Dishwasher plumbing question

[Lucy Walks]

I have a problem with my dishwasher and I’m told there’s an issue with my plumbing. I’m after some help with understanding what the issue is (and what the solution is).

My plumbing/appliance arrangement:
My dishwasher is plumbed in ‘downstream’ from my sink. They therefore share a pipe to drain the water to the outside drain. The dishwasher plumbing is directly behind the machine. There’s a drainage pipe that rises vertically from the horizontal(ish) one that drains from the sink. The dishwasher waste hose loops into the top of this vertical pipe.

The problem:
When my pipes become blocked ‘downstream’ from both the sink and the dishwasher (happens about once a year) I need to plunge my sink to remove the blockage. When I do this waste water from the sink rises up the vertical drainage pipe behind the dishwasher and splashes/overflows behind the dishwasher. Some of this water gets into the base of the machine through a gap at the back of the machine. This triggers an error on the dishwasher that disables it until the water in the base has dried out. Last time this took six days.
Here’s a picture of what I think is happening.
(Include pics)

Diagnosis:
Bosch say there is no fault with the machine and that the problem is with the installation and/or plumbing.
The installation team (from John Lewis) say that they cannot advise on plumbing.
It sounds like I need to have a change made to my plumbing but I need to know what that change needs to be.

Things I’ve thought of/tried:
Tipping the machine to drain the water in the base - isn’t practical as I’m disabled.
Covering the gap at the back of the machine so the water doesn’t splash into it - Bosch won’t tell me where the gap is.
Improving my plumbing so that the pipe doesn’t get blocked - the pipe is at a shallow angle and I can’t change that without rearranging my kitchen. I am doing all I can to keep it clear but it does just get blocked some times.

Help please!
Can anyone tell me what’s going wrong here, i.e. what’s happening that shouldn’t be happening?
… and how do I fix it?
Out of curiosity, why doesn’t this happen to everyone?

 

[Deleted member 120897]

Hi Lucy,

There’s no pictures attached. Are you able to discharge the dishwasher waste pipe outside?

 

[Ric2013]

I can't see your photo, so I'm having to assume your plumbing is basically correct (though it oughtn't block, really).

When you plunge, you are creating an alternating vacuum and pressure on the section of pipe leaving your sink. This may clear a blockage in the trap under the sink but when the blockage is downstream of both, the pressure is releaved at the nearest opening to atmosphere. In this case, it's your appliance trap 'standpipe' into which your dishwasher drain hose drops. As the water can't go down the blocked drain it gets forced up here.

In your case, the least-effort method is probably a good solution. Buy a roll of duct tape. Next time the pipe blocks, remove the drain hose from your dishwasher and duct tape over the upstand and you can plunge more effectively without sending water up your appliance upstand.

In the longer term, I'd be thinking poor pipework design (probaly overuse of knuckle rather than swept bends) is causing the repeated blockages and possibly worth getting sorted. Funnily enough, I have such a job on later this week!

 

[Basher]

It's not caused by the dishwasher, it's caused by the drain becoming blocked (presumably through poor design, or something stuck in it, or a build up of solids that still remains after your plunging activities). I don't think the plunging method of unblocking Is appropriate given the blockage is downstream of the dishwasher. I would reserve the plunger for when the sink trap is blocked.
You could try chemical unblocking liquids instead of a plunger, which would avoid the overflowing situation behind the dishwasher. It might be that 'preventative maintenance' of pouring drain unblocker down your sink every few months would stop it happening, and may be more effective at removing material than the plunger is.

You mention it would be difficult to increase the falll of the waste pipe, but would it be practicable to route a separate waste pipe from the dishwasher outside to the drain, or to a point past where it gets blocked. Then you could seal off where the dishwasher is currently connected, and future plunging activity would not affect the dishwasher!
That's not fixing the issue that the drain should really not be blocking in the first place.

Might be worth getting a specialist with a drain camera to investigate why it blocks, and if there's a simple fix?