In an earlier contribution to this discussion,
DavidM wrote:


Thanks for replies so far.
I've done some more checks:
1. Removed thermostat and checked it in a pan of hot water and with a
meter - switches correctly between 60 and 80 C. The old thermostat I
took out also works correctly grrrrrrrr - £33 needlessly spent!
2. Checked the electrical connections - the boiler gas valve is only
connected to the bioler thermostat, no where else, so nothing can be
bypassing the thermostat.
3. Checked that the thermostat bulb is pushed fully into the tube in
the boiler.
4. The gas valve turns off if the boiler supply is swithed off or if
the boiler thermostat is turned to the off position, so I'm pretty
sure it's not just the gas valve sticking (the whole gas valve
assembly was renewed last year anyway).

Any more ideas please?
David

So the thermostat works when you test it in water. The output from the stat
is the only thing connected to the gas valve - but when the boiler gets hot,
the stat doesn't switch off when it should. Is that correct?

If so, it can *only* be that the stat is not sensing the temperature of the
boiler for some reason. Try cleaning out its pocket with a suitable sized
drill bit (operated by hand!) to make sure there's no gunge in there that's
providing insulation and preventing proper contact. Then use some heat
conducting paste.

It that doesn't work, it sounds like there could be some sludge inside the
heat exchanger which is stopping the hot water from getting to the area of
the thermostat pocket.
--
Cheers,
Set Square
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