Syphon cistern keeps flushing


N

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?
 
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T

Tony Williams

Nick said:
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.
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.
 
N

Nick

John said:
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?
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.
 
N

Nick

Tony said:
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.
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.
 
G

Guest

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.
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?
 
N

Nick

On 16 Feb,


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?
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.
 
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N

Nick

Nick said:
I have an old cast iron toilet cistern with a cast iron bell inside. It
flushes fine but then it won't stop flushing.
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!
 
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N

Nick

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.
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.
 

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