How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
What is an auto vent, air admittance valve, or check vent?
Thursday, July 26, 2018
This device goes by a number of different names, including Studor® vent, in-line vent and mechanical vent. It is 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.
Although all the different names are often used interchangeably, there are actually two different technologies used to open the valve for air when needed and then close it afterwards. The older system 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 more recent design utilizes a rubber-like membrane that flexes to allow air into the drain system. It is an air admittance valve, 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. An air admittance valve made by Studor is shown at the top of the page, and an example of an AAV we saw under a kitchen sink at a recent home inspection is shown below.
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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