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My washing machine drain overflows from the standpipe. The waste pipe has kitchen sink, washing machine and dishwasher connected with dedicated P traps. The dishwasher and washing machine have dedicated standpipes. There is no overflow from the dishwasher standpipe. The waste pipe header goes down below the floor and then comes out to an outside sewer trap. I opened the sink P trap and washing machine P trap, cleaned them and also snaked the line all the way. But it doesn't look like a clogging problem as when I take out the washing machine P trap and leave the pipe open on that side, the kitchen sink drains well with no water backing-up even with a fully open tap. However, when I screw the washing machine P-trap back, the problem comes back. It seems there is air getting trapped somewhere as there is lot of gurgling. There has never been an air admittance valve (AAV) anywhere in the plumbing. The system has been working fine for many years until last few months so it can't be a plumbing design issue.

I noticed the drain hose was going deep into the standpipe so I raised up and screwed the hose clamp higher such that only about 2 inch of the hose goes into the standpipe. Not sure if this will solve the problem. Temporarily put a container below the overflow location to catch water if it happens again but this is not really a solution.

I am an engineer so keen to solve the problem myself before going to a plumber. Any suggestions on what could be the problem?

Thanks

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    Could be a vent problem. The vent could be blocked/partial block by outside stuff(leaves/birds/rats/santa). Should have a black(usually) pipe sticking out of your roof by a couple/few feet. This pipe is connected/extending from your main drain line.
    – crip659
    Commented Jan 4, 2023 at 21:47

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I had the exact same problem. kitchen sink and all the baths drained good, Washer standpipe overflowed. Turned out the main drain had a partial blockage. Plumber cleared the blockage. All is well

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    Washers do tend to dump a lot water fast, so they will show a partial blockage first, compare to other sinks/drains just draining the tap flow.
    – crip659
    Commented Jan 4, 2023 at 23:12
  • Since the kitchen sink tap drained without backing up with full tap open, it may be as high flow as a washer drain I guess. Both kitchen sink and washer drain through the same drain header into the outside sewer i.e. final disposal point to the sewer is common. If the kitchen sink overflow hole is partially blocked, could it cause this issue? The overflow hole also acts a vent.
    – Newbie
    Commented Jan 5, 2023 at 0:36
  • sink overflow vents the drain before the trap, so it only vents the short section before the kitchen trap
    – Jasen
    Commented Jan 5, 2023 at 2:26
  • I'm impressed, @Jasen, that you can see through the OPs walls to know how his plumbing is vented!
    – FreeMan
    Commented Jan 5, 2023 at 14:20
  • actually the sink overflow is almost always integral to the sink, if it is indeed plumbed inside the wall I am definately wrong.
    – Jasen
    Commented Jan 8, 2023 at 23:53

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