The hot water, whether the heating is on or off, fluctuates between hot and cold in a 30 odd second cycle. The secondary DHW heat exchanger has been cleaned and the electrical circuit board has been replaced (the very expensive bit). This was replaced because of another fault to a fan relay. The fault has been gradually getting worse with the water temperature varying slightly in the beginning, but now fluctuating between very hot to cold. The burner does cut in and out as if it is trying to regulate the temp. But not at the same rate of the water temp fluctuations. Generally the Heating is behaving itself but on the odd occasion after using the hot water the radiators get very hot all of a sudden. We then turn off the heating to let it settle down, and all returns to normal after.

Rob, January 2009

I have now changed both thermistors and the problem remains, further info available.... when turning the hot water tap off, the heating, when switched off, goes into anti frost mode, starts up and the 2 relevent lights flash. Still the water fluctuates between very hot and cool. Any suggestions would be really welcome.

Rob, March 2009

Sounds like the hot water thermistor or central heating thermistor is giving some dodgy readings. On hot water, the burner usually won't cut in and out. See fig 7 sensor D and E in your installation manual and check the connections or replace them, start with the central heating thermistor.

Jason, February 2009