How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
Is an air admittance valve (AAV) illegal by code?
Wednesday, February 26, 2020
Air admittance valves are approved by the International Residential Code (IRC), which is used in most areas of the country with only minor modifications, but not code approved in a few jurisdictions. One is shown above, and they are designed to open and admit air into a sink drain line when there is negative pressure (suction) in the pipe, which allows the drain to flow freely without gurgling for air. The one-way vent only allows air into the system for drainage, then closes to keep sewer gas from escaping out into a home. When the location of a plumbing fixture makes it difficult to run a regular (passive) vent pipe through the roof this type of vent is an alternative solution.
There is also a similar, but older, system that is spring-loaded and allowed in mobile/manufactured homes under the jurisdiction of HUD, but not site-built homes. It is shown below and correctly called a mechanical vent, auto vent, in-line vent, or check vent. The air admittance valve design utilizes a rubber-like membrane that flexes to allow air into the drain system. It is sometimes referred to by the acronym AAV. The Studor Corporation is the largest manufacturer of AAVs, so they are often referred to as Studor® vents or Studor® valves.
Two requirements for correctly installing an AAV are that it must a minimum of 4” above the fixture drain and readily accessible. Although they are rated for a minimum of 500,000 open-and-close cycles, which is approximately 30 years of usage, they do fail eventually and, occasionally, prematurely; so it’s important to be able to get to the AAV to change it out. They have a screw fitting, so switching out a dead air admittance valve is an easy plumbing chore.
Air admittance valves can also be installed in an attic, but must be 6-inches or more above the insulation. Because an AAV requires negative pressure to open, it cannot be installed to vent a sump pump, which generates positive air pressure when operating. Also, a home’s plumbing drainage cannot rely entirely on AAVs; a minimum of one vent through the roof is required by the building code.
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