After dishwasher drains and finishes its cycle (no problem draining), if we run water in sink, it trickles back into drain tube and builds up in DW, even with high loop. Strangely, if I disconnect the drain tube, water does NOT enter drain tube. I can literally leave it unattached and no water will come out of drain connection.

  • High loop (Yes)--
  • Air gap (No)--
  • Garbage disposal (No)--
  • Corner dead space (Yes) (not side by side)

Question/possible solutions?

  1. Does the high loop need to be higher? Pic represents height.

  2. Is the drain tube too long within corner dead space, possibly causing a pressure build up? Would shortening tube solve this issue?

  3. Should I be concerned if there is a large loop in drain tube in dead space?

enter image description here

  • 1
    Are you sure the water is coming from the sink drain? Maybe the dishwasher inlet valve is failing. My clothes washer recently had this problem.
    – isherwood
    Commented Apr 5, 2023 at 15:27
  • 1
    Do you know water is trickling in through drain tube, do you see it flowing through? Or do you know really only that you find water in the dishwasher when not in use? Because if the latter, I'd go with isherwood's theory. The inlet valves get calcified then leak.
    – jay613
    Commented Apr 5, 2023 at 15:31
  • " if I disconnect the drain tube, water does NOT enter drain tube. I can literally leave it unattached and no water will come out of drain connection." But do you still get water trickling into dishwasher (suggesting inlet valve as suggested in these comments)?
    – Armand
    Commented Apr 5, 2023 at 15:54
  • Is it possible that your main drain needs cleaning? I suspect that there is water in the drainline in the dead space between the DW and the high loop. If your main drain starts to run slow due to some partial blockage, with the drain setup you have under the sink, you're going to get back pressure when water is draining from the single sink. This may force water in the DW P-trap and the dead space back through the drain line and the DW. Make sure that the main drain line is running freely - it may need cleaning..
    – HoneyDo
    Commented Apr 5, 2023 at 16:16
  • Is there a reason you cannot install an air gap? An air gap will break the siphon and prevent backflow.
    – Chris O
    Commented May 5, 2023 at 20:07

2 Answers 2


The two p-traps is pretty jenky.

Always hard to judge elevation in these kind of photos. If the p-trap for the dishwasher is lower than the sink trap arm water could flow back into the dishwasher p-trap pressurizing the air in the dishwasher ptrap and causing the water in the dishwasher side of the high loop to push back into the dishwasher. I'd cut down the sink tail piece and get a vertical dishwasher connector and eliminate the dishwasher p-trap.

The other possibility is you have a partial clog between sink and stack but you'd see the water backing up into the sink before it is backing up the high loop into the dishwasher.


Seems like that connection scheme could permit siphoning water out of the trap, despite the high loop. I'd do like Codemonger suggests and reroute the connection to the sink tailpiece

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