There are flow temperature senders in the CH and DHW output water pipes - these are thermistors and the voltage passing through them is affected by temperature. This change in voltage is measured by the PCB (printed circuit board) which in turn modulates the pressure to the burners via the modureg in accordance with the temperature that you have pre set on the controls. If these fail then it is possible for the boiler to run erratically and for the water to be too hot. The sensors are easily changed as they are fitted in wet pockets and just screw out. They can also be bought in pairs (colour coded red for CH and blue for DHW). Cost is about £20.00. Other than that you may have a pcb or modureg problem, which unfortunately that can be expensive to rectify as specialist equipment is required to check them.
Phil the fluter, January 2010
