water in toilet being syphoned out

[hopelesslass]

Hi I wonder if anyone can help.
A friend lives in a flat and everytime her neighbour flushes his toilet, it syphons the water out of her toilet into his!! Her cistern then has to refill. His bathroom is directly below hers, and as they are purpose built flats, I'd guess that all the plumbing is linked into the same waste pipes.
She had a new bathroom fitted a few years ago, but the problem only started 2 years ago. It's a close coupled toilet and she did have a plumber out to fix it last year (I think he replaced the flush valve) but it's no better......apparantly now worse!!
There's 2 people in flat below and they use toilet a lot!! So her toilet is now constantly having to refill when they flush!
Wierd I know-has anyone any ideas??
Thanks

 

[45yearsagasman]

That sounds like a faulty air admittance valve.

 

[Space cat]

it syphons the water out of her toilet into his!! Her cistern then has to refill.

If water is syphoning out of the cistern, it has nothing to do with the drains. It sounds like inadequate water flow. When the toilet below flushes, the water pressure in the supply pipe drops low enough to suck water backwards through your friend's ball valve. Have a look in the cistern and see if there's a tube coming off the ball valve and dipping down into the water.

 

[neds]

the OP may be getting the terms wrong from the cistern and Bowl? Do you mean the toilet bowl needs filling? In which case you got a clear case of venting issue. If the mains supply pipe is sucking water that's a whole world of pain!

 

[sooey]

it syphons the water out of her toilet into his!! Her cistern then has to refill.

If water is syphoning out of the cistern, it has nothing to do with the drains. It sounds like inadequate water flow. When the toilet below flushes, the water pressure in the supply pipe drops low enough to suck water backwards through your friend's ball valve. Have a look in the cistern and see if there's a tube coming off the ball valve and dipping down into the water.

The ball valve washer is held tight shut by the float arm when the cistern is full, so the mains wouldn't be able to syphon the cistern.

 

[spraggo]

I think that the OP is confused and that he means that the toilet PAN is being syphoned out, in which case it would indecate that there is no open vent or Durgo valve on the soil stack.

 

[Space cat]

The ball valve washer is held tight shut by the float arm when the cistern is full, so the mains wouldn't be able to syphon the cistern.

I've got to admit, that puzzled me too. It would have to be leaking already - or else it's trying to fill the cistern above the overflow.

 

[TicklyT]

The ball valve washer is held tight shut by the float arm when the cistern is full, so the mains wouldn't be able to syphon the cistern.

A ballcock may stay sealed, but an equilibrium valve could get very upset if the water pressure drops below atmospheric

 

[hopelesslass]

Cheers everyone. I'm going to have a look at it for her next week, but from what she says, when he flushes his toilet hers empties and has to refill. Could it be a pressure problem with his??
Or as this has been happening for last couple of years (and got worse lately) could it be the fill level in her cistern do you think? If set too high and no air gap, could that do it, and create a vaccuum that sucks her water out?

Somebody else also told me to check that stop cock's fully open too and she's not running on low pressure.

Its driving her mad.......he uses his toilet alot

 

[hopelesslass]

I've just spoken to my mate, she had her cistern lid off and watched as neighbour flushed. Apparantly as he flushed his toilet, the water level in her cisten dropped quite low, then her cistern re-fills up but overflows passed the fill level. The water then overflows back into her toilet pan, before the water level settles again. So could there poss be a blockage somewhere??

 

[HERTS P&D]

Fit a check valve on the supply pipe to the cistern.

Andy

 

[Space cat]

The first step is to get the ballcock working properly. It should shut off the water before it reaches the overflow. Start by checking that it can shut off the water at all. Lift the float just a little. Minimal force should do it; DON'T keep pulling in a determined effort to stop the flow or you'll have a broken valve to deal with.

If you can't stop the flow, the valve is faulty. If you can, it just needs adjusting. There might be a screw at the valve end of the arm for this. Otherwise, if it's a metal arm, CAREFULLY bend it downwards. Use both hands and don't put any force on the valve itself.

Next, look for a tube running downwards from the valve into the water. You shouldn't need this (except maybe a very short length to prevent splashing). If you can take it off, that'll put an end to the syphoning problem.

 

[hopelesslass]

okaaaay so I went to look at her cistern, and her neighbour flushed his toilet....and nowt happened. Typical!! The fill valve and float looks to be sticking but I think its coz its a tight queeze in the citern-its very very slim.

However I did see that she's got water running into her pan, and could see that it's a leaking flush valve (combined flush with overflow-looked down the centre and could see water passing through it). This was apparantly replaced by a plumber only a year ago. BUT its a very slim close coupled toilet (slimmest cistern I've EVER seen) and I've fixed a couple in the past, and always had to drain down the cistern, uncouple it from the pan, replace the flush valve, put a new donut washer on, fix it all back together.
Apparantly the new valve was put in while the toilet was all coupled together-no water drained off-she said it took him 20 minutes . You can tell by the rust on the bolts that they weren't removed to uncouple it.
My question is....is it possible to do it this way? Can you fix a new valve without uncoupling the toilet?? Or emptying the cistern?? (coz if so I could,ve saved myself alot of bother in the past).

 

[Youllneverguess]

Depending on the valve yes.

 

[hopelesslass]

It looks likes a bog standard (scuse the pun) cheap, plastic, single flush push button valve. I've always seen them with a plastic back nut, fitting underneath the cistern. So how do you fit one, without taking the cistern off the toilet??
Or could it be taken out and replaced with one of those flapper valves on a chain fixed to the button.....the cistern doesn't have to come off for those does it?