I have a situation where the P trap to the washing machine is in the crawl space instead of in the wall. There is no vent.

I thought about adding an Air Admittance Vent in the wall (see diagram--dotted lines indicate places I'm thinking of adding an AAV) but I realized that would be before the P trap, so it may not do much there.

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To put the AAV after the P trap, it would have to be down in the crawl space, several feet below where the washing machine drain starts. Would this cause problems?

  • How frequently will you check the function of the AAV in the crawlspace? AAVs fail, with some dependability. You'll be venting sewer gas into the crawlspace from whenever it fails until you notice that it has failed...
    – Ecnerwal
    Commented Feb 25, 2021 at 22:32
  • What you've drawn is a running trap not a p trap. Is that what is there actually? P trap looks like the letter P when held sideways. What is the length of the standpipe( from floor to top of pipe)? Moving away from that washing machine standpipe what fitting does the pipe go into and how far is the length? Take a picture if possible. Also what size pipe is existing? Has the standpipe overflowed in the past when the washer drains?
    – Rich
    Commented Feb 26, 2021 at 21:32
  • @Rich I actually haven't seen the crawl space, plumber said it was a P trap, so maybe I should have drawn it over where it comes down. floor to top of pipe is about 3.5 ft. It's 2". I don't think it's overflowed seriously, though there's evidence of water getting behind the paint a little near the drain inlet. Commented Feb 27, 2021 at 1:23
  • Many times p traps are installed in crawlspaces as you describe. I would be inclined to leave it be until you've had an overflow issue and have a definite picture of what the layout is in the crawlspace. If it's a matter of splashing at the opening that can be a issue with using too much soap occasionally. It happens. The foam(bubbles) impede the flow of water enough that you can get some splash. Powdered soaps are more prone to this. They even have a name for it- suds pressure. When and if you get down there take a picture for confirmation. One pic from a distance and one closer up helps.
    – Rich
    Commented Feb 27, 2021 at 1:59
  • @brentonstrine I am very much having the same issue. Would you mind sharing what you ended up going?
    – divB
    Commented Nov 3, 2023 at 4:52

1 Answer 1


It is acting as a vent, it should be above the the sanitary tee.

In a washing machine drain set up the Drain goes to a Ptrap and then into the side inlet of the san tee, Vent goes out from top of tee to vent or an AAV.

  • Although I have always thought it crazy to require a vent on a washing machine drain they do need one and it needs to be before the trap as Alaska man states. Not all areas allow AAV’s so check on that and it helps to be higher than the drain line itself.
    – Ed Beal
    Commented Feb 25, 2021 at 21:05
  • I'm confused by this answer. There is no sanitary tee right now. Also, what does "it" refer to when you say "it is acting as a vent"? Commented Feb 25, 2021 at 23:58
  • I have the same question as OP and I am exactly the same way confused about this answer. It would be great if the answer would be made more understandable. Especially with respect to the setup given by OP.
    – divB
    Commented Nov 3, 2023 at 4:54

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