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 🙂

  3. Katie Peck says:

    Hi there! We have been struggling with some plumbing problems for a few months now, even with licensed “Master” plumbers doing work. We can’t seem to get straight answers from anyone.

    Here is a summary of our situation.

    Our house has one main drainage line from the kitchen that runs perpendicular to the sewage main exit from the house, in the basement the sewer main is about 5 ft above the floor. The line from the kitchen was moved to allow for remodeling but has been properly graded. About 40′ from the kitchen sink there is a laundry sink and lift pump to pump the laundry water up and into the sewer main. We have a studor vent under the kitchen sink and a studor vent for the lift pump. This work was just completed and what is now happening is a back up of air pressure in the laundry pump. It results in a slow drain so the water is backing up in the sink, threatening to overflow if we aren’t there to watch and release the pressure in the pump by removing the cap on the pump.

    We called the plumber who is on his way over now. He says maybe the guy who installed the pump didn’t read the directions and the thing can’t work with a studor vent and may need to be vented outside.

    I’m not paying for any more plumbing work from anyone until I know for sure what is going on here. We used the pump with NO issues before with no studor vent, just an open pipe. Can we not just return to this set up?

    Any suggestions or expeirence with studor vents and laundry lift pumps????? HELP!

  4. Marti McNamara says:

    I live above a restaurant. Sewer gas has been an issue for three months. There are two AAV’s in my condo, one under the sink and the other under a wash tub in the laundry room.
    The restaurant sewer line and my line has to have a connection somewhere along the line. Is it safe to cover the the two AAV’s? This is the third AAV replacement in my laundry room. I cannot live like this. Help!

    • Charles Buell says:

      If you are having a problem with AAV’s you need to call a plumber pronto—and it sounds like not the one you have used in the past. I have never heard of an AAV failing so I am thinking there must be something else going on. Are you sure they are AAV’s and not Auto-vents? It is certainly not ok to cover any vent.

      • Marti McNamara says:

        The last plumber called them auto-vents but it is the same as the one you have displayed at the top of this column. I have had three plumbers replace these vents in the past three years. Are these vents suppose to be connected to a roof vent?

  5. Marti McNamara says:

    I have a picture of the installed AAV. How can I send it to you?

  6. Toni Buchanan says:

    I have an old cottage that we now have a grinder on all the plumbing except the kitchen sink which just goes to a old discharge in the yard. We have been having issues with our sink backing up. So we rented a drain snake and we had no resistance or even nasty stuff come out it was actually pretty clean. So we change the piping under the sink and go to the well box to be on a slant for better drainage and we put one of those air valves like you were showing and raised the pipe that it is on up to about six inches under the ceiling in the room its in. My question is are the gases going into that room out of that vent dangerous to breathe and our drain still backs up just not as fast. If we go all the way out the roof do you think that would solve our drain problem. We are renters and my landlord has been trying to avoid is going through the roof. What do you think.

    • Charles Buell says:

      From everything you say the only thing that is very clear is that you need a licensed plumber to figure this out. I suspect there are other more serious issues going on. Don’t mess with this problem yourself please.

  7. Toni Buchanan says:

    Thank you but at this time that does not seem to be an option because that would be up to my landlord and she keeps putting that off. That’s why I was wondering if the gases were coming back into my house.

  8. Patrick Stefan says:

    This is really helpful! I installed an oatey AAV for a kitchen sink that did not have its own venting from when the previous owner did a remodel. Everything seemed great, but every once and a while I get a hint of sewage smell that lingers for about 30 min. Is the AAV bad?

  9. We have an AAV under our sink where a disposal is also present. We get sewer gas smell from time to time – is there a way to check this valve or is the method to throw it away and replace?

    • Charles Buell says:

      Pat there are so many other sources it could be. I would be hesitant to blame the AAV unless it is actually an Auto-vent. You should have a plumber look at your setup if you are indeed smelling sewer gases.

