heating will come on for a few days but no constant hot water

martin turnbull, December 2010

Possibly one of two things:

With the heating off turn on the hot tap, does 'd' appear in the display, if no, the HW thermistor is at fault.
If yes, put your hand on the CH flow pipe whilst tap is running, does it get hot? If so, diverter/actuator fault

Carefree, December 2010