I bought a new house about a month ago, and have since noticed some plumbing issues in the laundry room:

  • laundry tub drains slowly
  • occasional "musty" smell, which seems to come from the drains

I did some research online and found that lack of drain ventilation could cause both of these problems. So I checked the plumbing and it looks like both the washing machine and adjacent wash basin drain into the same pipe. As far as I can tell, there is no ventilation on these drains (photo below); it looks like a stacked pair of S-traps.

laundry room drains

The laundry room is in a little addition at the back of the house. So I probably cannot tie this back in to the main house venting stack without a lot of work. I gather the simplest solution is to locally install an air admittance valve (AAV). Does that sound like a reasonable approach here?

If so, am I on the right track with one of these:

My plan is to remove the upper S-trap. Then install a sanitary T, going to: AAV (top) and P-trap (horizontal). Does that sound right?

aav schematic

1 Answer 1


The washing machine sends water into that drain under quite a bit of pressure and you want air moving freely through the pipe. You would be better off if you could run a new vent pipe up that wall and out the roof, or you could go as high as possible then out through the wall, presuming it isn't too close to a door or window that opens (since sewer gases exit the vent pipe on a regular basis).

Barring that, however, you should certainly put an AAV on this. I've had a plumbing inspector approve exactly this kind of setup for a washing machine drain on a remodel. The inspector required 4 feet of vertical pipe between the AAV and the wye that tied it into the drain pipe, though.

Your diagram may not be intended to be to scale, but I'd say you definitely want the AAV higher than what you're showing. I certainly wouldn't try to get away with putting the AAV under the sink.

  • Thanks for the feedback. I had intended the sketch to be (roughly) to scale, so it is very useful to know the AAV should be higher up. I may need to relocate the shelving and work around the water inlets to do this. Can you clarify the benefit of the added separation?Also, you mentioned a wye to the drain pipe - I think that was just for the application you mentioned and I am supposed to use a sanitary tee here, correct?
    – Roberto
    Commented Dec 29, 2015 at 13:48
  • 1
    If I remember correctly, the AAV simply has to be above the flood level if the fixture (e.g. above the rim of the sink). Also, not all areas allow them, so you'll have to check with the local building department.
    – Tester101
    Commented Dec 29, 2015 at 14:52
  • @Tester101 I know that's the case for venting sinks and dishwashers (kitchen islands), but clothes washers expel quite a bit more water, under more force. I'm not sure the same rules apply. I'm also not sure they don't. I just know I was required to run the AAV 4 feet up the wall to keep it clear away from the flowing water when I had it on a drain line with a clothes washer. Also note that the first AAV device the OP listed only explicitly states that it's suitable for sinks. Personally, I'd go with the second one, which has a higher capacity, since it's attached to a clothes washer. Commented Dec 29, 2015 at 16:38
  • @Roberto I ended up looking at this again all these years later. The Wye just has a sweep that directs the water the right direction down the pipe under pressure so it isn't splashing or flowing back up the pipe and coming out the AAV or drains -- whatever is upstream that you don't want that water splashing out of Commented Dec 16, 2023 at 1:02

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