Overflow from expansion tank

I know there have been a number of posts regarding this topic, however I thought I may as well ask again.

I'm not very technical but as far as I can see we have the following:

A two storey house
An Ideal boiler on the ground floor
A hot water cylinder on the ground floor with an expansion tank on top of it (a combined cylinder I think)
A further expansion tank in the roof space, I'm not sure what this is for.
A pump of some description adjacent to the hot water cylinder.
A Zone Valve

The house used to be two flats so the plumbing is very messy.

We went on holiday for a week last week, and by Thursday we had a call from the neighbour to say our overflow was leaking warm water from the expansion tank above the hot water cylinder. When we got back and checked, the water level in the expansion tank was very high, and the water was warm. Once we has used some water the level dropped back down to normal and has stayed there since. We left the heating on for a few hours a day whilst we were away.

Does anyone know what might have caused this? The main problem is the cylinder and expansion tank are built into a cupboard so the only way to see in is via a mirror and a torch, which isn't helpful. I would just like an idea as to what it might be, as we will need to remove half the bathroom to get to it and do any major repairs.

I'd say you have a slow drip from the float (ball) valve.

the expansion tank fills and eventually overflows very very slowly while the water is cold. When the water in the main body of the cylinder warms up and expands, it causes the expansion tank to overflows a lot more.

While your in the house using the hot water all the time you may not notice any overflow because the drip does not get time to reach the overflow level.

Because it is in such a tight space you will need the expert services of an experienced tradesman who can contourt himself into the space and change the whole valve or perhaps just the rubber seal.

youm qick off the mark tonight
or am i getting old

There are two distinct possibilities here. Both of them will cause water to come up into that tank from the hot cylinder below.

1) Water from your heating system is leaking into the hot cylinder through a hole in its heating coil. That tank in the roof space will be a feed/expansion tank for the heating system.

2) Mains pressure water is forcing its way into your hot water pipes. This can only happen where hot water meets mains fed cold water, something that should never happen inside the pipes.

To check the first hypothesis you will need to shut down your heating for a while and tie up the ballcock in the roof space tank, or turn off a stopcock feeding it if you have one. This is a warm weather job. If the upper tank level falls as the lower one rises you have a leaky heating coil.

To check the second hypothesis, turn off your main stopcock. If this cures the problem you have a badly fitted mixer somewhere. The only kind of mixer that must use mains cold water is a kitchen mixer and, if you look carefully at these, you'll see they have two distinct exit holes. They aren't really mixers at all. All other mixers must take cold water from the same tank as feeds the hot water system.

Can you see the pipe leading from the bottom of your downstairs cold tank to the bottom of the hot cylinder. This is your cold feed. If there's a stopcock in it turn it off and see what happens. My guess is that water will come out of the vent instead. The advantage of this is that it's much easier to see whether it's flowing or not.

Thank you for all the responses. We had a problem again last night and enlisted the help of the experts (not a very romantic Valentine's Day). He thinks the problem is the float valve and is going to contort himself into the cupboard this weekend. The tank in the loft hasn't changed level throughout. Incidentally, all our taps etc. are mains fed, but we haven't made any alterations to them and the problem has only just started.

There is one final thing that I was wondering about. From what I could make out using the mirror and torch method, the expansion tank was very full to quite a way over the overflow, but water was only trickling very slowly out of the overflow pipe. Given the lack of access it is difficult to be sure but is this right or might there be a blockage somewhere. Our neighbour has built a lean to against the wall, so when it overflows it goes into this, so I wouldn't put it past him to have minimised the flow.

Sorry, I completely overlooked the possibility of a leaking ball valve. I was misled by the fact that the tank was full of WARM water but of course there's a perfectly good explanation for that - it's sitting above the hot tank! The words felix and plonker come to mind.

And now for your overflow. A blockage is something worth checking but there's another possibility. The pipe might have a slight upwards slope somewhere along its length. The purpose of the overflow is to prevent a flood if your ball valve fails so test it. Reach into the tank and push the float down into the water until water runs freely into the tank. Does the level ever reach the top? If it doesn't then your overflow is working. You will also know it's not blocked because water will be pouring out of it. If the level keeps rising and threatens to come over the top THEN you have a problem.