Dishwasher plumbing to prevent drains smells?

We have a really annoying thing with our dishwasher where if you don’t empty it within about half an hour of it finishing a cycle, the contents end up smelling of drains (with the glasses first to suffer). We frequently clean the filter and do machine clean cycles with both dishwasher cleaner or white vinegar and bicarb.

The drain hose goes up to a clip near the top of the machine, then through a hole just under the worktop and then into one of those tapered push connectors next to the sink waste. There’s a mini sink waste about foot across from that and then they all go through a u-bend and then down and out sideways (along through the utility room and to the main drain).

They used to go out to an open drain by the back door before the utility room was built and the main sewer had to be bridged and manifold/manhole moved.

Ironically the dishwasher doesn’t beep yet needs emptying immediately whilst the washing machine and tumble dryer beep lots but can’t be ignored without issue…

Is there a better way to plumb this drain? I’m sure the dishwasher or washing machine hose used to drop into an open vertical pipe.

I think what’s meant to happen is the dishwasher hose siphon itself mostly clear and the height of the loop stop the dirty water running back/in from itself or the sink.

Any suggestions of better routing, traps, pipes, valves etc I can do/add?

We used to have the same problem with ours. Turned up the pipe had disconnected from the top and once put back in place it’s been fine. Maybe the pipe needs to drop from the high point to work properly? Sounds like yours doesn’t?

It does loop up but not that far above the sink plughole, only about 4”.

they all go through a u-bend

Is this u-bend a trap that always contains water to stop nasty niffs coming in from the sewer?  If so, and the trap isn’t being ‘syphoned’ empty by something else then your niff must be coming from elsewhere on the dishwasher side of the trap.

I’m sure the dishwasher or washing machine hose used to drop into an open vertical pipe.

Quite possibly but there should still have been a trap at the bottom.

From the setup you describe, if you aren’t getting the smell from the plug hole I’d be suspicious of it emanating from gunge within the dishwasher

Is it a drain smell or a U-bend smell?

You can get inline one-way valves but they shouldn’t be needed. It’s something else to go wrong, jam, need cleaning, etc. Example https://mcalpineplumbing.com/traps/bath-traps/in-line-non-return-valve-wasteflow

Thanks! We never notice any smells from the sinks…

is the hose loose in the drain or somewhat sealed? if its sealed, my money would be on the dishwasher sooking the water out of the basin trap

either that or general oomsk in the sump of the machine

“is the hose loose in the drain or somewhat sealed?”

The hose is completely sealed onto the under-sink plumbing.

I had this with my washing machine/ utility room sink (less used). I cut a piece out of the waste pipe on the wall, fitted a specific trap/ upstand pipe and plumbed washing machine waste into that. No smells now.

“ I cut a piece out of the waste pipe on the wall, fitted a specific trap/ upstand pipe and plumbed washing machine waste into that.”

I wish I could do that but this being the kitchen all the pipes are hidden under the worktop starting at the sink, and then run along low down behind the utility room units.

So I’m thinking the best solution is to add a standpipe trap behind the dishwasher, to prevent siphoning or reverse flow.

So you can see the dishwasher hose on the right. The standpipe can fit behind the dishwasher or at the back of this under sink cupboard, there’s space either way.

The puzzle is, if I add a standpipe and trap like this:

Where should I be connecting the outlet from the trap side of the standpipe/trap unit to?

That waste pipe heading down to the bottom of the cupboard then goes back for about 6” before dropping down for an inch or so and then turning left and heading sideways out of the kitchen and through the utility room and out (about 15’ or so).

Run a cycle but put the dishwasher waste into a bucket instead of the drain, will require you monitoring said bucket for overflow.  If the dishwasher still stinks then it’s coming from the dishwasher somewhere as the dishwasher pipe wasnt connected to the drain to get smelly water from.

If however the dishwasher smells fine then it was coming from the plumbing. I’d do this before changing any pipework just to be sure.

Where should I be connecting the outlet from the trap side of the standpipe/trap unit to?

The U-bend will probably end up below the cupboard floor, so cutting will be needed. It’ll be much easier to explore the more pain-free options of cleaning, etc. first ^^

Follow all manufacturer’s recommendations for height, etc. Copy the first 90° section from the sink waste in your photo (but much shorter, obvs) into a swept tee in the waste pipe that connects to the drains. Make sure that the swept tee sweeps in the direction of flow

You don’t have to make everything a 90° turn as in your sink photo; rotating the sections will allow a shallower, easier angle with a smaller number of joints; you’ll see what I mean when you start to construct.

Choose a system and stick to it. Solvent-weld (glued) fittings need solvent-weld pipe (and solvent-weld adhesive), nominally it’s the same size but the reality is different 🙂

Our dishwasher stank the house out every time it was turned on. I did all the under-sink plumbing/fitting valves/u-bend mods etc etc.

Turns out there’s a condensation tank that takes up one complete side of the dishwasher – I guess it’s part of the drying cycle in that steam heads out of an outlet at the top of the dishwasher and condenses on the side of this tank. A valve at the bottom lets it out at the right point (but it easily jams open). Our condensation tank had filled with the kind of white crumbly stinking (fat?) that causes fatburgs. Any water condensing in there would immediately stink of the drain / fart emissions of the devil. I took it apart and cleaned it as best I could (bleach in there for about 3 hours and careful rising through a teeny tiny hole). It cured the problem.

So I say do the bucket thing mentioned by NeverEnoughBikes ^^^ up there before you begin dismantling the plumbing.

Another vote to do the bucket thing.  If you think it’s siphoning through the u bend you could fit an air admittance valve under the sink, I will always add one now after my last two DIY sink fits resulted in glugging (I had to do the job twice to fit one!)

So following the advice on here I decided to try to clean out the drain hose and also checked to see if we got any water running into the dishwasher sump from when the sinks are filled to the max and then emptied simultaneously. Conclusions – no sink water gets over the high loop back into the dishwasher; and you can’t pour water through the drain hose into the dishwasher, so there must be a non-return valve.

I think all the points to it being a dishwasher problem, not a plumbing problem.

I looked into how to get into the dishwasher and it seems a tiresome job with a lot of dismantling in illogical ways so as a stopgap I’ve tried squirting a small amount of white vinegar into the sump/filter at the bottom after each wash. That appears to be making a big difference!

Will see how we go…