In an earlier contribution to this discussion,
wrote:

I have a Myson boiler. The water boils, the overflow pipe make a noise
like thunder as hot water overflows into the header tank in the loft
with a tremendous gush. The overflow pipe is very hot to touch, and
the water in the header tank is hot and steaming. Is this a boiler
thermostat problem? The boiler does this after a couple of hours of
being on for HW and CH and is up to temperature. When I turn the
temperature control on the boiler front panel down to off, it stops.
The pump is working. What's the likely cause, and is it a DIY job to
fix it?
Thanks
Kevin

What happens if you turn the temperature control down but not off? If the
boiler keeps running, the thermostat is almost certainly duff.

The thermostat is actually a pressure switch, connected via a capillary tube
to a bulb containing a liquid, which is plugged into the side of the heat
exchanger. The switch responds to changes in vapour pressure from the liquid
which depends on temperature. If the bulb or tube leak, they no longer
produce any pressure - so the stat doesn't operate. Is there any sign of
leakage? The bulb needs to make good thermal contact with the heat
exchanger - usually using a conductive paste. If this has dried up, or if
the bulb has come out of its socket, that would also stop the stat from
working properly.

However, it's probably more likely that the electrical bit of the stat has
failed.
--
Cheers,
Roger
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