In an earlier contribution to this discussion,
Ross Galvin wrote:

Gents,

After taking all the useful advice on board I turned the pump
to it's lowest setting and bailed out the expansion tank,
to the tune of 26 pints in fact. I left the tank with
approximately 5" of water in, which was the point at which the
ballcock kicked in.

I've been popping back up, on and off, over the last couple of
days and the water has been consistently cold. However, the
water level has crept back up, in fact it has risen by 3 or
4 inches! I then noticed that the rust coloured high water
mark was ABOVE the level of the open vent exit. I bailed the
tank out again, then help a cup of water over the end of the
OV. I could then see Brownian motion at the entrance to the
cold feed pipe with my torch, and after poking my index finger
in the cold feed I could feel hot water emerging. I took the
cup away and the flow quickly stopped.

My gut reaction is that the ballcock is gently leaking, the
tank is filling up, submerging the OV, and the system is
then seesawing. I suspect a new ball cock and some inhibitor
are called for, what do you think?

Regards

Ross

The ballvalve *could* be faulty - but there's another, perhaps more likely -
and expensive(!) - explanation.

Do you have a tap on the supply side of the F&E tank? If so, turn it off so
that no water can enter through the ballvalve, and see whether the level
still rises. If not, just put a piece of wood across the top of the tank,
and tie the arm of the ballvalve to it.

If the level still rises (other than just by expansion when the system gets
hot) you could have a leak in the indirect coil inside your hot water
cylinder, which is allowing domestic hot water to enter your primary heating
circuit.

Do you have a large header tank for the DHW in the attic? Is the water level
in this *higher* than that in the F&E tank? If so, and if there *is* a leak
in the coil, the higher static pressure in the DHW system will cause water
to flow into the primary circuit.

If this *is* the problem, you'll need a new hot water cylinder. If you *do*
need to replace the HW cylinder, make sure you get a fast recover one (with
a large heat exchanger inside). Unless the existing cylinder is fairly new,
the new one will almost certainly be better insulated - so you'll end up
getting faster hot water, and wasting less heat.
--
Cheers,
Set Square
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