Both c/h and dhw keep tripping off after a certain period of time. I have replaced overheat thermostat as an initial measure but the problem will not go away. the heater fires up when the system cools down but the amount of available hot water is variable sometimes only a couple of minutes. grateful for any assistance. Jim.

jim Rothwell, January 2008

Hello, There is a possibility that your boiler heat exchanger is sludged up with either limescale or rust from the radiators.
This would slow the flow down through the heat exchanger causing the boiler to overheat.
A quick repair would be to add a flushing agent to your system like fernox. This would only cure your rust (ferrous oxide) problem.
If the cause is down to limescale the heat exchanger would have to be powerflushed. Quite an expensive job.
Regards,
Nick

Nick Frost, January 2008