This question is a follow-up to my earlier one about a sewer gas issue in my house.

Since posting, and with the help of some of the comments, I've isolated what I believe is the problem: When the first floor bathroom toilet is flushed, it siphons water out of the shower p-trap. I have confirmed that when I notice the sewer gas smell, pouring some water down the shower drain makes it go away. Then, if I flush the toilet in the same bathroom, it comes back.

Because this portion of the drain system is not vented and instead uses an air admittance valve (AAV), I'm concerned that there may be an issue with the AAV. Is it broken? Too small? Are the drain lines not properly installed?

Here's what the plumbing looks like from below:

Photo of drain plumbing

Here's the AAV:

Photo of AAV

  • the first thing i'd check is if the aav is working. have someone flush the toilet and have some smoke or something that allows you to see the aav pull air into it. aav's only have so many activations and the cheaper one fail quickly. Sep 17, 2022 at 21:21
  • @FreshCodemonger I took it off and tried to test it a few different ways. If it were totally broken, wouldn't that keep all of the drains from working? It's about three years old and there are only two of us using this bathroom.
    – LShaver
    Sep 17, 2022 at 21:23
  • 1
    No drains will still work even without vents, there will just be bubbling as air is pulled down the drain with the water. the traps siphon, and it drains slower. Sep 17, 2022 at 21:29
  • 1
    The diagram in your previous sketch had the relative locations of the toilet, sink with AAV and shower on the drain line out of order. You should have a spare AAV anyway, so take the simple approach and swap that to see if it gets better - if not, you have to get better venting on the shower line. If it does get better, eventually buy another one to be the spare on hand.
    – Ecnerwal
    Sep 18, 2022 at 12:11


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