Your plumbing system should not pass gas indoors!

Lets get something clear right off the bat.  This is about your houses plumbing—not your personal plumbing.

When I first started doing plumbing in 1971–at least for money–venting island kitchen sinks was a bit of a problem.  Every plumbing fixture needs a means of venting the drain line so that water doesn’t get sucked out of the trap of the fixture—-which could allow sewer gases to enter the home.  Somehow sewer gases and the smell of fresh baked bread or homemade pizza just don’t go together.

We had to create all kinds of strange assemblies of pipes to create this venting–it worked, but was time consuming to construct and took a lot of additional pipe.  Pipe and time equal money.

Along came the “auto-vent” or “air-vent” (sometimes called a “cheater” vent).  This mechanical device had a spring loaded gasket inside that would allow air to flow into the pipe but not let sewer gases out–or, in the event of the pipes flooding, the device would not allow sewage out either.

When you think about it, keeping sewage out of the home is a better idea than keeping gases out!

These vents could be installed right at the fixture location—-saving tons of time, pipe and money.  Unfortunately most jurisdictions did not allow them–except in the mobile home industry–where they are still allowed–go figure.  Here is a picture of one of these vents that I found installed at a kitchen sink in new construction that had to be replaced after I found it.

Air vent

Air Vent or Auto-Vent

If you can see a spring inside the cap, it is an Auto-Vent.

Spring visible inside an Auto-vent

Spring visible inside an Auto-vent

At some point in the 90’s, a new type of air-vent could be found in the neighborhood–called the Air Admittance Valve–or AAV.  AAV’s quickly gained acceptance in most jurisdictions and are now widely used.  These valves are different from Auto-Vents in that there are no spring mechanisms that can fail and they have screens to keep out critters.  They have a simple EPDM diaphragm (trust me–want the initials stand for is not all that important—synthetic rubber for short) that allows air into the drain under negative pressure and seals tight under positive pressure.

AAV’s, to meet approval for use in residential construction, have to demonstrate that they can go through 500,000 cycles without ever passing gas—-roughly equivalent to 30 years–man I sure wish my plumbing system could do that!  Here is a picture of a modern AAV on a laundry sink drain.

Air Admittance Valve

Modern Air Admittance Valve

While a really simple device, they must be installed within 10 degrees of vertical, they must be installed on the welded-pipe side of the trap, they have to remain accessible and they cannot be the only means of venting the plumbing system.

Air Admittance Valve

Air Admittance Valve (AAV)

These Air Admittance valves come packaged with a rubber band that needs to be removed at the time of installation–I sometimes find them with the rubber band still in place–rendering them non-functional of course.

AAV with the protective rubber band still in place

AAV with the protective rubber band still in place

So, never–ever–pass gas again!

Now what fun would that be?Gas puts a smile on your face!


By Charles Buell, Real Estate Inspections in Seattle

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  1. I install a new toilet. When the toilet flushes it sucks the water out of the trap of the bathtub. Can cheater vents be used on a bathtub to solve this problem?

  2. Mr. Buell:

    Can I install a Studor Mini-Vent or similar vent on a laundry-tray pump setup with the drain line 7′ above the pump? The new rig is a Zoeller Model 105 laundry tray pump that uses an M53 sump pump — a very durable pump as I’m sure you know — inside a plastic box that has a 2″ vent port. It is replacing a 25+ year-old Westinghouse model that used only a 1/2″ hose port for a vent, from which I ran a piece of garden hose up the side of the laundry tub and into the tub. That worked fine but of course did not shut off the air flow to or from the pump box below the tub.
    I never had a problem with sewer gas smell, possibly because the Westinghouse float switch was set to leave a few inches of water in the box, and the discharge was at the bottom of the box, so I assume that little bit of water functioned like a trap to provide a seal. Also, I of courese had a check valve on the discharge line. I’d love to get the same Westinghouse or similar model, but I can’t find anything like it.
    The non-vented laundry tray pump by Bur-Cam uses an electronic switch, but I prefer a float switch, which nearly all vented models use. Also, I know Zoeller makes good pumps, but not sure on Bur-Cam.
    So — can I use the Studor Mini-Vent? Venting outside is not an option.
    Thanks much and your site is a fine one.

    • Charles Buell says:

      Send me some pictures of the installation privately and I will see if I have an opinion for you :)


  1. […] Mechanical vents are not allowed in Minnesota.  These are often referred to as cheater vents, and they come in two varieties – an air admittance valve and a check vent.  A discussion of the different types is a moot point for Minnesota, because neither is allowed.  If you’re curious though, check out this post by Seattle Home Inspector Charles Buell on check vents vs. air admittance valves. […]

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