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Air admittance valves (AAVs, or commonly referred to in the UK as Durgo valves and in the US as Studor vents and Sure-Vent®) are negative-pressure-activated, one-way mechanical valves, used in a plumbing or drainage venting system to eliminate the need for conventional pipe venting and roof penetrations.
In the United States, plumbing codes usually provide strict limitations on how far a trap may be located from the nearest vent stack. When a vent cannot be provided, an air admittance valve may be used instead. These devices avoid negative pressure in the drain pipe by venting room air into the drain pipe (behind the trap).
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The valves are normally installed with the first installation of a system, then remain in place for the life of the system. They are generally reliable and require no maintenance. If they do eventually fail, they are replaced rather than repaired. A leaking valve commonly shows a dribble of water from the air vent, or stains of rust or limescale.
The "Venting to atmosphere" section is great! Of course, diagrams and picutres would make it even better... But the "Air admittance valve" section does not make sense -- if it only allows gases in, not out, how does it prevent back-pressure when a toilet flushes?69.87.193.156 13:05, 6 September 2006 (UTC) It doesn't.
Valves stop (or regulate) the flow of liquids or gases. They are categorized by application, such as isolation, throttling, and non-return. Isolation valves temporarily disconnect part of a piping system to allow maintenance or repair, for example. Isolation valves are typically left in either a fully open or closed position.