Hi guys,

I thought I'd draw a diagram of my problem as well as describing it! (Busy at work as usual! :D )

So, here is the setup;

http://i30.photobucket.com/albums/c317/JtotheBtotheS/kitchen.gif

As you can see (hopefully), my washing machine drains into the outlet pipe of our kitchen sink. The snag is, water and bubbles come back up into the sink when the washing machine empties.

I took the pipes apart under the sink, but there's no blockage (the sink drains fine).

Can anyone tell me why this is happening? Is it just a bad setup? :confused:

Yes, that is a bad setup!

Especially if you are using a conventional, top-loading, high-capacity washer... those things put out a lot of water. That diagram looks like a front-loader though. (ahh you're in the UK. Probably frontloader then!)

Anyhow, there is also a minimum height that the outlet hose has to be, which will vary by washer type and design. Conventional toploaders usually don't even have a valve on the outflow, just a pump. So if the hose is lower than the water level in the machine, the water will come out when it's not supposed to.

Check your washer manual or documents, if you don't have them you can probably find them online... I'll help if you need it.

In any case, I would personally install a separate 2" drain for the washer. You may get away with 1.5" (depends on the DFU of the washer, and the code you're under), but 2" would be a lot better. IF you have to attach it to the same horizontal branch that the sink uses, make sure you use a Wye (not sanitary T) and point it in the right direction. The washer drain must have it's own trap, and remember that the trap will slow the flow, so you need to have plenty of pipe ABOVE the trap to prevent exactly the same thing happening, just without the sink to catch the mess! Washer drain should also be vented, just like any other.

Upon further reflection, I really don't know what I'm talking about.. you'd better verify any/all of this with someone more experienced, I am NOT a plumber, nor do I play one on TV.

Concur that it's a bad setup. What's happening is that the washer is discharging faster than the pipe can take it; since the sink allows an "outlet" for the building pressure that's where the "extra water" goes. But that's about as far as my expertise can take it.

Well, it's only bad because it's the kitchen sink... this is one the things that laundry tubs are expressly designed to do, take on a high volume of water rapidly, buffer it and let it drain at the rate the pipes can handle..