What is an Air Admittance Valve ( AAV ) in the waste piping system of my home?

Occasionally I run into air admittance valves (AAV) in homes during inspections and when I explain them to clients they get a sour look on their face.  So why the sour look when they are useful items?

[tab: The Waste System]

The waste plumbing system in your home is all but hidden and rightfully so.  After all who wants to see or think about what it is used for?  But when we start smelling those funky odors coming from around sinks, and other drains, we get a chance to learn a little about how the waste system works.  Part of that system might well be an Air Admittance Valve that is in plain site and possibly the cause of those funky odors.  But first let’s look at your waste pipe system so we can understand why an Air Admittance Valve might be needed.  This will be a simplified explanation for those who might never have seen this.  So for those of you who are fully aware of how the plumbing system works keep that in mind while reading this.

Laundry standpipe

This is a typical laundry standpipe showing the vertical drain vent. (Click for a larger image).

For this description we will use a typical laundry standpipe used to drain a washing machine.  This is displayed in the image to the left.  When the washing machine drains the water it will send it down the standpipe (Arrow 1).  When it reaches the bottom of the pipe it will flow through the trap (Arrow 2), which we will discuss momentarily.  It then flows down into the sewage system running under the home (Arrow 3).

The trap is a very important part of the sewage system.  Every plumbing fixture (tubs, washing machine, sinks, toilets, etc.) has a trap installed.  Some you can see such as under a sink, where others you can not such as the laundry standpipe, toilet, tub, or shower.  When the plumbing fixture is not in use the trap will hold enough water from the last use to create a seal which blocks the flow of sewer gases back up the standpipe, in this example, and out of the standpipe into the home.  In a sink it prevents the gases from returning back through the drain and into the house, and the same with a toilet, shower, etc.  Since the trap acts as a seal preventing the backflow of gases into the home it causes the waste system piping to be a sealed system.  With any sealed system pressure can build up and cause unpredictable results.  Some of those can be slow or failure to drain, back-ups out of the drain, the trap can be back-siphoned and emptied, as well as other issues.

Roof termination of a plumbing vent pipe.

This is how the plumbing vent pipe terminates at the roof level. (Click for larger view)

To prevent unwanted effects from a sealed system there must be a way to relieve the pressure.  This is done by installing plumbing vent pipes as shown in Arrow 4 of the picture above.  The plumbing vent pipes are routed straight up to, and through the roof as displayed in the picture on the left.  These vent pipes perform two major functions.  The first is to provide an opening in the waste pipe system to help prevent the pressure build-ups just described.  Now the waste system is no longer a closed system and pressure can not build up and cause the ill effects mentioned.  The second major purpose is to provide a method to vent off the sewer gases that do build up in the plumbing waste system.  These are routed above the roof so you don’t have to deal with the sewer smells in the home.

Before we move on to what an Air Admittance Valve is let’s touch on another important point of these plumbing vent pipes.  These open, uncapped pipes should never vent into the interior of the home anywhere, ever!  Obviously they should also never terminate into a closed wall either.  These vents do emit waste gases which include methane gas as a natural byproduct of human waste.  Methane gas is highly flammable!

So now let’s look at the purpose of an Air Admittance Valve.  Select the next tab for more.

[tab: The AAV]

Island fixture venting.

This is an example of how an island sink is vented. (Click picture for a larger view)

As you can see the waste piping, along with the vents for the waste piping, are for the most part concealed in walls and floors.  Some times it is not possible to to install a waste vent pipe.  For example an island sink, in the middle of a kitchen, might not have a wall nearby or any other means to route a vent pipe.  There are two methods used in a case like this.  One is the island sink venting method displayed in the picture to the left and the other is an Air Admittance Valve.

With the island sink venting method a loop is made in the drain plumbing that provides a place for sewer gases to back-up temporarily until they can be relieved by a nearby vertical vent pipe.  In this arrangement the output of the sink trap is fed into the drain at Arrow 1.  The water then flows down to the sewer pipes at Arrow 2 and out towards the sewer at Arrow 3.  While water is flowing from the sink, or at the nearby plumbing fixture, any built up sewer gases are allowed to back-up into the extra loop displayed by Arrows 4 – 6.  Once water stops flowing from the sink, and the nearby plumbing fixture is not draining, the sewer gases then are allowed to flow from Arrow 6, into the under slab drain pipe displayed by Arrow 3.  The gases will then flow to the vent pipe serving a nearby plumbing fixture, such as a laundry or powder room.  In these cases this is the preferred method of handling the sewer gases as they stay in the sewer piping system and are vented up and through the roof to the outside.

