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Community and Q&A

DWV Connection – Drain to Vent

mritterjr | Posted in Mechanicals on


I am wondering if this is an incorrect setup for our drain line and its connection to the vent?  I have seen multiple posts online that a wye should not be used in this application, but it should be a sanitary tee.  For the plumbers in here, can I get confirmation of this?  My contractor, who is not using a licensed plumber but operating under the license of one, is adamant this is sufficient.  My understanding is the way it’s setup now with the wye could allow for a strong enough siphon to pull water from the trap and allow sewer gas out the “wrong end.”  It also appears to almost make an s-trap, but I think that’s debatable.  



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  1. Patrick_OSullivan | | #1

    Not allowed and likely clearly evident in whatever code you're under. You are correct in saying it should be a sanitee.

    1. mritterjr | | #3

      Thank you for the confirmation! Glad to know I'm not crazy. Everyone should bear witness to the totality of this plumbing job and let it serve as a guide on exactly how not to plumb a house. It's so sloppy and bad.

  2. Expert Member
    BILL WICHERS | | #2

    Yep, should be a sanitary tee. There is a thread on almost exactly this setup on the Terry Love plumbing site:

    JLC's field guide for venting may also be helpful here:

    I think you're pretty close to having an S trap here too, but if they replace that wye with a sanitary tee raised up a bit, then they'll probably have enough of a horizontal pipe section to get you back into P trap territory. Basically replace that one fitting, and swap the position a bit using a coupler, and you can fix both issues at once.


    1. mritterjr | | #4

      Thank you Bill for the reply and links. They will serve very helpful in my conversation. I also found a plumbing company that will come do a full inspection, so at least will get some peace of mind before everything is covered up and it's too late.

  3. Expert Member
    DCcontrarian | | #5

    Not a plumber. But the reason a sanitary tee is required there is there has to be a continuous layer of air between the top of the pipe and the vent, otherwise the pipe can get airlocked under heavy flow.

    Fixing it is less than $20 worth of fittings and a few minutes.

    1. mritterjr | | #6

      Thanks DC, that's my instinct too but all I get is pushback when I ask, cause I'm just a lowly homeowner. It's a pattern of behavior from our contractor and we're just safeguarding ourselves from future issues and trying to get to a point where we can let him go.


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