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Thread: Dishwasher Siphoning water

  1. #1
    DIY Senior Member rsmith99's Avatar
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    Default Dishwasher Siphoning water

    I have a dishwasher with a drainline that runs into the bottom of the sink cabinet and into the garbage disposal.
    When we don't use the dishwasher for a few days it starts to stink and we notice about an inch of dirty water in the bottom.

    I recently installed a new garbage disposal and did the usual checks for leaks, which included running the dishwasher.

    A minutes after the dishwasher had stopped I looked at the dishwasher drainline and I could see bubbles and water running backward from the bottom of the loop in the drainline, back toward the dishwasher.

    The dishwasher drainline is looped about 4" higher than the disposal drain.

    What am I doing wrong?

    Thanks!
    Ron
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  2. #2
    Plumber jimbo's Avatar
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    Your DW drain is BELOW the bottom of the sink, WAY below the flood rim of the sink. Mother Nature ( gravity) will assure that water flows back into that hose. Your code may or may not require an air gap....but installing one would be good. At a minimum, the drain hose should be high-looped up against the bottom of the countertop.

    Last edited by Terry; 11-10-2012 at 09:48 AM.

  3. #3
    DIY Senior Member rsmith99's Avatar
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    But it is higher than the disposal drain line. I thought that's what mattered.

  4. #4
    Plumber jimbo's Avatar
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    Quote Originally Posted by rsmith99 View Post
    But it is higher than the disposal drain line. I thought that's what mattered.
    Do you never have water in that sink higher than the disposal outlet???????????????????????????????

  5. #5
    In the Trades Gary Swart's Avatar
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    What was it about "high-looped up against the bottom of the counter top" didn't you understand? As already point out, some areas require an air gap be installed, while other areas permit high-looping. Gotta be one or the other.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Two weeks ago I added an air gap to Sue's dishwasher. Her dishwasher drain looked just like yours. By the time she was running the dishwasher, there was so much gray water in there from the kitchen sink it was almost pouring out the door.

    It's a simple fix, and your wife will think you're a God.

  7. #7
    Moderator & Master Plumber hj's Avatar
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    It has to loop above the connection to the disposer, NOT the drain line. Ideally, it should loop up to the bottom of the countertop, but even that will NOT prevent siphonage, if the water in the sink is above the dishwasher's connection to the disposer.
    Licensed residential and commercial plumber

  8. #8
    DIY Senior Member rsmith99's Avatar
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    Ok. It's up against the bottom of the countertop. Couldn't go any higher without drilling a hole in the countertop.
    I understand a high loop! When I watched it siphon water, the sink, disposal, and dishwasher were empty. It was pulling water back out of the dishwasher drain line that didn't quite make it to the garbage disposal. Now all the drain line is above the disposal outlet.

    Thanks for the friendly help!

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    There's a reason why all commercial dishwashers and many locales require an air gap on residential units...to specifically stop what you were seeing. Also, keep in mind that the disposer acts somewhat like a pump when you do run it, stuff drains to the area with the least restrictions. Second rule, gravity always wins!

    The instructions on ALL dishwashers say to high loop as high as possible unless you're using an air gap. They do that for a reason.
    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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