In an earlier contribution to this discussion,
John Andrews wrote:

3. Ball valve has got stuck up in header tank (smaller
tank), and water level has dropped below the top of the
hot water cylinder coil so it no longer circulates.
Check the small header tank.


Good idea, I hadn't thought of that. But no, that's not the problem.
Water level and ball valve are fine - in fact I replaced the ball
valve assembly about six months ago. The important point is that the
hot water supply to the cylinder is not working - i.e. cold.

Cheers
John

I think that you may have misunderstood the point. You should have *two*
header tanks - a big one and a little one.

The big one feeds cold water into the bottom on the cylinder expelling hot
water out of the top to your hot taps. Is this the one with the new ball
valve?

The small one keeps the primary heating circuit full of water. This is the
water which circulates from the boiler by gravity to the heating coil inside
the hot cylinder, and pumped action to the radiators. If the water in *that*
circuit becomes depleted for some reason, the first thing to suffer will be
the gravity circulation to the heating coil - followed by the central
heating.

Are you *sure* that you have located the *small* tank, and that it has water
in it?

--
Cheers,
Set Square
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