Combi boiler and overheat stat trip problems


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 replaced!).
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.
Kev
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"Ret." <xxx> wrote in message

You could check the fault finding part on the manual.
http://www.worcester-bosch.co.uk/cache/file/117/installation-and-servicing-instructions-for-240-bf--discontinued-dec-1997.pdf
Adam
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ARWadsworth wrote:

http://www.worcester-bosch.co.uk/cache/file/117/installation-and-servicing-instructions-for-240-bf--discontinued-dec-1997.pdf Yes - my son still has the hard-copy and we've just been through it. It appears that there could be other explanations for this problem. They've decided to go for a new boiler!
Kev
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The heat exchanger became scaled up on our combi a few yeas ago so I removed it from the combi and carefully poured some brick acid into it and in no time at all it was clear of scale and refitted. It's worked perfectly ever since.
Ash
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