Kitchen sink vent problem

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by pcrudolph, Oct 20, 2011.

  1. pcrudolph

    pcrudolph New Member

    Messages:
    3
    Location:
    Ohio
    We recently bought an older house, with plumbing that was updated about 10 years ago. A problem we found was that the kitchen sink drains very slowly. When I looked at the DWV plumbing, I found something very strange: there was what looked sort of like a loop vent...but with no vent connection, it just looped back to the drain line. Similar to the diagram of a Chicago loop on Wikipedia, if you crossed out the vent connection. Was there a reason for this that I don't understand, or was this as pointless as it looked?

    Anyway, I guessed that the slow draining was caused by a lack of proper venting, so I took out the loop, planning to replace it with an air admittance valve. (Yes, they're permitted here.) I tried the drain while the vent lines were wide open, and it drained great. But with the AAV in place, it drained as slow as before.

    By temporarily removing the AAV and taping a plastic bag over the vent line instead, I discovered that, when the sink drains, a POSITIVE air pressure is created at the vent, which must have caused the AAV to close, so that I still effectively have an unvented drain whenever the sink is in actual use. Now I'm not sure what to do. Installing a real vent out the roof would mean tearing out cabinetry and opening the wall, which I really want to avoid. (Plus, there's a wide window over the sink, which the vent line would have to go around.) But I can't think of any other options, either.

    Anyone have any suggestions?

    Thanks,
    Paul
  2. nelrossen10

    nelrossen10 New Member

    Messages:
    8
    Location:
    Atlanta, Georgia
    The mechanical vent installed below your sink is the right idea, but those do wear out (they're spring-loaded). I'm not sure what kind of mileage you can get out of them, but try replacing it. , and simply screw in (no glue).

    The leak is a different problem... Either not tightened correctly or not installed correctly.

    Of course, SpeedBall is the full expert here so listen to what he says!

    Good luck,

    James
    Last edited by a moderator: Oct 21, 2011
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,048
    Location:
    Cave Creek, Arizona
    You have a blocked or partially blocked drain. When the sink drains the air between the sink and the blockage creates the positive pressure keeping the sink from draining. Fix the stoppage and your sink will drain properly, either with the "loop" or the AAV. IF they had installed the loop properly with a "foot vent" it MIGHT have drained properly even with the stoppage, at least to a point.
  4. pcrudolph

    pcrudolph New Member

    Messages:
    3
    Location:
    Ohio
    Could this have been a response to a different post? I have no leak problem.

    Paul
  5. pcrudolph

    pcrudolph New Member

    Messages:
    3
    Location:
    Ohio
    I don't think the drain is blocked at all. I've run a snake all the way to the main stack, and it comes back perfectly clean. Also, when I had the vent lines cut open, I could run the water full blast and it drained perfectly.

    However, in the basement the drain line does have a long (20+ ft) near-horizontal run from under the sink to the stack. I suspect that, because the wastewater moves more slowly here, this section easily gets filled with wastewater. Once it does, any additional water entering the drain has the effect of pressurizing the air in the vent near the sink. That's my theory, at least.

    Paul
  6. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,267
    Location:
    Maine
    So you are saying that the drain is blocked


  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,048
    Location:
    Cave Creek, Arizona
    quote; Once it does, any additional water entering the drain has the effect of pressurizing the air in the vent near the sink. That's my theory, at least.

    It is a bad theory. Once the line is full of water there is NO air to be pressurized and the water will move as fast as possible taking the "new" water with it. The ONLY way the air can be pressurized is if there is NO WAY for it to escape, and for that to happen BOTH ends of the pipe have to be obstructed by water. IF you have a septic tank it could be overloaded. In this case the pipe WILL be full of water, but NOT plugged, and WILL drain by itself, but as far as the sink drain is concerned it will cause the same problem. There are MILLIONS of 20', and longer, horizontal drain lines which do NOT have your problem.
    Last edited: Oct 22, 2011
  8. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    It is usually a misconception to conclude that a bad, or lack of a, vent causes slow drainage. ALL fixtures are vented automatically....air can enter behind the water right through the sink, tub, etc drain. The problem with that is that is takes the trap water with it!
    A blocked vent can cause some gurgling in OTHER fixtures, etc. but in general the stuff will drain. A pumped discharge like a washing machine, etc. can sometimes cause an issue with a studor. But it sounds like you have a clog issue
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