Inconsistent hot water temperature - Combi Boiler - Worcester 28CDi

[Chris]

I wonder if anyone can advise?

I have a Combi Boiler - Worcester 28CDi - installed 2001 - so nine years old.

I have carefully adjusted the hot water temperature so that it is just right for a hot bath without using the cold tap at all. (This is very convenient because I can go away and do other things while the bath is filling and know that the temperature will be exactly right.)

That setting has worked for years ... until recently. Now the temperature is inconsistent: sometimes too cold; sometimes too hot.

Please could anyone advise on possible causes and cures? Any such help will be valued and appreciated.

[cynic]

Its 9 years old - old age brings its problems with it.

[Martin Pentreath]

It could be that the secondary heat exchanger is scaled up. Does the temperature oscillate up and down while the water is running? That's a classic symptom. Once the heat-exchanger gets coated with enough scale the water passing through it can't carry away the heat being generated by the gas quickly enough, so the boiler's internal thermostat makes it cut out momentarily until it cools down a bit, then the cycle resumes. The solution is to get the heat exchanger out and dump it in some suitable descaling solution for a couple of hours.