  10. I have a sewage pit in my basement to service a bathroom and a small hair salon. The vent stack is 1 1/2″ leading to the basement. The sewage pump is a 3/4 HP with 2″ discharge. My concern is that my vent stack won’t handle the vacuum of the pump without siphoning from my traps. Is the aav a good option as a complete vent or should I still tie to the stack?

    • Charles Buell says:

      Sorry, Carl, but I cannot adequately answer your question due to all the variables. Consult with a licensed plumber.

  11. Ray and Judi Jelinek says:

    After 23 yrs we replaced our tub with a shower and changed the vanity. We now have a sewer gas odor coming up through the sink. We can’t afford to spend any more money on this bathroom and are hoping it is a problem we could solve.

  12. We had to have 50 feet of galvanized sewer pipe removed from our very old home and had new plastic put in along with pex water lines and a new venting system put into our upstairs bathroom. We put in a new toilet, new sink and new tub. After paying a substantial amount of money we find that the venting system is not working. When draining the tub or sink the toilet will gurgle and spit water. This does not happen all the time but usually every morning at the least. The plumbers have been back several times and last time they extended the venting system up to the attic with AAV. This did not work so the AAV was taken off only until they come back in next day or two so they can extend it even further up through the roof. We have been dealing with this mess for exactly 5 weeks, What are the chances that this will work? Cost is another 375.00 on top of the $4,000.00 we paid previously, Any suggestions?

    • sorry was not galvanized it was cast iron

    • Charles Buell says:

      Sounds like one or more of the vent caps is still capped off from testing perhaps. The plumber should be able to easily sort this out for you. But it sounds like you might need a different plumber 🙂

  13. Justdoit says:

    Hi Charles,
    I’m replacing my 6″ deep kitchen sink with a 10″ deep sink that needs all the space it can get in my 36″x24″ sink cabinet. I have the AAV vent you described and I noticed it is on the way. Is the vent absolutely necessary? Is it really a must have? Can I just cut off the pvc the vent is attached and put some sort of mesh on the pvc (to prevent critters as you put in your post)?
    The sink MUST be installed (for reasons I cannot get into here). Now it’s just a matter of what should move and what should be removed.

    • Charles Buell says:

      If you have an Auto-vent now, you will need a vent when you change the sink. It should be the AAV type though and you should be able to figure out a way to make it perhaps not as high as it was before. Without seeing your configuration I can’t say for sure though. Your plumber will be able to answer that.

  14. Bob Goldsmith says:

    Great article! Now I have a question for you. Along these same lines at my last 2 homes we had septic odor coming out of the roof stacks. Can these vents solve this type of problem installed on the roof vents? Odors were particularly noticeable after someone used the shower, etc. Also warm weather seemed to make the odor worse.

    • Charles Buell says:

      This is very common with homes that have septic systems and typically it might be my first indication that the home has a septic system if the listing doesn’t already state it. I hear there are charcoal filters made for these vents above the roof to deal with these odors but I really don’t know much about them—or even whether they are code approved.

      • Bob Goldsmith says:

        Yes, did I not mention we had septic? It was pumped anew for the sale so I’d think that was not the issue, rather is the direction the odor is traveling indicate a structural (bldg. or otherwise) anomaly that is making this odor noticeable? Where is the gas generated in the tank supposed to go and via what path?

  15. Dealing with an icky smell in old farm house. It is not the septic system, just had it pumped. Farmer plumbing for washing machine, no p trap, but pipe goes up over a joist then back down. The toilet is tight. Just replace an old can trap with p trap under tub. The washing machine joins into tub drain. Just had plumber put AV under tub and under vanity in bathroom. Still seem to have some odor. Septic man with ‘smell tester’ said the smell is blackwater smell, not sewer. So what is the next step. This house is in a location that does not really have to meet any codes.

    • Charles Buell says:

      Carol, sorry, I have no advice for you. If the plumber was not help, I recommend asking a different plumber.


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