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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).
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.
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A common type of this style fitting is a "hex bushing" with a hex head for installation with a pipe wrench. A double-tapped bushing is more compact than a reducer but not as flexible. While a double-tapped bushing has a more minor female thread concentric to a larger male thread (and thus couples a smaller male end to a larger female), a ...
One side is connected to a flexible supply hose; the other may be attached to a manifold, valve, tool, or another hose. A female coupler is used on the supply side, and a male nipple is used on the receiving side. Fitting profiles have identifiable geometry on the male end, but care must be taken to use a compatible coupler.
An atmospheric vacuum breaker consists of a check valve and an air vent. The devices allow air into the system to prevent a siphon at the point of use. Hose connection vacuum breakers are installed on faucets and hose bibbs to prevent backflow into the water supply.
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.