"Andrew L" com wrote in
message ...
Dear all,

I need some help diagnosing a problem I have with my hot water system.

I've got a cylinder tank in the airing cupboard which has above it two
square tanks, one large one small. I believe the larger is called the
feeder tank. As hot water is drained from the cylinder tank water is
drained from the feeder tank into they cylinder, which in turn is filled
up from the cold water mains supply up until a certain level (about a
1.5 inches below the overflow) at which point the float valve closes.

This all seems to work as described. If I don't use any hot water for
about 4 hours though the water level in the feeder tank has reached the
overflow and starts trickling out. It's a tiny, tiny trickle, so much so
that the water doesn't pour out of the end of overflow pipe but trickles
out the end, back along the bottom and is absorbed by the wall. The
plaster on that wall is blown. :-(

At first I thought that the float valve was shot, so I put in a
replacement, but that hasn't changed anything.

If you have replaced the float valve AND there is no sign of it dripping
when up to level then you may have a leak in the coil inside the cylinder
which is allowing water from the heating system to make its way through into
the hot water (tap) side. This may be evident from a dribble from the
heating header tanks ball float valve without attendant rising level in that
tank.
If this is the case then you will have to replace the cylinder. Continued
leakage will cause damage to the inside of your boiler and rads etc and
contaminate your hot tap water