Typical air admittance valve under a kitchen island sink.

This is a typical air admittance valve located under a kitchen island sink. (Click imager for a larger view)

Unfortunately there are times when it is not possible or cost effective to tear out the sheetrock, and other materials, to install a proper vertical waste vent pipe.  This is a major cost issue during many home renovations.  In rare occasions for island sinks in new homes there might be a technical reason that the island sink venting arrangement can not be used effectively.  This is where the Air Admittance Valve comes into play.  A typical Air Admittance Valve is shown in the photo to the left.

An Air Admittance Valve is nothing more than a normally closed, spring loaded valve.  When the negative pressure builds up enough in the drain pipe, if there were no means to equalize the pressure, then that pressure would back-siphon the water in the trap into the waste piping.  That in effect opens the waste piping system to the homes interior and allows sewer gases to flow into the home.  During a normal positive pressure the valve stays closed so no sewer gases escape.  When the negative pressure builds the valve opens only momentarily to draw air into the system and equalize the pressure and then closes again.  As you can see this is not an ideal condition but is sometimes used when the cost of installing a proper vent pipe is prohibitive, or there is no other way to install a proper vent pipe.

There are requirements on the placement and location of Air Admittance Valves.  We won’t go into all of the technical specifications on locations.  The allowable locations are determined by the manufacturer’s installation instructions and by the model building codes in use today.  Essentially they are allowed to be installed in any ventilated space that can provide air to enter the valve.  These valves can even be located in the attic area as long as the attic is properly ventilated.  The key here is that they must be accessible as they can fail and require maintenance.  If located in the attic they must be 6″ or more above the insulation to prevent insulation from being drawn into them.  Unlike a completely open vent pipe (as described before) thesse valves are normally always closed until needed and as such do not constantly release methane based gases into the attic area.

So why did I say an Air Admittance Valve might be the source of those funky odors that you smell near sinks?  Well just like any mechanical device they are subject to problems and failures.  If the spring mechanism becomes corroded, weak, or clogged, or the seal around the valve is damaged or blocked then you can experience the following issues.

  • If the valve fails to open and stays sealed shut then the negative pressure can siphon off the water in the trap and draw it back into the sewer system.  Now you have no water seal and the sewer gases can exit at the drain.
  • If the valve does not properly close and seal off the waste pipe you can have sewer gases leaking out and into the surrounding air on a constant basis.

So if you have one of these traps under a sink and start smelling sewer gases intermittently then the Air Admittance Valve should be checked and replaced as needed.  If you smell the gases constantly then the valve might have completely failed.  In Texas the builders place the air handlers for the heating and cooling systems up in the attic.  If you have Air Admittance Valves in the attic and they fail to close then it is always possible that the air handler can draw in the sewer gases as they escape and distribute them through the house.  When you buy your home pay special attention when the Home Inspector indicates that one of these valves is present and where it is.

Another issue to take into consideration is that these valves can be damaged by mishandling.  The valves need free air space around it to be able to draw in air.  I have seen these valves under sinks where the owners have totally packed the underside of the sink with “stuff”.  Some sinks are so packed you can’t even see the valve when you open the sink door.  You need to be careful about storing lots of stuff around these valves so that you don’t accidentally damage them by constantly bumping them, and you provide air for it to draw in.

All in all they are functioning devices that will work well and perform the task they are designed for.  Obviously it is preferred that when a new home is built, or when major remodeling is performed, that a proper vent system, or island sink venting, is used instead.  Also it really is not a big thing to extend a vent pipe through the roof of a home and only is a little more money.  So I don’t really understand why people place these valves in attics?  Any work that is being performed where these would be installed is normally permitted by the local municipality and requires inspections by the Building Inspections Department.  Some municipalities allow there use and some don’t under any circumstance.  Some municipalities who don’t will also allow them on a case by case basis.

So if you have one of these Air Admittance Valves and start smelling sewer gases have a licensed plumber check it out.  Some of these are screwed into a fitting and you can even replace them if your so inclined.

[tab: Links and References]

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