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Thread: Strange Drain Problem

  1. #1

    Question Strange Drain Problem

    I just installed a 15" round hand sink (w/o overflow) and a grid drain in a new commercial remodel. There appears to be a "back pressure" at the grid drain which makes for very sluggish drainage until the water reaches a certain height in the sink and balances the "back pressure". At this point the sink fills no further, but the water level remains high. A quick stroke of a plunger starts a siphon action that drains the sink in a flash, but once it empties, the cycle starts all over.

    The plumbing is all new (6' of 1 1/2", 30' of 2", 10' of 3") to the main drain. I installed a vent pipe about 2' from the drain when I did this installation. I suspected a vent problem, but after installing an auto vent at the trap, there was no change. I even increased the trap size to 1 1/2" to no avail. I'm not a plumber, but have done many, many installations and never seen this. Any ideas?

  2. #2
    DIY Senior Member Mike Swearingen's Avatar
    Join Date
    Jan 2005
    On Albemarle Sound In Northeastern NC


    Are any of the drain pipes less than 1/4" per linear foot slope toward the main drain?

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default drain

    The key words in your descsription are "w/o overflow". The overflow is how the air trapped between the water in the trap and the water in the sink escapes. Without the overflow it is caught and prevents drainage until you plunge it, or the water depth is enough to overcome the backpressure's resistance. The openings in the grid strainer are not large enough to permit the air to rise while the water is falling. Often drilling the holes larger is enough to allow the two to pass through at the same time and permit drainage.

  4. #4

    Default Strange Drain

    All drain lines are pitched at 1/4"/ft. Hj, You're right about the trapped air not being able to escape the grid drain. I can see it (an air bubble) in there while the drain gurgles. If I take a soda straw and poke it through one of the grid holes while the basin is filled, I can suck the air right out. If I suck hard enough (my wife found this amusing), the drain suddenly roars to life. I'll change out the sink with one equipped with an overflow and/or drain. Thanks for the help!

  5. #5
    DIY Junior Member mrjetskey's Avatar
    Join Date
    Apr 2005


    Your wife found it amusing?I just fell off my chair,that sounds like something I would do!Have A great Day!!


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