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I am installing plumbing for a dishwasher that is not near a sink. We already have a dishwasher that is intended to be installed free-standing. My plan was to install a washing machine box in the wall in the kitchen and connect it to a drain/vent stack in the basement, putting a P-trap there, but was wondering if the height will cause an issue. There isn't space in the wall to install a trap and definitely can't install a self-venting cap or a vent pipe there either.

drawing of dishwasher plumbing In the picture, red is the hot water, the solid green is the drain would install and the dotted green is the existing drain/vent. The vent stack is actually on the other side of a door from the location of the dishwasher.

Also posted on Reddit: https://www.reddit.com/r/DIY/comments/18og80r/height_of_ptrap/

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  • I don't think the height of the p-trap will cause an issue if it meets all other considerations for an effective p-trap. It simply may be annoying to unclog if that particular area in the basement is not easily accessible. In fact, the code may require a cleanout, but count that as a good idea anyway.
    – AdamO
    Dec 22, 2023 at 14:53
  • I was planning to put a union on each side of the trap. It will remain accessible in the basement.
    – yakatz
    Dec 22, 2023 at 15:08
  • May want to add a vent close to the p-trap to avoid vacuum clogging. Check local codes, but within a couple of feet of the trap and further than 4-6".
    – RG Hughes
    Dec 22, 2023 at 15:13
  • It just comes down to critical trap arm length. jlconline.com/how-to/plumbing/… Dec 22, 2023 at 17:48
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    @FreshCodemonger That won't be an issue - the drawing leaves out some detail of the exact vent location, but it is close enough that the slope of the pipe will not cause it to go below the trap.
    – yakatz
    Dec 22, 2023 at 20:48

1 Answer 1

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Some plumbing codes specify a maximium standpipe height (above the trap weir) of 42". Dishwasher manufacturer instructions may specify a minimum discharge line height above the floor. This could get you caught between a rock and a hard place. I've done this before, trap ended up near the basement ceiling, worked fine, passed inspection.

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  • Since OP has mentioned installing a dishwasher outlet box, I imagined the standing pipe height would meet the minimum discharge line height. The drawing supports it. But you're correct, it would be a problem if it's too low.
    – AdamO
    Dec 22, 2023 at 19:24

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