A one-way mechanical venting device installed on drainage systems to allow air into the system when negative pressure develops. An air admittance valve serves the same purpose as a traditional vent stack (the pipes you see sticking out your roof). Still, it offers greater flexibility and installation cost savings, particularly in tight or retrofit situations. In residential settings, these valves are most often found beneath kitchen sinks, bathroom fixtures, or island installations where extending a vent through the roof would be impractical. They may be used in branch or remote fixture assemblies in commercial applications. Unlike passive vent stacks that rely on gravity and natural airflow, an air admittance valve opens a rubber seal only when internal pipe pressure drops, closing again to block sewer gases and odors from re-entering the building. Check with your local code officials before you make a purchase.
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