An Air Admittance Valve (AAV) is an engineered device used in plumbing drainage systems to allow proper venting without requiring a direct connection to an external vent stack. AAVs work by automatically opening under negative pressure to admit air into the system, equalizing pressure and protecting trap seals. When not in use, the valve remains closed to prevent the escape of sewer gases. Typical applications include island sinks, remodels, or fixtures located far from a conventional vent stack.