Andrew L wrote:
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. :-(

Thanks for all of the replys to the original post. Having done some
experimenting I've done/found the following out:

- the level at which the float valve cuts off is about 3 inches below
the overflow pipe.

- the feeder tank fills up irrespective of whether the cold water feed
is turned on or off. ie Overnight wednesday it filled up with the
stopcock for the cold water feed turned on. Last night it did the same
with the cold water feed turned off.

- the water in the feeder tank is lukewarm. Not boiling hot, but
slightly warm to the touch.

- there's no water coming from the steam vent pipe.

- there are no hot taps or pipes above the level of the tank.

Does anyone have any further ideas as to what may be happening/what I
can try before I call out a plumber?

Cheers,

Andrew