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Thread: Slow draining basin

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  1. #1
    DIY Junior Member Malibu Jim's Avatar
    Join Date
    Oct 2007

    Default Slow draining basin

    About 1 & 1/2 years ago, we had our bathroom renovated and the company doing the work replaced the wash basin, among other things. Since then, it seems our sink is sometimes slow to drain. While running water in it, it will drain OK, then suddenly stop draining until it's 1/2 to 2/3 full, then drain again normally. I don't believe it's a venting problem, since it was OK previous to their work. I have taken the trap apart, but never found "anything" in it. I did notice, that it appears, the drain pipe from the sink attach to the connection at the sheet rocked wall is a little smaller than the pipe in the wall. Could this create some kind of a vacum problem ? The sink has never come close to overflowing, but it's just annoying.

  2. #2
    DIY Senior Member kreemoweet's Avatar
    Join Date
    Sep 2009
    Seattle. WA


    There is nothing improper about using a larger size drain in the wall, nor could it cause slow drainage. It would appear there is some kind of intermittant
    partial blockage in the pipes downstream from your lavatory. I doubt anyone not on premises will be able to tell you what it is.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    What does the drain in the sink look like? If it's a grid drain with small holes, does the sink doesn't have an overflow? Sometimes, with sinks like that, it gets airlocked, sort of like when you put your finger over the end of a straw...the water won't flow out.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Janitorial Technician nestork's Avatar
    Join Date
    Aug 2012

    Default It's a clogged sink overflow drain.

    I'm with Jadnashua on this one.

    I regularily have to clear the overflow drain on my bathroom sinks to keep them draining properly.

    But, I don't think it's an air LOCK issue. From what I can see, if the overflow drain is clogged, then when you pull the plug on a full sink of water, the air that was under the plug gets compressed under the clog in the overflow drain.

    As the water drains out of the sink, the pressure on that compressed air diminishes and it starts expanding into the sink drain, thereby pinching off the water flow. Watch closely, and if you see the drainage rate slow to a crawl, and then see a big bubble come out of the drain, and then the sink drains quickly again, it's definitely a clogged overflow drain.

    I have a special 1 1/4 inch trap adapter that's cemented directly into a removable cap. I put a 5 gallon pail under the sink, put a board across that pail, and then put that special adapter on the sink drain, and tighten the nut at the top so it doesnt' leak. Then I fill the sink full with water until the water covers the overflow drain and plunger the sink. Since the water can't drain, the water surge goes through the overflow drain, and I end up plungering all the rotting putrifaction in the overflow drain into the sink through the overflow drain hole. Then I remove the board, take the removable cap off the bottom of my trap adapter and the water and crap in the sink drain into my pail. Then I'm good for a long time again.


  5. #5
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    If it is a vessel sink without an overlow, it will airlock until water starts flowing good, but if the faucet is left running, the aerated water WILL restore the air lock before the sink empties.
    Licensed residential and commercial plumber

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