Dishwater filling from waste pipe (Indesit DFG 15B1 UK)

[conny]

In a nutshell, your waste pipe going through the back wall needs to be below the height of the spigots.
As oldbuffer says, your effective water seal is above the level of the two spigot outlets and will therefore syphon back to the dishwasher.
Why/how this has not been happening in the past is a mystery, unless you have made alterations to the waste pipework/dishwasher outlet arrangement recently.
Arguing with people on here who are, clearly giving you suggestions/sound advice, is not going to solve your problem if you don't listen to what is being said. and try some of the ideas.

 

[oldbutnotdead]

I'm quite surprised that lot isn't leaking or reeking- the long drops from the sinks mean there's a lot of water trapped in the pipework.
I'm also surprised the thing hasn't been back-syphoning for ever- if you have a read of the dishwasher installation manual it'll almost certainly advise a 600mm or so rise for the waste pipe.
Your tee into the horizontal idea should work (in terms of breaking the syphon) and hopefully the water held in the dishwasher waste pipe will seal any drain whiff coming back from an untrapped attachment.
Ideally you'd be shortening the whole sink trap arrangements so the only permanently wet bit is the trap but that could be a right faff.

 

[DBMandrake]

1. The inlet spigots for the dishwasher waste and the unused one on the left hand side of your picture are in both cases below the level of the waste pipe going out through the wall.
2. So effectively you have a very deep trap (U bend), which will hold more water than it should.
3. Hard to tell from the photographs but:
3.1 If you can shorten the pipes coming directly from the wastes to raise the trap by enough to raise the spigots above the final outlet pipe, that should clear your problem.
3.2. If there is not sufficient length to shorten, you need to move the final outlet pipe lower. Will mean removing the back of the cabinet and drilling a further hole through the wall. Quite a lot of work.
4. If you raise the spigots, you will have a clear air break above the U bend to break any siphonage effect.
5. I suspect you need to check the drainage out anyway. For a problem like this to start with no plumbing changes suggests to me you have a partial blockage further down the drainage path so that the waste spigots are being immersed in U bend water for longer than previously.

Thanks for the great response. I've actually solved the problem before coming back to read this thread, (I'll post a photo in another post) but you make some good points so I'll comment on them as it might help others facing a similar problem.

1 - Yes, this is the fundamental problem, the waste exit through the wall is too high, that's what I was trying to explain in my previous post, but this was being ignored and I was just being told to raise the loop of the dishwasher pipe - which won't help.

2 - Yes, it's a deep U-bend, not much I can do about that without drilling a new hole in the wall which means the entire kitchen counter unit would need to come out. Lots of work. With a slightly different U-bend (a U instead of a J) I could raise it a little but not enough to solve the syphoning, and I wasn't able to find a suitable U as all the kits I could see used J bends.

3.1 - Not quite enough room unfortunately, that is one option I considered but the spigot would still be a little below the settling level of the trap and still be enough to prevent an air break.

3.2 - Yeah, a lot of work, can't feasibly do it unfortunately.

5. You would think this but I've checked it thoroughly and the exit pipe (which just runs into an open ceramic drain pot outside the window) is clear and can take a high flow without problems.

 

[DBMandrake]

In a nutshell, your waste pipe going through the back wall needs to be below the height of the spigots.
As oldbuffer says, your effective water seal is above the level of the two spigot outlets and will therefore syphon back to the dishwasher.
Why/how this has not been happening in the past is a mystery, unless you have made alterations to the waste pipework/dishwasher outlet arrangement recently.

Yes, I don't know why I wasn't noticing this issue in the past, it's clear that the problem should have been there from day one...so I don't understand that part of it.

Arguing with people on here who are, clearly giving you suggestions/sound advice, is not going to solve your problem if you don't listen to what is being said. and try some of the ideas.

As for arguing with people giving sound advice, was it actually sound advice ? I don't think so.

It was obvious the advice to just raise the drain pipe up higher than the sink and everything would be fixed was incorrect, especially when I had already tried what was being suggested, and stated that I had already tried it. Raising the drain pipe higher does NOT fix the issue of syphoning. I was then told that water can't flow uphill.... err ok...

 

[DBMandrake]

I'm quite surprised that lot isn't leaking or reeking- the long drops from the sinks mean there's a lot of water trapped in the pipework.
I'm also surprised the thing hasn't been back-syphoning for ever- if you have a read of the dishwasher installation manual it'll almost certainly advise a 600mm or so rise for the waste pipe.

The drain pipe was 600mm high - it was above the bottom of the sink originally, although I have it a bit higher now.

The main reason for trying to do something about all this was smells yes, but from the dishwasher not the sink. It has not always been smelly. Perhaps as the pipework has gunged up in the traps below the sink it has got worse. If the dishwasher has been syphoning mucky water back out of the sink trap into the dishwasher to sit in the bottom then a mucky sink trap would cause smells in the dishwasher.

I usually disassemble the whole lot and scrub it clean and reassemble it every year or two, this time however it was pretty bad so as I was buying new pipes to reconfigure the piping I've ended up just replacing 90% of the pipework with new stuff.

Your tee into the horizontal idea should work (in terms of breaking the syphon) and hopefully the water held in the dishwasher waste pipe will seal any drain whiff coming back from an untrapped attachment.

Yeah, that's what I've done:

The dishwasher is now connected on the final slightly downhill run and this is working perfectly, despite the excessively deep trap. There is a good air break at the point of connection - so good in fact that if I leave the dishwasher disconnected I can drain the sink without water even coming out the open hole - which I've deliberately placed facing upwards. Even while draining the sink there is still some air in the top of this pipe run.

The dishwasher doesn't need to be connected before the sink trap - when it stops pumping out an air break immediately forms at the point of attachment to the horizontal run and the contents of just the dishwasher drain hose syphons back into the sump of the dishwasher. It's below the level of the filter but if you remove the filter there is a good 2-3 inches of water depth after this syphon back - the dishwasher is providing its own trap so there shouldn't be any drain smells. (None yet anyway)

Putting it after the sink trap means no more gurgling or noise through the sink when it pumps out (no noise at all, it was quite noisy before) and no dirty water from the dishwasher sitting in the sink trap. There is basically no inter-mixing of sink water and dishwasher water at all now.

Ideally you'd be shortening the whole sink trap arrangements so the only permanently wet bit is the trap but that could be a right faff.

Yeah - if I could find a U pipe instead of the J pipe depicted in the photo above with the right fittings I could shorten the runs from the sink bowls a few inches and still line up with the level of the exit pipe making the trap less deep - but I've been unable to find a suitable fitting so far as all the kits I've looked at come with unequal length J sections. That is certainly a further improvement that could be made.

It's not until now that I've realised just how many different incompatible variations of pipe fittings there are for drain pipes - it was quite a challenge trying to find a way to connect the T that I ended up with as none of the readily available fittings are designed to do this. That T is actually one I removed from the original sink that was ripped out when we moved into the house years ago.

 

[Old Salt]

I am glad you have it sorted but bear in mind that people are trying to help, free of charge and working from photographs and sometimes a poor description of the problem. Whilst not all suggestions may be helpful, or some have already been tried, they are all worth considering even if it's only to eliminate that particular scenario.

In the vast majority of cases, making sure the waste is fitted iaw the instructions provided with the appliance, will prevent issues arising in the future. In my experience a stand alone drain, reaching just under the worktop with the drain hose inserted in the top will be effective and far less trouble than using an under sink spigot joining into the sink waste.