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Adding a plumbing vent to old plumbing—too much work?

maine_tyler | Posted in Mechanicals on

Ok plumber types. I understand why venting is now required, but many older systems lack individual trap vents. Moreover, older systems often have a single cast iron vent stack that is nearly impossible (impractical) to tie into on a trap by trap basis.

So what is the reasonable solution when remodeling a kitchen or bathroom sink?

In my situation, the previous bathroom sink had an old drum trap with no horizontal run- straight into the basement. I am happy to change to a P-trap and add the needed inches of horizontal pipe post-trap, but getting venting in that location seems nearly impossible without significant surgery to the surrounds. Furthermore, P-traps are more susceptible to siphoning than the old drum traps. So am I better to simply stick with a drum (acknowledging they collect debris easily), or should I be looking into something like an air admittance value (AAV), or other?

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  1. Expert Member
    BILL WICHERS | | #1

    I'd go with the AAV. I'm not a huge fan of the things generally, but if you're in a pinch with an existing system, they're a good option.


  2. Expert Member
    NICK KEENAN | | #2

    Air admittance valve. If the sink is in a cabinet it just goes in the cabinet. So long as there is a roof vent somewhere in the system you can use the AAV to solve vent-trap distance problems.

    I'm a little puzzled by your statement, "I am happy to change to a P-trap and add the needed inches of horizontal pipe post-trap." The only horizontal pipe you need is between the P-trap and the tee for the vent or AAV. There's no minimum on that length, just a maximum. If your drain goes into the floor under the sink an AAV works well because you don't have to re-route the drain.

    The AAV does need to be mounted somewhere accessible, which can be an issue if you have a pedestal sink.

  3. maine_tyler | | #3

    Thanks Bill and DC.

    DC, I thought there was a minimum length that the waste arm extension (horizontal pipe downstream of the trap) needed to be to be considered a P trap vs. an S trap. 1.5 to 2 times the pipe diameter or something. Am I making this up , or misunderstanding something?

    1. Expert Member
      NICK KEENAN | | #5

      This article explains it:,a%204%E2%80%9D%20trap%20arm%20minimum.

      It's two pipe diameters -- so 3" for inch and a half and 4" for two inch -- but it's measured not from the edge of the fitting but from the vertical section of pipe. If you use a tee it's almost impossible not to get the required distance.

  4. Expert Member
    PETER Engle | | #4


    There is no minimum length for the trap arm. It would seem that the "P" trap starts to look like an "S" trap when the arm is short, but that doesn't account for the vent. A "P" trap with a short arm leading to the tee fitting (vent up and waste down) doesn't work like an "S" trap because the vent breaks the vacuum that can siphon the seal out of an "S" trap. This is also true with an AAV mounted above the tee. The AAV opens when there is a vacuum in the line, admitting air and breaking the vacuum before it has a chance to suck the trap seal dry.

    Be careful to get an actual AAV. There are similar "mechanical vents" that are spring loaded. These are not code approved for residential construction. FWIW, most AAVs are white and most mechanical vents are black. Possibly just coincidence but maybe not. Just read the package. The descriptions of the parts will be specific - Air Admittance Valve (AAV) or Mechanical Vent.

  5. Expert Member


    Install the AAV with a rubber coupling. It makes it much easier to put in, or service if necessary.

  6. maine_tyler | | #7

    Thank you all.

    I don't plan to, but I assume one wouldn't want to install an AAV in a wall, correct? For serviceability and leak issues.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      Yes. The two requirements for correctly installing one are that it must a minimum of 4” above the fixture drain and readily accessible. That rules out burying them in walls, but does mean they can go in attics.

    2. Expert Member
      NICK KEENAN | | #9

      If it's in a wall you have to have a panel or other access path to get at it in case it needs to be serviced.

      It shouldn't ever be exposed to wastewater so leaking shouldn't be a concern.

      1. Expert Member
        Deleted | | #10


    3. Expert Member
      BILL WICHERS | | #12

      The classic installation location for AAVs is in a cabinet under the sink they serve.

      I'd be reluctant to install one in the attic, mostly because I don't entirely trust the things to start with, and in an attic you're unlikely to notice any issues. If you can install one in an attic though, why not just extend the pipe up through the roof and make it a "real" vent anyway?


      1. Expert Member
        MALCOLM TAYLOR | | #14


        And what are the chances of anyone ever finding it up there if it does go wrong?

        1. Expert Member
          BILL WICHERS | | #15

          Probably pretty low. It would become one of those building details that will be lost in the mists of time, like mysterious electrical junction boxes and your favorite screwdriver.


  7. user-6623302 | | #11

    You can use a loop vent or oversize the drain pipe. Look at techniques for plumbing an island sink.

    1. Expert Member
      MALCOLM TAYLOR | | #13


      I agree. I much prefer Chicago Loops to AAVs.

    2. maine_tyler | | #16

      The loop vent still requires tying into the cast iron vent stack, which sounds difficult to me, but perhaps I'm wrong. I hadn't heard of simply oversizing the drain. Is that a reliable way to ensure proper drain action? Would it need to be oversized for the entire run to the main drain?

      1. Expert Member
        MALCOLM TAYLOR | | #18


        Leaving plumbing codes aside... Just as over-sizing the drains can work, so does a loop-vent with no connection to the vent stack.

  8. user-6623302 | | #17

    Why don't you post some pictures so we are not lost in words.

  9. maine_tyler | | #19

    This article get's into it:

    The oversized drain pipe (combination drain/vent) sounds convincing. Can't think of why not to do that. Thanks for the heads up Jonathan.

    I don't really have any worthwhile photos. I think any conceptual discussion on the 3 options discussed here thus far (and in that link) would apply to me so far as I can tell. I've drawn the line at not cutting into the old cast iron if I don't absolutely have to.

    Malcolm, every photo of the loop vent I see looks different from the one you've posted. They all tie into a vent. Yours appears to be using that main drain as a sort of wet vent. But then why not just oversize the vertical connection to the fixture and do away with the whole loop thingy?

    1. Expert Member
      MALCOLM TAYLOR | | #20


      You don't find many examples of either loop-vents without a connection to a stack , or over-sizing the drains, because until recently neither were code approved. That's why I added the disclaimer "plumbing codes aside". Both work well, although the loop-vent still isn't legal. It does seem like in y0ur situation a larger drain is the easiest option. I agree - cutting into the cast iron pipes is definitely something to avoid if possible.

  10. user-6623302 | | #21

    Cutting into cast iron branch drain is not that difficult if you have a chain cutter but the easiest way is to attach at a clean out. Use a pvc bushing and a sanitary tee. I am no pro but have done both.

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