Syphon cistern keeps flushing

[Nick]

I have an old cast iron toilet cistern with a cast iron bell inside. It flushes fine but then it won't stop flushing. It will keep filling and flushing for hours unless I turn the inlet tap off. If I then turn the tap back on, the cistern fills normally and there are no leaks. The cistern never fills as high as the downpipe inside the bell. The top of the downpipe is higher than the top of the cistern, so it cannot fill that high.

After the main flush, there is a continuous trickle of water going into the bowl, and as the cistern fills, the trickle gets larger until suddenly there is a complete flush and the cistern empties completely again. The downpipe between the cistern and toilet is about 6 feet long, and it appears that after a flush, there is a low pressure in there that continues to suck water out of the cistern. There is a 0.25 inch hole about 1.25 inches from the bottom of the bell, which is obviously intended to let air in after a flush. But it seems that it doesn't let in enough air to allow the water to completely drain out of the downpipe. There is no sign of any valves anywhere and I don't think they are needed.

It's a heritage building and I don't want to replace the cistern, which looks fine. I should not have to do anything major such as drilling more holes, because clearly the cistern has worked OK for 50 years or so. However it has not been used for the last 40 years. I cannot see anything missing. Remember, it does flush OK.

Any ideas?

[Tony Williams]

That description sounds like water is leaking from the cistern, directly into the down-section of the inverted U assembly. As the cistern fills, the leak increases, enough to fill the narrower downpipe with a 'piston' of water, which pulls the required vacuum for a flush.

[John]

Could it now be filling faster than earlier. The cistern never empties enough for the siphon to be broken due to the surface of the water going below the bell. Throttling back the inlet might help. Have you replaced the ball valve at some time?

John

[Nick]

I have tried turning the tap mostly off but it doesn't seem to help. I'll check that again when there's daylight. The ball valve cannot be replaced as the nut is rusted on. I'll restore the whole building and cistern properly one day but I have workmen who would like to use the toilet in a few hours.

I can hear the air going into the bell when the cistern is nearly empty. But there is a lot of water in the downpipe, and before that has emptied completely, the hole in the bell is covered, thus stopping more air going in. The downpipe really needs a separate air vent pipe connected half way down. I can't understand how the thing ever worked the way it is. Maybe I could drill another hole further up the bell. At the moment, after 1.25 inches of water, the hole is covered.

[Nick]

But I said "If I then turn the tap back on, the cistern fills normally and there are no leaks. The cistern never fills as high as the downpipe inside the bell. "

If I turn the inlet tap off before a flush, then turn the tap on after it has completely finished flushing, everything works fine. Clearly the incoming water is stopping enough air getting into the bell for the downpipe to completely empty. And I have tried reducing the input flow to a trickle. I shall time how long it takes to empty the downpipe. It may be as long as 10 seconds.

[<me9]

IIRC there should be three bumps on the botton of the bell at 120degrees apart, to stand off the bell from the cistern floor, which let air in and stop the flush. either they have worn off or there is enough crud in the bottom of the tank or around the bell to not let enough air/fluid through to stop the flush.

It also could be a partly blocked flush pipe or inlet to pan. Is the flush as poweful as other high level bogs?

[Nick]

The flush is huge, maybe because the top of the cistern is 7 feet from the floor and there's about 20 litres of water in the cistern. The downpipe is 5 feet long and is thin and made of lead, but since the flush works fine it should be OK.

There are three bumps on the bottom of the bell, and the bottom of the tank is rather rough. If I have any more problems I'll put a spacer in the bottom of the tank to lift the bell a bit. I have turned the bell around so that the 1/4 inch hole near the bottom is away from the inlet water stream, and I've turned the inlet tap almost off. Now it works OK, but one time there was a very small stream of water that never stopped. There was definitely some weird syphon effect happening sometimes. There is definitely no leak in the downpipe. When I take the bell off, no water goes into the bowl at all.

[Nick]

I have just discovered that the lead downpipe where it goes into the toilet pan is not horizontal, but goes down about an inch and then back up again. So it is acting as a trap and there is always water in it. Air is unable to enter the pipe from the bottom to clear the vacuum in it, so the water inside the syphon bell in the cistern is raised enough to overflow into the downpipe continuously. Sometimes that is enough to cause a complete flush.

Holding the chain down after a flush until the downpipe is empty does fix the problem, but the builders using the toilet have enough trouble working out how to use a syphon flush as it is.

I could try to straighten the lead pipe but it doesn't look easy. I think it's been wrongly installed for at least 100 years. Or is it supposed to have a trap at the bottom? I have not been able to find a diagram of how an old syphon cistern is supposed to work. If you think it's easy, it isn't!

[Colin Stamp]

You could try drilling a small hole (say 2mm) half-way up the syphon. It shouldn't let in enough air to kill the syphon during a real flush, but it should have time to equalize the pressure whilst the cistern re-fills back up to the hole.

Cheers,

Colin.

[Nick]

That is certainly the easiest solution, which I will try. I suspect that the trap at the bottom of the downpipe is needed to stop air getting in while the syphon bell is lifted. If that happened, the bell would lift no water at all. The principle of operation of a syphon cistern is really quite complicated. As I said before it's a heritage house so I don't want to get rid of the cistern.