Does an AAV belong in your home?

When you’re looking at the systems of a home, some systems are easy to understand and some are not so apparent.  Drains: easy, every water appliance has to have a drain.  Vents: why do we need a vent system?   Vents work in conjunction with drains to allow drains, whether they are gray water (sinks and showers) or black water (sewage) to flow easily and quietly, and they help keep sewer gases out of the home.  A video from This Old House shows what an unvented drain sounds like and what a proper drain should look like.  At the end of the video, they introduce what they call a mechanical vent, which is in actuality an Air Admittance Valve (AAV) or Studor Valve, which is one brand name of the AAV.  Sture Ericson developed the AAV in Sweden in the early 1970s to help solve plumbing issues, and introduced the valve to the United States in 1986.  It was distributed under Studor, Inc. in 1988, which was acquired by IPS Corporation.  (History)  This little invention could be your answer to a difficult plumbing problem…or it could be a problem waiting to happen, but you’ll need to research it thoroughly first.

In the 40 years since AAV’s were introduced to the US, acceptance and use has been mixed!  They are allowed under International Plumbing Code, but some localities may not allow them.  This video shows detailed applications and installation of 5 AAVs in one house.

Here are some of the pros of AAVs: 

• Less drainpipe and plumbing hours required.

• Less roof penetrations and associated leaks/maintenance required, also vents are “unsightly” on a roofline facing the street.

• Helps solve vent problems in tricky places (islands, walls that don’t have an easy path to the roof, etc.)

This Green Building Advisor thread was very helpful to illuminate some of the pros and cons of AAVs.  It mentions a video series entitled AAV Valve Death By Plumbing Code.  In Part 1 of this video series, the creator (master plumber and plumbing company owner) explains the inner workings with a cut-apart valve.  Here are the things he mentions against AAVs:

• Anything mechanical can and will break.

• The rubber seal can rot away

• Bacteria can accumulate on the rubber seal and cause it to stick open or closed

• Insects (ie. cockroaches) can get stuck in the valve, keeping the valve open.

• Insects can gather on top of the valve and keep the valve closed.

In parts 2, 3 and 4, the creator demonstrates a typical installation of AAV in the home, and goes through scenarios if the valve fails.  Here are some more negative aspects of AAVs:

• Most people don’t know what AAVs are, if their house has them, how they work and how to maintain them.

• An AAV is a mechanical device.  If it fails and causes damage, the home insurance company probably won’t cover damages because the mechanical device was not maintained.  The manufacturer’s warranty is limited and short and would only cover manufacturer defects.

• People or visitors in the home might get sick from sewer gases (such as cholera) coming back through the device if it fails in the open position.

• If the device fails in the closed position, the venting function fails, and the P-trap becomes essentially an S-trap (which is illegal), which will siphon out the liquid seal and sewer gases can come out after the water seal siphons away.

• If the AAV valve fails open while the sink drain is plugged, any water going into the sink will start coming out the AAV valve (if it’s below the sink, which is a normal configuration).

• If the water coming out of the AAV is small, it could be a small drip or trickle which can quickly cause mold, insects, etc.  The mold/diseases formed can harm young children, older people, people with respiratory illness most.

• Worst case in scenario (see video 4 in the series): There was a gas tanker truck delivering gasoline to a neighborhood gas station in the DC area in the early 80’s.  The gasoline delivery driver dumped the load of gasoline into the station’s sewer cleanout, which generated a huge amount of gases.  The older homes near the station, which were connected into the city sewer system by older cast iron pipes (notorious for cracks anyway), and the gas fumes backed up into the basements of the homes, where they could be ignited by furnace pilot lights, etc., burning down the house.  Similarly, if the AAV fails, methane gases in the sewage line will come up into your house under the vanity and continue to accumulate, and could ignite, causing an explosion. The video shows this in graphic detail at minute 3:40!.

This master plumber and plumbing company owner is rightfully concerned because he’s in Florida (warm climate), and most people do not want to maintain their plumbing systems.  A commenter on the Greenbuildingadvisor thread stated that he built a new home fully fitted with AAVs and has ended his first year with them with no problem.  Also he “wouldn't have gone with the traditional vent lines based on prior experience with them in a northern climate. We had two sizes after a remodel, a two inch and an old style iron 4" with the lead boot flashing. Both would clog up frost and cause issues when we had very cold temperatures. The pitch of the connected vent lines did not prevent warm moist air from getting to the vertical exit. Perhaps a flaw of that house only.”

In this well-written article, an experienced builder/inspector in Mississippi laid out what AAVs are, how they work, the pros and cons, how to install one, and how to inspect one (although we are in agreement with him–if you’re not sure it’s working properly, for an average $15-30, it’s probably best to replace it).

So, in summary, the cons of this appliance are:

• If the valve fails to open or close (gets stuck), you risk sewer gases, water or  insects coming into your home, which could cause mold or disease or (worst case?) an explosion.

• They can be tricky to fit under small vanities, or a larger sink cabinet full of water hoses, sink drain pipe and garbage disposal drain pipe.

• They can be stinky at times. Another commenter on the Greenbuildingadvisor thread stated that when their home is surrounded by a temperature inversion (cool air), the AAVs tend to leak sewer gas, which seemed to be a consequence of the atmospheric pressure and wind conditions.  In general, Greenbuildingadvisor editors seemed not to be in favor of them.

After taking ownership of a home, anything that happens to it and in it really is the homeowner’s responsibility, so we need to inform ourselves of every little detail, even down to how the drain system is vented!  Is an AAV right for your home or vent situation?  It could save you money and time in the short term, and even be the right decision in cold climates where condensation can freeze up and block a traditional vent, but we would advise thinking every possible scenario through with thorough research.  If you decide to install one or several AAVs, or buy a home with them already installed, consider inspecting/replacing them on a regular basis to make sure that everything works to keep the sewer gases where they belong…in the sewer.

Photo by Timur Shakerzianov on Unsplash