Dirty water flowing back into dishwasher - have I fixed it?

Postik

I came to put the dishwasher on last night and noticed the floor of the dishwasher was covered in filthy water.  I removed the filter (which was clean) but the drain where the filter goes was also full of water.

I turned the machine on for 10 seconds, then opened it and all of the water had been pumped out.

I checked the waste pipe and it was evident why this had happened - because the dishwasher waste outlet was at virtually the same height as the waste for the sink, and the elbow to the dishwasher pipe was pointing downwards with the pipe trailing along the bottom of the kitchen cupboard.  I filled up a few bowls of water and tipped them down the sink and sure enough the dishwasher drain started filling up again.

What I don't understand, is why this hasn't happened before?  Although I admit we rarely use the sink so perhaps it has been happening, just very slowly and we never noticed (we use the dishwasher every 2 days).

I have since managed to remove the kickboards, untangle the pipe from underneath and pull more slack through.  I've repositioned the elbow so it points upwards instead of downwards, and created a high loop (as high as I can make it).

Do you think this will resolve the issue?  Also, would it be safe for me to cable tie the dishwasher flexible pipe to the sink waste pipe to stop it sagging when water is pumped through?

Attached is a photo with the sink waste in purple and the dishwasher waste pipe in red.

Eldi_Dos

It is amazing how many fault potentials are between that sink and the discharge pipe. Modern plumbing.

Can you put a screw or two into wall of unit and attach dishwasher pipe to them with a clamp, attaching to pipework might increase vibration to pipes

ThisIsWeird

Nice job, Postik, 'specially the photo colour-IDing. Nice

Almost certainly you have sorted it. And, yes, you can lightly cable-tie the hose's upper loop to that horizontal section of the purple pipe to keep it 'up'.

Why did it happen? No idea, but if the connecting spigot for the hose was pointing downwards, could it have slowly turned that way over time?

If you look at where the purple pipe joins the silver, there's no 'sweep' to assist the sink's downward flow to go left as it should, so half will presumably try and go right, straight into the d/w hose.

Almost certainly fixed. If it isn't, then there may be a partial blockage further along the silver pipe, which reduces flow and causes a backlog. But, I suspect you have fixed it

Postik

ThisIsWeird said:

Nice job, Postik, 'specially the photo colour-IDing. Nice

Almost certainly you have sorted it. And, yes, you can lightly cable-tie the hose's upper loop to that horizontal section of the purple pipe to keep it 'up'.

Why did it happen? No idea, but if the connecting spigot for the hose was pointing downwards, could it have slowly turned that way over time?

If you look at where the purple pipe joins the silver, there's no 'sweep' to assist the sink's downward flow to go left as it should, so half will presumably try and go right, straight into the d/w hose.

Almost certainly fixed. If it isn't, then there may be a partial blockage further along the silver pipe, which reduces flow and causes a backlog. But, I suspect you have fixed it

Yes my thoughts exactly.  It was particularly bad last night because I washed up a bowl of curry sauce in the washing up bowl and tipped it down the sink, so the water in the bottom of the dishwasher was mixed with Korma sauce :-(

As to why it only just happened - perhaps we didn't use the dishwasher for 3 days instead of 2, and I poured more water down the sink than usual.  Perhaps it always filled up, but not to the point of noticing before the dishwasher was used.

Just one thought - but why do dishwashers not have a one way valve to stop this from happening?  Could it be that mine did have such a valve which has gone faulty?

Eldi_Dos said:

It is amazing how many fault potentials are between that sink and the discharge pipe. Modern plumbing.

Can you put a screw or two into wall of unit and attach dishwasher pipe to them with a clamp, attaching to pipework might increase vibration to pipes

Yes I did have concerns about vibrations, but not sure where to buy such a clamp/bracket?

CSI_Yorkshire

Postik said:

Just one thought - but why do dishwashers not have a one way valve to stop this from happening?  

Because that would cost extra money and be another component that could go wrong.  With the drains all pointing the right way, they don't need one.

ThisIsWeird

No valves, as just not needed. And imagine how easily they'd grime up?!
D/Ws and W/Ms simply use the high point of the hose as that 'valve' to prevent it draining when not required. The  water cannot drain accidentally out the machine as it has that high hill to climb. But once the pump pushes the expelled water up over the high point, it'll then drain easily aided to some degree by syphonic action. Similarly, waste water shouldn't be able to make its way back up the pipe as it has that same hill to climb. A nice example of KISS.

I wouldn't be concerned about vibes, as they'll barely exist at that end of the hose. The hose material is also quite hard and tough, and it isn't in contact with anything abrasive either. So loosely tie-wrap the upper bend to the nearby horizontal pipe. By all means, if you have any concerns, wrap a few layers of insulating tape around the hose where the tie wrap will be, but really do not be concerned. 

These hosed appliances are usually supplied with a curved clip in order to hold the top part in a nice unkinkable arch - yours not sitting in a cupboard, is it?

ThisIsWeird

By a funny coincidence with this story, two days ago I had to repair sil's W/M hose as it had split - water everywhere. A new hose, a double-connector, and a couple of jubilees from SF had the job done. Yesterday she called again - the inlet hose is now spurting, in the same area where it comes through the hole in the unit side.

What?!! Caused by bad vibrations? At the same time? Surely not!

No, "I also have a rat in the house..."

coffeehound

I wonder if a restriction, i.e. partial blockage, could have occurred downstream in the drain, meaning the waste water from the sink took longer to drain away, and therefore had time to find its way into the dishwasher?  Might explain why it only recently became an issue.  I'd be tempted to check for obstructions in the U-bends, etc.

ThisIsWeird

I think it's sorted.
The angled hose spigot connection was pointing downwards, and the hose itself fell even further down from there on its way from the DW. In short, it would have defied physics if waste water from the sink had not found its way into the machine...
Almost certainly sorted.

Postik

coffeehound said:

I wonder if a restriction, i.e. partial blockage, could have occurred downstream in the drain, meaning the waste water from the sink took longer to drain away, and therefore had time to find its way into the dishwasher?  Might explain why it only recently became an issue.  I'd be tempted to check for obstructions in the U-bends, etc.

Yes this was one of my concerns but the sink is free and fast flowing.  As ThisIsWeird pointed out, it would have defied physics for water to not make its way into the dishwasher.  Very odd that we never noticed it before.

I did observe that the issue didn't occur, or occured much more slowly, as soon as I moved the spigot connection so it pointed upwards instead of downwards.  Perhaps the spigot had gradually rotated around from an upward to downward position.  Although I think it's more likely the issue was always occuring, and as we don't use the sink a great deal, we just never noticed.

If it happens again after repositioning everything, I will provide an update!

Thanks everyone.

coffeehound

One other possiblilty is that e.g. heavy rain had flooded the drains, raising the water level.  I've seen that happen during a deluge where water started issuing forth from a washing machine.  Anyway just academic.  Can I ask what package you used to colour the components?  It's a great effect