An air admittance valve, or AAV, provides a viable way to vent a sink drain P-trap in the event that hooking up the drain to the building's vent network is prohibitively costly or impossible. Situations in which an AAV makes a plumber's life easier include installation of a kitchen island sink or a bathroom lavatory in a basement on the side of the house opposite the vent stack. If you're a remodeler who is fairly new to plumbing, it helps to understand AAVs, their importance as alternative venting solutions and how to install them.
Proper Venting Is Critical
Plumbing vents were developed soon after the introduction of the P-trap in the mid 1800s. The purpose of a P-trap is to hold a pool of water that seals noxious and flammable sewer gases in the pipes — but early traps had a habit of emptying whenever water flowed through the drain. The reason this happened is that flowing water creates negative pressure that sucks water out of the traps, just as you can draw water up a straw by sucking it.
The solution, discovered in 1874, was to relieve pressure by introducing air through a system of vent pipes that connect to a main vent stack that terminates outside the building. Since that time, every building in North America and elsewhere features at least one 2- to 4-inch plumbing vent stack, which is usually on the roof, and venting requirements are written into every building and plumbing code, such as the International Residential Code (IRC), the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC).
The most basic code requirement is that every fixture drain must have a P-trap and a vent, with no exceptions. If air can't get into the pipe to break the suction created by flowing water, water slows down or stops altogether, and it empties the trap. You usually hear gurgling from a drain with poor venting, which is the sound made by air being sucked through the trap water by the negative pressure in the pipes.
The Studor Valve to the Rescue
What do you do when you can't install standard vent piping? If your local codes allow it, you install a Studor vent, properly known as an air admittance valve, which has been an option in the United States since 1986.
Sture Ericson and his wide Doris first marketed the air admittance valve in Europe in 1975, combining their names and calling it the Studor valve. They introduced it to the U.S. 11 years later. It's a simple mechanical device with a gravity-controlled valve that typically connects to the P-trap waste arm within the distance from the trap mandated by code. The valve remains closed when the pressure in the pipe is balanced or positive, as occurs in front of a head of water, but it opens when the pressure is negative, as occurs after water has passed the valve, to allow air into the pipes to equalize the pressure.
The Studor valve is the original AAV, so the names have become synonymous, but today, many manufacturers market AAVs, including Oatey, Tuuber and Webstone. The dome-shaped cylindrical fittings are about 3 or 4 inches long, and they are inexpensive, costing less than $15. They are typically made of PVC plastic and sized to fit on a 1 1/2- or 2-inch drain pipe.
Check Your Local Codes Before Installing an AAV
Because an air admittance valve is a mechanical device, it can fail, and that led many state and local plumbing authorities to prohibit the use of AAVs when they first appeared, especially since earlier ones were spring-loaded. Many authorities have since lifted the restrictions, but some haven't. For example, home inspector Reuben Saltzman, writing in the Star Tribune, notes that AAVs are illegal in Minnesota because the current code contains no language rescinding the prohibition in earlier versions.
The situation varies from state to state. The 2016 California Plumbing Code, published on UpCodes, is based on the UPC and contains no language related to AAVs, making it important to consult with local authorities before installing one. On the other hand, states such as Florida, Michigan and New York typically allow AAVs because their plumbing codes conform to the IPC, which contains a section on their use. Your plumbing system probably won't be red-tagged if you illegally install an AAV, but you could run into problems when it's time to sell the house, so check first.
Air Admittance Valve Installation Instructions
The most common way to install an AAV is to tie it into the horizontal waste arm between the P-trap and the drain outlet in the wall behind the sink. The plumber typically cuts into a section from the pipe and glues in a straight tee (not a sanitary tee) followed by a short length of pipe that terminates with the AAV.
According to IPC regulations published on the International Code Council website, the minimum distance between the AAV and the fixture drain trap is 4 inches, and the minimum height from the AAV to the top of the pipe is also 4 inches. The AAV can rise as high as the underside of the sink countertop as long as it is at least 4 inches from the sink.
It's also possible to install an AAV in a riser connected at the point where the waste arm drops to the drain as long as the distance between the AAV and the trap doesn't exceed the maximum allowable for a conventional vent pipe, which is 6 feet for 1 1/2-inch pipe and 8 feet for 2-inch pipe. When you do this, you must use a sanitary tee for the connection with the sweep sloping down toward the drain.
Important Things to Keep in Mind
An air admittance valve must always be accessible because you may have to service or replace it, so if you install it behind a sink wall, you must provide a removable access panel. The AAV must also be in a location that allows air to enter, so it can't be in an airtight enclosure. Most sink cabinets satisfy this requirement.
You can use an AAV as a stack vent to provide venting for more than one fixture. When you do this, the AAV must rise at least 6 inches above the flood level of the highest fixture being vented, and it must be accessible. You can extend the vent stack into the attic, and if you do, the distance between the AAV and the attic floor must be at least 6 inches.
To be legal, an air admittance valve must conform to standards established by the International Association of Plumbing and Mechanical Officials (ASSE) in section 1050. Most major brands do and are identified as ASSE 1050 compliant on the packaging. If you choose an off brand and you don't want to run into any problems with the plumbing inspector, make sure this compliance is listed.
Venting a Dishwasher or Washing Machine
A dishwasher drain typically connects to the drain tailpiece of a kitchen sink, and the dishwasher wastewater empties through the sink P-trap. So, if the sink drain is serviced by an AAV, the dishwasher drain is properly vented as long as the AAV is installed the way it should be.
A washing machine, on the other hand, has its own drain trap connected to the stand pipe into which the washing machine drain hose empties. You can use an air admittance valve to vent this trap, but as plumber Terry Love points out, you should choose one that is designed for this application.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.