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

[DBMandrake]

Hi All,

We've had Indesit DFG 15B1 UK dishwasher which we have had for about 6 years.

Very gradually it has started to have an issue with syphoning back the water from the sink trap.

I first noticed this about 6 months ago where after the dishwashing cycle was finished the sump would be full up to the top of the filter food filter with waste water.

Catching it immediately at the end of the cycle the sump would be clear (so it is definitely draining) but over about 30 minutes the level would gradually creep up.

Since then it has got steadily worse, and now after a drain cycle ends either during operation or at the end about 10 seconds later there will be a gurgling from the sink.

If I force it to drain by starting a new wash empty, opening the door and cancelling the wash cycle (hold the power button for a few seconds) which continues to run the drain pump for about 20 seconds you can see the sump completely empty, after the pump shuts off the level immediately starts going back up and within about 30 seconds dirty waste water is back up to the top of the filter.

I've verified it's not leakage of incoming fresh water by turning off the cold supply when doing the same test.

The pipe arrangement we have is not ideal as the waste pipe in the back of the kitchen unit is quite high and too high to use the U-bracket supplied with the machine (which is long gone now) however I have always had a loop of the drain pipe cable tied to be well above the level of the outgoing drain:

I've disassembled and cleaned all the pipe work in the photo a couple of times since the problem started, and I've also tested the drain pipe heading through the back of the unit is clear and will accept a high flow rate without backing up, and nothing has changed about the configuration of the pipework in years.

So the problem seems to be the machine itself but after having the machine apart (and not for the first time) I'm at a loss to explain how it wouldn't syphon back ???

If you search for this type of problem online nearly all hits refer to a problem with the "air vent" in the drain hose, but this seems to be a very North American thing where they seem to have an actual air vent that is actually mounted in the counter top to allow the waste water from the dishwasher to separate at the vent and drain back separately to the sink trap and dishwasher after the pump shuts off to avoid syphoning.

But we don't have those kind of counter top air vents in the drain hoses of dishwashers in the UK so none of those articles are relevant. Since there is no air vent I assumed there must be a solenoid operated valve that shuts the flow off completely after the pump turns off.

But no, there is nothing of the sort in the machine. The water literally goes from the sump directly to an impeller style pump and then to the drain hose. No solenoids or one way valves or anything that could prevent syphoning the sink trap back into the sump of the dishwasher.

So what am I missing here ? Looking at the pipe layout it does occur to me that the point where the dishwasher attaches to the trap will be lower than the natural settling level of the trap - which will be in line with the bottom of the right angle bend in the foreground - so there is certainly potential for syphoning there despite the high arching loop, but if so, how did it operate for 5 years without syphoning back into the machine ? What changed ?

Unfortunately the height of the drain pipe through the back of the unit is above the level of the trap and I can't change that.

About the only thing that I can think of that might help would be to change the pipe section with the cable tie on it to one with a joint for the dishwasher to connect there, downstream of the trap level, but I'm not convinced that that would completely prevent syphoning.

Any suggestions anyone ? Is it the machine at fault or the pipe layout ? And if it's the pipes why did I not see a problem for 5 years then suddenly its got much worse very quickly ?

Currently the machine is partially apart on the middle of the kitchen floor and now that I'm not sure whether the machine or the piping is to blame I'm not sure what my next step is..

 

[foxhole]

Have you tried connecting waste to spare spigot on the left of the photo? You could change spigot to and anti symphony type ( has a flap on it which falls shut when not pumping).

 

[JohnD]

The run of the waste hose is wrong. You have to loop it up to the worktop. Tie it in place with string and a cuphook or something.

It can also be achieved by drilling a hole in the side of the sink unit at the top, and passing the hose through that. Yours comes in at the bottom which is wrong.

The top of the loop needs to be higher than the water level in the sink when you pull the plug out.

 

[DBMandrake]

Have you tried connecting waste to spare spigot on the left of the photo? You could change spigot to and anti symphony type ( has a flap on it which falls shut when not pumping).

Hi,

No I haven't tried the left waste spigot (not sure the hose would reach) however due to the high exit pipe after the trap the effective settled level of the trap water will be above both spigots so I doubt that would help.

I've been browsing Screwfix and B&Q plumbing listings and haven't seen any "anti-syphon" spigots listed - where are they available ? I'd prefer to solve the problem without that approach if possible though.

 

[JohnD]

Raise the loop.

 

[DBMandrake]

The run of the waste hose is wrong. You have to loop it up to the worktop. Tie it in place with string and a cuphook or something.

It can also be achieved by drilling a hole in the side of the sink unit at the top, and passing the hose through that. Yours comes in at the bottom which is wrong.

The top of the loop needs to be higher than the water level in the sink when you pull the plug out.

There isn't actually any gap between the back of the sink and the back panel of the kitchen unit, nor to the right hand side - how the pipe is in the picture is the highest the loop can go without putting the loop completely outside the kitchen unit, and I'm not sure there is enough room or hose length to do that without a lot of modifications.

I should be clear though - the syphoning problem is not related to draining the sink - it happens even when the sink is empty. It's syphoning the standing water out of the sink trap.

