A local plumbing company (big name) said that anything draining from upstairs would suck my p-trap dry. I asked about the mechanical vents, and he said it would not work. He went on to tell me that connecting was not advised and he would not accept the work.

Basically, I just want to tie into the 3" drain from upstairs with a sanitary Tee for a small sink. Adding the mechanical air vent before the P-Trap should eliminate any concerns around draining it, no? He also said if there was a backup, I'd have waste water issues, but isn't that the same for any other drain further down the line (like my master bath)?

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  • Welcome. Please take the tour to learn how this site works.
    – isherwood
    Jun 16, 2020 at 21:17
  • Very nicely done for a first post! Welcome!!
    – FreeMan
    Jun 17, 2020 at 13:16
  • @Homercliez - while model codes allow the use of mechanical vents (AAVs) in some cases, many local codes remove that part of the model code in adopting their local code, and nobody I know with any practical experience of them likes them one stinking bit. Fail and stink they do... Run a real vent line or don't bother. And don't cheap out on the smallest possible vent line size.
    – Ecnerwal
    Mar 14, 2021 at 1:07

1 Answer 1


Air admittance valves aren't allowed because they tend to fail over time and because they can leak sewage in a backup. Obviously such a situation can result in overflow elsewhere, but that's the rule.

As you've implied, flow from upstairs creates vacuum as it rushes past such a connection. If there's an opening for air to enter, as there always is with a drain, atmospheric pressure pushes the water out of your trap. Too much flow from above can result in backflow without adequate prevention.

There are ways to make this work, but you'd need at least a partial vent line attaching higher up on the stack.

  • Thanks for the information. I actually may be able to run a proper vent up that wall. I don't know what a partial vent line is, but I will start Googling that too.
    – Homercliez
    Jun 16, 2020 at 20:15
  • Typically you can rejoin the stack up above the entry points for the floor above. You may not need to go all the way through the roof.
    – isherwood
    Jun 16, 2020 at 21:17
  • That makes a lot of sense. Going to peek into the attic this evening and see what I find. Thanks!
    – Homercliez
    Jun 16, 2020 at 21:33
  • 1
    To ensure I understand correctly (for future knowledge): The partial vent would run from the blue pipe in the OP's drawing, up the wall, then would connect back into the stack above the highest drain entry point in the "upstairs" bathroom, tying it back into the upstairs bathroom's vent line?
    – FreeMan
    Jun 17, 2020 at 13:15
  • That's what I've done in the past, but I'm not enough of a plumber to know whether that's exactly right for this case.
    – isherwood
    Jun 17, 2020 at 13:31

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