The boiler is working and provides both hot water and heating but runs continuously, even with the programmer removed, unless the power is turned off. Both the boiler and the pump run although the room stat and cylinder stat are turned right down and are not calling for heat. Fault was intermittent to start with but now seems permanent.

Les Webb, April 2008

One of your zone valves most likely has stuck open, supplying demand power to your boiler, these valves open and close via your room and cylinder stats, but if stuck open or the micoswitch in them has failed in the on position it will power your boiler.

T, April 2008