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Thread: Dishwasher Air Gap with Under-Sink Dishwasher

  1. #1
    DIY Junior Member AugustSecond's Avatar
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    Question Dishwasher Air Gap with Under-Sink Dishwasher

    Hi all, I have a very peculiar setup. I have an under-the-sink space-saver dishwasher but when it drains, water shoots out of the airgap into the sink. After that initial spurt, the next time the dishwasher tries to drain, the sink drain actually floods up (just lower than level with basin) and water dribbles out of the airgap. That behavior continues for the rest of the times the dishwasher drains throughout its cycle.

    After running the dishwasher, the sink doesn't drain well (it bubbles when draining) until I blow on the airgap (using a handy paper-towel roll as a "straw"). When I blow into the airgap, water shoots up out from the sink's drain. After that water comes up, then the sink drains fine again--at least until I run the dishwasher again.

    As long as there is no water in the air gap discharge, the sink drains just fine. I've attached a photo of the plumbing with the dishwasher pulled out.

    Name:  UnderSink.jpg
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    BLACK HOSE: Air gap discharge
    White Ribbed Hose: Dishwasher discharge
    Silver Hose: Cold water inlet to Dishwasher

    I had a handyman who told me to remove the air gap and have the dishwasher discharge directly into the p-trap (which was previously hidden behind the drywall). He said this would cost $90 to fix.

    I then had a master plumber, who told me the only solution is to remove our silestone counter, remove both cabinets to the right of the washer, and tear open the drywall because all of the plumbing lines need to be redone. He wanted to move the p-trap to the left of the dishwasher (to make it accessible) and change the height of the horizontal line through the drywall until it reaches the vent about 2-4 feet away (there is a window directly above the washer). (Is this setup called a "dirty arm"?) This was thousands of dollars.

    My question: would the handyman's simple answer fix my problem? Is the present installation not up to code--hence the reason for the major reconstruction from the master plumber? Does anyone have input for my situation?

  2. #2
    DIY Senior Member bluebinky's Avatar
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    What a mess!

    One thing is that the hose from the air-gap to the drain should go down hill all the way. No low spot like it has how...

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    Plumber jimbo's Avatar
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    You have far from an ideal set up, but I would probably scrap the dishwasher, before I tore out the countertop and cabinets!

    If you can make it fit with the DW, I would put a wye on the vertical riser out of the p-trap. Sounds like this is what the handyman suggested. Be VERY careful on that set-up. You cannot have any kinks or funky bends on that very stiff black hose.

    I suspect this never worked right...is that correct? The fitting and connection where the black hose connects to the horizontal drain tube looks too small and may represent a restriction, which is what causes your symptoms. And as binky pointed out, that belly, which will always be full of water, in and of itself represents a restriction for the gravity drain from the airgap.

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    DIY Junior Member AugustSecond's Avatar
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    Thanks, Binky, Jimbo.

    Yes, Jimbo--you're correct that this setup never worked. When I purchased the home, our inspector told us the dishwasher drained into the sink, probably because of a seed caught in the line. He told us a wire-hanger would solve our problem. Boy, was he wrong.

    I also agree that the fitting at black hose/horizontal drain is likely too small. I think it's about 3/8", coupled with the 90 degree bend. Add to that--once the water makes it around the 90 bend, it has the option to either flow forward (headed back up into the sink) or backward (toward the wall and then toward to the right to the trap). It's all horizontal, so I imagine it can't tell which way to go.

    The handyman suggested using a wye (I believe) but he was going to have the white, ribbed hose drain into the vertical riser out of the p-trap. The stiff black hose would be removed. Bypass the airgap entirely.

    If there are kinks or bends, is the concern that wastewater would flow back into my dishwasher? Or is there an even scarier scenario that could play out?

  5. #5
    Test, Don't Guess! cacher_chick's Avatar
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    It looks like whoever plumbed that did not understand the concept of gravity.

    Jamming a proper drain and a dishwasher under a sink is a tough job.
    Last edited by cacher_chick; 03-13-2013 at 09:00 PM.

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    DIY Junior Member AugustSecond's Avatar
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    Hi cacher_chick, thanks for the help! I'm new to this so I'm not sure I understand. What do you meant by a "dishwasher tailpiece"? Where are you envisioning the trap?

    To explain, the dishwasher sits directly in this cubby and the existing horizontal PVC (under the "sink basket" if I understand that term correctly--right under the basin) rests on top of the dishwasher (there's basically no clearance). There's only 1" clearance behind the dishwasher, so the only room for a trap that I can find is to the left of the washer, where I have about 3" width.

    I was told I could fit the trap on the left, but then the copper trap would need to be removed (obviously) but also the copper piping would need to be re-done to be higher up before it meets the vent (which is to the far right of this photo). I was told it cannot "drop" down that much (the vertical copper pipe in image) and was told it needs to continue horizontally at a higher level until it reaches the vent. Once it hits the vent, then I was told it could drop. Is that not entirely true?

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    DIY Senior Member kreemoweet's Avatar
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    I vote for the Modified Handyman Fix: leave the airgap as is (I presume it's there because the local plumbing codes require it; if not, it can go). Somehow get a vertical section
    of drain with a 7/8" wye inlet (standard size for dishwasher drain) directly above the p-trap. Alter the sink drain to go straight over to that vertical and 90 down into it.

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    DIY Senior Member Hairyhosebib's Avatar
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    I think the sole reason these things even exist is to protect the potable water inlet supply in the event that the contents of the dishwasher were somehow sucked back into the water main. This is what a backflow preventer does. I don't understand why a horizontal spring loaded check valve cannot be installed after the shutoff valve and not even use these things. They almost never work properly and they end up ruining cabinetry from water damage.

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    Moderator & Master Plumber hj's Avatar
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    quote; I think the sole reason these things even exist is to protect the potable water inlet supply in the event that the contents of the dishwasher were somehow sucked back into the water main.

    Incorrect. It is to prevent drain water from getting into the dishwasher and "contaminating" any "clean" dishes in it. The water inlet has an "air gap" to protect the water supply line.

    I assume your dishwasher is one of GEs "better ideas". The sag in the air gap drain WILL cause water to spill out of it, but NOT to backup water into the sink. You either have a partially clogged drain, or a "lousy" drain installation, or maybe a little bit of both. Ideally, the air gap drain should connect to the riser above the "P" trap. If water backs up into the sink it will also go into the air gap drain and prevent it from draining properly, regardless of the sag.
    Last edited by hj; 03-14-2013 at 06:51 AM.
    Licensed residential and commercial plumber

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