards and code rules
AAVs are tested to two ASSE standards. ASSE 1051 covers individual and branch valves that serve one or a few fixtures. ASSE 1050 covers larger stack valves. A listed valve also carries an IAPMO mark. Code limits where an AAV can go. It cannot vent a sump or tank unless an engineer designs the system. The Uniform Plumbing Code, which Phoenix follows, does not permit AAVs by default. It allows them only as an approved alternate method.
AAV vs other devices
An AAV is easy to mix up with two other parts. A vent stack is a real open vent pipe that runs to outside air. An AAV only stands in for that pipe at one spot. A vacuum breaker does a different job. It stops dirty water from being siphoned back into the clean supply. An AAV protects drain traps, not the drinking line. Common uses are island sinks, additions, and remote fixtures. For a plain-language overview, see what is an air admittance valve.