I think I'm just going to have to change where the spigot is attached so that it's not below the level of the water in the trap. The best place would seem to be to install an appliance feed T facing upwards in the final run of pipe that goes back through the hole in the cabinet - that would be after the trap and above the water level in the trap and allow a natural air break to form.

The difficulty doing this is there is a mixture of pipes in use - from the sink until the front elbow it is mostly threaded 40mm with built in o-rings, however it then goes to 44mm with compression fittings for the rest of the run - which is a legacy of the drain pipes in the house that go to the outside.

All the appliance feed spigot sections I've seen to purchase are for 40 (or 32) millimetre threaded pipes not the 44mm compression type, so I'm going to have to change some of those pipe sections to be able to use a threaded pipe in that part of the run...

 

[DBMandrake]

Raise the loop.

I don't think it will solve the problem when the appliance spigot is below the water level in the trap - there is no chance for an air break to form, so the entire run of hose back to the machine will always have water in it. The fact that it syphons back shows that.

 

[JohnD]

I don't think it will solve the problem

It will.

 

[JohnD]

The top of the loop needs to be higher than the water level in the sink when you pull the plug out.

 

[DBMandrake]

It will.

Thanks John but with all due respect I don't think you understand how syphoning works...

If a syphon pipe is already fully primed with water and no air gaps it doesn't matter how high the top of the loop is, you can still syphon the contents out of the lower container. (The standing water in the sink trap in this case) That's how syphoning works.

The hose all the way back to the dishwasher is remaining fully primed by the sink trap as there is no way for an air break to form when the spigot connects below the float level of the trap. (This is a problem caused by the exit pipe into the wall being too high relative to the trap, it should really be lower than the trap not higher, but that's outside my control unless I wanted to pull the entire kitchen unit out and knock a new hole in the wall...)

To be honest I don't know how this ever worked properly before...

I'm pretty confident connecting an upwards facing T in the final run towards the back of the cabinet will solve the problem as it will connect the dishwasher after the trap instead of before it (which is good for smell reduction anyway as it won't leave dirty water from the dishwasher sitting in the sink trap) and the remaining drain pipe to the outside of the house will provide an air path to allow an air break to naturally form after the dishwasher finishes pumping water out. When draining the sink the sink water will take the easier downhill path of going out the main drain pipe and won't have any reason to go up the T joint into the dishwasher.

The problem is it's going to take a bit of cobbling together of various adaptors due to the types of pipes in use, hopefully I can find what I need.

 

[JohnD]

Thanks John but with all due respect I don't think you understand how syphoning works...

I understand enough to know that water will not run into a hose if the top of the loop is above the water level in the sink when you pull the plug out.

But never mind, if you're so convinced that you won't even screw in a cup-hook to tie up your hose, feel free to wander down blind alleys

Goodbye.

 

[foxhole]

Thanks John but with all due respect I don't think you understand how syphoning works...

If a syphon pipe is already fully primed with water and no air gaps it doesn't matter how high the top of the loop is, you can still syphon the contents out of the lower container. (The standing water in the sink trap in this case) That's how syphoning works.

The hose all the way back to the dishwasher is remaining fully primed by the sink trap as there is no way for an air break to form when the spigot connects below the float level of the trap. (This is a problem caused by the exit pipe into the wall being too high relative to the trap, it should really be lower than the trap not higher, but that's outside my control unless I wanted to pull the entire kitchen unit out and knock a new hole in the wall...)

To be honest I don't know how this ever worked properly before...

I'm pretty confident connecting an upwards facing T in the final run towards the back of the cabinet will solve the problem as it will connect the dishwasher after the trap instead of before it (which is good for smell reduction anyway as it won't leave dirty water from the dishwasher sitting in the sink trap) and the remaining drain pipe to the outside of the house will provide an air path to allow an air break to naturally form after the dishwasher finishes pumping water out. When draining the sink the sink water will take the easier downhill path of going out the main drain pipe and won't have any reason to go up the T joint into the dishwasher.

The problem is it's going to take a bit of cobbling together of various adaptors due to the types of pipes in use, hopefully I can find what I need.

John is completely correct , you are completely confused .

 

[DBMandrake]

I understand enough to know that water will not run into a hose if the top of the loop is above the water level in the sink when you pull the plug out.

But never mind, if you're so convinced that you won't even screw in a cup-hook to tie up your hose, feel free to wander down blind alleys

Goodbye.

If you don't understand how syphoning works you shouldn't be giving out advice on a forum. Sorry.

Anyway, I've already tried temporarily raising the height of the hose as you suggest on previous occasions and it does not solve the problem.

This image should give you a clue why the height of the loop isn't relevant in this scenario:

As long as the trough in the bottom of the dishwasher is lower than the sink trap - which it is, unless an air break can form it will syphon back. An air break can't form because the exit drain is higher than the attachment point of the spigot for the dishwasher.

You keep going on about the water level in the sink when pulling out the plug - I've made it very clear that the problem is happening with an empty sink. You don't seem to understand the problem.

You want to get huffy and leave, fine.

 

[DBMandrake]

John is completely correct , you are completely confused .

How am I confused ?

 

[oldbuffer]

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.