Air admittance valves (AAVs) are pressure-activated mechanical vents which have been used for decades in plumbing venting systems to eliminate the need for conventional venting and roof penetrations. As wastewater is discharged through the drain line it causes the AAV to open and release the vacuum to allow air to enter plumbing vent pipe for proper drainage. When not in use the valve remains closed, preventing the escape of sewer gas and maintaining the trap seal. Using AAVs can significantly reduce the amount of venting materials needed in a plumbing system, increase plumbing labor efficiency, allow greater flexibility in the layout of plumbing fixtures, and reduce long-term maintenance problems associated with conventional vent stack roofing penetrations.
Standard plumbing systems use water trap seals to perform the critical function of preventing sewer gas from emanating into living areas, with fresh air pipe venting commonly used to prevent siphoning of traps. Although this method is simple and reliable, it requires each plumbing fixture to have a lateral return vent that passes through wall studs to a central stack, or to have its own vertical vent that passes through the wall, ceiling, attic, and roof. Air Admittance Valves are mechanical devices designed to maintain trap seals without the need for additional vent piping. They are one-way valves that open only under negative pressure (created when a toilet is flushed or a drain stopper is opened). When the water flow stops, gravity closes the valve, preventing the escape of sewer gasses under conditions of equal or positive pressure.
Internal view of an air admittance valve
AAVs are typically made from polyvinyl chloride (PVC) plastic materials with ethylene propylene diene monomer (EPDM) rubber valve diaphragms. Valves come in two sizes: one for fixture venting and a larger size for system venting. The valves fit standard diameter pipes, ranging from 1-1/4 to 4 inches. Screening protects the valves from foreign objects and vermin. ASSE (American Society of Sanitary Engineers) standards require that AAVs be tested to reliably open and close a minimum of 500,000 times, which is equal to at least 30 years of use, with no release of sewer gas. Some manufacturers claim their units are tested for up to 1.5 million cycles, or at least 80 years of use. Air Admittance Valves have been effectively used in Europe for more than two decades. U.S. manufacturers offer warranties that range from 20 years to lifetime.
Because air admittance valves eliminate the need for vent stack penetrations and flashing at wall assemblies, attics, and roof decks, they reduce the potential for air and water leakage due to vent stack penetrations.
Typical installation of an air admittance valve under a sink.
U.S. Code Acceptance
AAVs are accepted by major model building and plumbing codes, including IRC 2003, for single or multifamily residential construction, and by American Society of Sanitation Engineers (ASSE). However, some local code authorities may be unfamiliar with Air Admittance Valves or reluctant to accept them despite code listing. Prior to using an AAV, an installer should contact the local building code official for specific approval status. In the United States, one open-air plumbing vent per structure is required.
Most municipalities require that the AAV be accessible such as being placed under a sink or in a wall with a removable grill. Many AAV manufactures also make a recess box with a removable grill that looks similar to a return grill.
AAV Recess Box (Studor, Inc.)
AAVs cost about $15 to $40. However, AAVs typically reduce vent pipe material requirement and can also reduce labor costs. Depending on the plumbing fixture, there may be additional piping required to make the AAV accessible or to elevate the valve to a point above the fixture which is required in many codes.
Plumbing contractors can install AAVs using conventional tools. They are attached to 2" or 4" vertical lines in place of a vent stack in the attic or in a wall accessed by recessed box. Some manufacturers make units for interior use only while others offer insulating collars for exterior use. The smaller single drain vents come with tapered threaded connectors that allow chemical welding (PVC cement) to 1 ½" or 2" pipe. Adapters are available for 1 ¼" pipe diameter. Single AAVs are often placed on drain lines inside cabinets below sinks, between the trap and the wall. The location must be accessible, and in a space where air can move freely. Devices are compatible with both PVC and ABS piping.
Manufacturers should supply exact installation guidelines for laying out the system. Although Air Admittance Valves can eliminate the need for multiple roof vents, one fresh air vent per structure is required in the United States, especially when used with a septic system.
Twenty years to lifetime, depending on manufacturer.
Benefits and Costs
For plumbers and contractors, efficiency of installations can be greatly increased, and the amount of materials needed for the system is significantly reduced. AAVs eliminate the need for lateral return vent runs that require cutting several holes through wall studs, or long runs of vertical piping that must pass through the ceilings, attic, and roof. Fewer bends and tee connections are required, and there is no need to cut, flash, and seal a hole through the roof deck. The appearance of the roof is improved from having fewer protruding pipes, and a common source of leaks is eliminated as the roof ages and deteriorates. AAVs can also eliminate the need for firestopping materials at floor/wall penetrations, and improve workers' safety, since they do not need to climb on the roof.
Designers have much greater flexibility in laying out rooms and fixtures. AAVs offer simple solution for venting kitchen islands, or fixtures connected to walls underneath open living spaces on the floor above. Remodelers can more easily change layouts or vent bath and kitchen additions without disturbing existing living space.
AAVs should be maintenance free after they are installed.