I fit a dishwasher at weekend, unfortunately due to length of waste pipe and the, ahem, previous administration’s plumbing work I’m getting “backwash” from the sink into the dishwasher, (setup is a bit like this but the u bend outlet is slightly higher still). I, figure the easiest way is getting an inline “none return valve” (see 7.3 here) for the dishwasher waste pipe but I’m having difficulty finding one. Any ideas? The proper name for them may help.

Next options are fitting a longer hose and raising it higher than ubend outlet or mucking about with the Ubend/sink plumbing, would prefer the valve tho.

the dishwasher should have it’s own internal U-bend set up to prevent this. Ours had a problem last year that was causing it to not drain caused by a little piece of plastic off the clip of dishwasher freshener stuck in the only bit of the U-bend assembly that was not visible. The waste trap at the bottom of the dishwasher also should have a non-return valve (normally a rubber flap) to keep the water in the internal U-bend section.

Is the dishwasher new?

Also the problem could be caused by poor draining past the u bend in your kitchen plumbing and it only just coming to light now and the dishwasher, being lower than the sink is showing the problem. Having the U-bend above the dishwasher conntection sounds odd too. I would want the top of the U bend to be below the dishwasher inlet and make sure it is free flowing when water is draining from the sink.

DW is new-ish, 1 careful short term owner. Definitely draining OK, at the end of the cycle the bottom of DW is fine, couple of hours washing hands and the odd pot in the sink and the DW has a puddle in the bottom. I was a little surprised that the DW doesn’t appear to have it’s own valve.

Having the U-bend above the dishwasher conntection sounds odd too.

yeah, as with a lot of stuff in this house it’s a bit of a convoluted route, it can be tidied up to get the outlet lower than DW waste pipe but that’s more work than I wanted to do TBH.

Mine has a non return valve fitted to the ‘stem’ you connect the dishwasher waste too in your pic, so you could buy a complete trap and use that.

Or do what everyone else in B&Q had done which was tear open the pack and nick just that bit – took me a visit to PlumbBase to find a complete one!

You shouldnt be getting any backwash into the bottom of the dishwasher due to the valve it should have. Of course yours might not have one but I would doubt it as they all seem similar.

Jump to around 3 minutes for the rubber check valve flap: https://www.youtube.com/watch?v=-b5Z1SjaVQg

Jump to around 3 minutes for the rubber check valve flap

😯 jesus! Interior looks very similar to mine but **** a duck! you have to almost entirely dismantle the thing to get at the valve. Is this the DW equivalent of removing most of your engine to replace a headlight bulb?

Or do what everyone else in B&Q had done which was tear open the pack and nick just that bit

unfortunately I couldn’t do that, more a worry of getting caught than lack of moral turpitude 😉
So I guess no-one sells them separate? you have to buy the whole frickin lot to get the ~50p valve?

Checking the screwfix site I could get this but it doesn’t say whether it has a valve
or this and this to extend the waste pipe and make a loop (n-bend?) higher than the ubend?

bump