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In article ,
(Andrew Gabriel) writes:

Let's try to guess what might cause the temperature in the cabinet to
overheat. Perhaps blocked air intake or flue, burst/split/disconnected
internal flue pipe, burst/split/disconnected internal condensate pipe
before U-trap (would also show up as water leaking from casing), failed
burner gasket, failed heat exchanger insulation (internal or external),
excessive combustion rate.

I just checked the fault finding section, and it gives another
possible cause as pump overrun not working although I'm not sure
what you could do about that as it's controlled by the fan
control board. You could perhaps have a system design which had
no water path for overrun (e.g. no 3-way valve and no bypass) or
the bypass loop is too short to absorb the required energy.

Also, are there any external influences which might heat up the boiler
casing, such as positioned over a radiator or some other heat source?
This could have blown the thermal fuse by overheating the boiler when
it's not running and there's no airflow through it.

Another thought -- has the thermal fuse actually failed at all?
Check it, and if not, look for a broken wire connecting it back
to the fan control board. It could also be a fault on the board
itself.

--
Andrew Gabriel