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Thread: Stubborn kitchen sink venting problem

  1. #1
    DIY Junior Member Meta's Avatar
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    Default Stubborn kitchen sink venting problem

    I did a full house remodel a few years back, including all new plumbing. The new kitchen sink has been plagued by a very slow drain relating to what I think is a flaw in the way the air vent was installed. Both sinks routinely fill with water while washing dishes and drain at a snails pace. The flow can me marginally improved by turning on the garbage disposal that seems to inject air into the system and allow it to flow at a slightly faster snail's pace. Removing the air vent entirely allows the sink to clear and flow very rapidly. Bad air vent? Nope. My plumber replaced it after I asked him to fix the problem. He even we so far as to experiment by installing two vents, thinking one was not providing enough air. My thinking is that we have a design problem with the system that's causing the vent to lock up with water before it's able to allow air into the system. I've attached some pics for the gurus here to have a look at at. My plumber is suggesting drilling through the wall to the outside to utilize a traditional open air vent. If rather not have an ugly pipe outside my house in a conspicuous location and would prefer to solve this more intelligently. Any thoughts here? Much appreciated!


  2. #2
    DIY Senior Member Smooky's Avatar
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    I have disassembled several types of AAV’s and some have very small openings. Some are very simple and the price is low but may not allow enough air into the pipe. Some that cost a little more have better designs and allow more air in when needed.

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    DIY Junior Member Meta's Avatar
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    That's what my plumber thought, that it was not getting enough air. So he temporarily installed two vents where now you see only one. Two vents made no difference at all and the sinks continued to be extremely slow so he went back to a single vent. I suspect the vent is locking up with water immediately and then the system "breathes" through the sink drains. The drains gurgle with air constantly as it very slowly drains.

  4. #4
    In the Trades Tom Sawyer's Avatar
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    You have a sag somewhere in the drain line
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Quote Originally Posted by Tom Sawyer View Post
    You have a sag somewhere in the drain line
    Do you mean a sag between one or both of the traps and where it goes into the back of the cabinet and then down the waste pipe in the wall? I didn't look at the pitch carefully, but could a small negative pitch towards the trap over such a small section cause the problem? What explains the good flow without the air vent? I'm still struggling to conceptualize this problem.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    The vent prevents the traps from siphoning, not generally a factor in whether the pipes drain or not. An unvented S-trap drains too well, siphoning itself, which is why they are illegal. A garbage disposal can put a lot of crud into the line - it's likely, as Tom said, there's an issue, most likely with the slope or linearity of the drain line and there may be a partial clog.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    Your drain line is partially plugged. When the sink tries to drain it has to push the air out of the drain line. The obstruction prevents it from moving down the pipe, and the air vent WILL NOT let it vent under the sink. IF you had a conventional vent, the line would still be restricted, but the air would be released up the vent pipe. The disposer, (NOT a "Disposal"), is a pump that forces the air down the pipe. It is not "letting air into the pipe".
    Licensed residential and commercial plumber

  8. #8
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by jadnashua View Post
    The vent prevents the traps from siphoning, not generally a factor in whether the pipes drain or not. An unvented S-trap drains too well, siphoning itself, which is why they are illegal. A garbage disposal can put a lot of crud into the line - it's likely, as Tom said, there's an issue, most likely with the slope or linearity of the drain line and there may be a partial clog.
    As Jim said, the purpose of the vent is to allow air in, to break the siphoning. If air needs to be let out of your drain to get it to work, it suggests that there is a problem with your drain lines somewhere after it disappears into the wall.

    The best analogy I can think of is trying to fill a pop bottle. As you try to pour water in, the air needs to come out the same opening.

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