In an earlier contribution to this discussion,
wrote:

Can you tell us a bit more about your system? Does the primary hot
water circuit (from the boiler to the indirect coil in the hot
water cylinder) circulate by gravity (convection) or is it pumped?

Gravity feed.

When you mentioned the thermosatat above, were you talking about
the knob on the front of the boiler, or a room stat somewhere?

Before testing the central heating I turned the room and boiler stats
to maximum because it was a warm day. The heating worked.

The boiler stat was also set to max for the hot water test, with the
results described in my previous post.

Tony.

OK, so it's the gravity circulation which isn't working - suggesting an
air-lock or blockage in that part of the circuit. You've probably got two
header tanks in your attic - a large cold storage tank which feeds the
domestic hot water system, and a small fill & expansion tank which feeds the
radiators and primary hot water circuit. How much water is in the *small*
tank? There should be a pipe from the bottom of this tank, comnnected into
the primary circuit somewhere - probably near the hot cylinder in the airing
cupboard. Is there a tap or gate valve in this pipe, and is it fully open?

When did the hot water system last work properly - and did it suddenly stop
working?
--
Cheers,
Set Square
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