How & Why Air Admittance Valves Are Used

Learn how air admittance valves work and why they're needed in plumbing systems

Air admittance valves can make dreams come true! Well, sort of. In the course of designing a new home - or renovating an older one - it's not unusual to have ideas challenged and plans scrapped. Quite often it's the plumbing situation that determines whether dream becomes reality - and it's not always about water.

Venting is an important part of plumbing: not only does it remove unpleasant and dangerous gases from the structure, it also equalizes air pressure within the drain system, preventing vacuums and the siphoning of water out of traps. But to vent properly involves cutting holes in the roof and installing pipe at code-specified locations, which can be inconvenient (to say the least) or even impossible in some situations. It's in those situations where an air admittance valve can save the day.

Air Admittance Valves (AAV) are one-way vents installed after the trap on the drain line of a fixture. They're designed to address negative air pressure in the drain system, created when a fixture is drained. The wastewater being sent down the drain - referred to as a "slug" - pushes air in the pipe in front of it, leaving behind an area of negative pressure.

Failure to bring air back into the pipes can result in a vacuum (causing slow draining and gurgling), or even the siphoning of water out of traps (allowing sewer gases to enter the home through drain openings). Where standard venting is undesirable or unfeasible, an AAV will safeguard against those scenarios.

Pro Tip: AAVs deal only with negative air pressure in drain pipes. Positive pressure can also be a problem, though usually only in multi-story residences and high-rises. To address positive pressure when standard external venting is not adequate, we recommend using a P.A.P.A. (Positive Air Pressure Attenuator).

Operation is simple: when a fixture drains and creates negative air pressure in the correlated drain line, the valve is opened, drawing air back into the pipe. Once the air pressure has been equalized, the valve securely closes (preventing the escape of gas) until the pressure drops again.

AAVs can be used to service an individual fixture or several - what's permitted depends on local code, and the model being used - units like the Redi-Vent can accommodate a mere 20 DFUs (Drainage Fixture Units) while large units like the Maxi-Vent can handle up to 500 DFUs. AAVs are most often found under sinks (especially island sinks) and in basement bars or bathrooms.

Installation guidelines can vary depending on code, but in general AAVs serving a single fixture should be at least 4" above the center of the trap arm. When serving multiple fixtures, they should be a minimum of 6" above the flood level rim of the highest fixture, and should only vent fixtures that are on the same floor level. Because they rely upon gravity to close, AAVs must be installed in a vertical position.

Caution! In any dwelling, there must be at least one vent open to the atmosphere outside (per code). An external, passive vent relieves positive pressure in the lines. No building should exclusively use AAVs - significant problems will result.

As we mentioned, AAVs are often found under sinks - as are a myriad of cleaning products and who knows what kind of junk. To ensure proper operation of your AAV, make sure that the surrounding area is free of obstructions, and things that can knock against it. When an AAV is installed in an attic, make sure that it's at least 6 inches above insulation - any less, and there's a good chance insulation will be drawn into the unit, severely affecting performance.

AAVs should be installed indoors whenever possible. When an outdoor installation is necessary, a cap of some sort (in addition to any insulation included with the valve) is necessary to prevent degradation and lackluster performance due to freezing temperatures or excessive UV exposure.

Note: Modern AAVs are sometimes confused with the older "Automatic Air Vent" - a spring-operated device that accomplished the same task, but suffered from unreliability owing to its mechanical nature. Automatic air vents are no longer permitted for use; air admittance valves are not defined as mechanical devices, and are permitted to varying degrees in most plumbing codes.

AAVs can be a great solution for venting problems, but should never be the first choice. In fact, they should be the absolute last possible choice - a traditional, passive, external venting system will always be superior (especially when it comes to positive pressure), and is often required by code. AAVs should always be considered a last resort.

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