Dishwasher drain plumbing, again..

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by cmbroth, May 23, 2011.

  1. cmbroth

    cmbroth New Member

    May 23, 2011
    I've checked many of the forum topics on this, and I think this might be different..

    My dishwasher is a Whirlpool Quiet Partner IV and has worked perfectly for 2-3 years in my old house. We moved it to the new house and I installed it. The supply plumbing and wiring was simple, but in the old house we had the drain hose installed over the disposer (although not correctly) and the dishwasher still worked correctly. In this new house, the drain hose goes from dishwasher to high loop to basement where it meets a dedicated p-trap. My question is -- is all of that necessary? The dishwasher doesn't drain correctly and I wonder if all these backflow preventions are causing drain problems. My other option would be to run the 8 feet of hose around to the disposer. I'm thinking the trap prevents gasses, but does it prevent sewage backflow per requirements? I've also heard rumors some newer dishwashers have internal hose loops? Many questions.. help, please and thank you.
    Last edited: May 23, 2011
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    A DW must hook to the drain with an indirect path (an air gap - although some places allow high looping, all places allow an air gap which is safer). Hard plumbing it into a dedicated trap is just wrong, and unsafe. Also, having it below the DW, could under some circumstances, allow it to siphon. When it is hard plumbed into the sewer system, if there ever was a backup, it could push waste back into the DW. If it is vented properly, there's no real chance of gas pressure forcing its way into the DW.

    FWIW, my DW has probably about a 10' run to the disposer...the pump has no problem pushing the water that far.
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  4. cacher_chick

    cacher_chick Test, Don't Guess!

    Jan 5, 2008
    Test, Don't Guess!
    Land of Cheese
    Because of the drop you have between the high loop and drain, it will quite surely siphon. Installing a dishwasher air gap unit on the sink or counter would prevent this.

  5. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona
    People tell me that some dishwashers have a built in "high loop" but I have NOT seen any so far. They do have a "loop" for the water feed, however. Your drain will siphon the water out of the dishwasher since it does NOT have an air gap above the water level in the unit.
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