Washing machine filling with sink water

31 Oct 2013
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United Kingdom
Hi, I appreciate this is probably going to make me look stupid but having no clue about even simple plumbing I've come here to ask the experts!!
We've recently noticed the drum of the washing machine filling with water that goes down the kitchen sink. I've had all of the pipes off under the sink and given them a good clean and while they were off, had the hose down the pipe that leads outside to the drain as I can't take it off and the water ran nicely through.

The problem I'm noticing is that water comes from the sink straight to the u-bend (i've learned it's called a trap :) ) in between the down pipe from the sink and the u bend is the inlet where the washing machine hose joins the waste system. Water from the sink is quickly backing up from the trap and backfilling the washing machine waste pipe. Only recently noticed as we've had the tap running a lot longer so it's had chance to fill up the machine and start to be visible in the drum.

I'm at a loss as to what to do? Any help please?
I've included a photo of the pipe work under our sink with the waste pipe taken off. When connected, it goes from the inlet down to the bottom back left corner of the cupboard to the machine which is right next to it. The light grey pipe is from the dishwasher.

Sorry if I'm being dumb.
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In a setup where the washing machine joins into the sink waste there should be a one-way valve to stop back flow into the washing machine. My guess is your setup either doesn't have such a valve or it's faulty.​
the easier way:

the flexible waste hose for the washer must loop up, and the top of the loop must be higher than the water level in the sink when it is full.

If you attach the loop to the underside of the worktop, that will probably do. Use a couple of cuphooks and a piece of string. or you might feed it through a hole in the side of the unit.
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As JohnD says, make sure the hose is as high up as possible under the units. I believe most washing machines these days have a built in non-return valve to prevent this from happening but there are some makes out there that don't. If that doesn't work, look for any partial blockages in the sink waste/traps.

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