"Set Square" wrote in message
...
In an earlier contribution to this discussion,
Andrew L com wrote:

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

Is the water level in the small tank higher than that in the large one?
[The
small tank is the fill & expansion tank for the heating/hot water primary
circuit]

The water in the primary circuit is heated by the boiler, and is pumped
through a coil inside your hot water cylinder - enabling it to heat the
domestic hot water without mixing with it. However, if that coil develops
a
leak, the waters *do* mix - and water flows from one circuit to the other.
The direction of flow is determined by the relative pressures in the
circuits. Whichever circuit has a higher water level in its header tank is
at a slightly higher pressure than the other one - and water flows from
high
to low pressure.

If your cold header tank continues to fill with the water supply turned
off
then - discounting a small rise due to expansion - the water *can* only be
coming from the other circuit. The fact that the water in the cold header
is
lukewarm re-inforces the notion that it is coming from the primary
circuit.

You say that when it reaches overflow level, the drip rate is very slow.
Could it be that the overflow level in the big tank is only *just* below
the
static level in the small tank. If so, the pressures would be very nearly
equal by the time it rose to this level.

If this *is* your problem, you'll need a new hot water cylinder!
--

One other possibility comes to mind - is there a leak between a cold mains
fed input and the hot water service via a defective mixer tap or temperature
control mixing valve located anywhere in the house?
Try turning off the cold water main at the stop tap and observe the header
or feeder tank as you call it.