My son has a Worcester Bosch 240BFcombi installed in his house. The boiler
is some 15 years old and has been causing them problems for some time.
Although the boiler runs the central heating without problem, as soon as a
hot tap is turned on, within seconds the over-heat thermostat trips and the
boiler shuts down.
In recent months the gas valve, thermocouple and overheat stat have been
replaced but the problem persists (in fact it is worse since the stat was
It seems to me that the over-heating is being caused because there is
insufficient flow of water through the heat-exchanger. We know that this is
not an issue with water pressure because this has been checked. The only
other thing I can think of that may be reducing water flow is that the heat
exchanger has become scaled up and requires replacing (although this is not
a hard-water area and kettles here don't scale up).
Because no gas connections are involved in this job, it is probably
something I can tackle myself - and, surprisingly, the heat exchangers for
this boiler are not over-expensive at between £119 and £170.
The big question is, however: is it worth spending out even more money in
trying to keep a 15 year old boiler going, or would the best course of
action be to replace the boiler? I don't know for certain that the heat
exchanger is the problem - and it would be annoying in the extreme if we
replaced it only to find that the overheat problem was still occurring.
Any advice welcomed - including any other suggestions as to why the overheat
stat keeps tripping when hot water is demanded.