Boiler that turns itself on out of hours!

[phil1963]

My boiler is set to automatically come on between the hours of at 5.30am-7.30am and 5.30pm-10.30pm. At the moment I have turned the central heating switch off, so only the water gets heated. Last night at 11pm and now at 9.45am i.e. BOTH OUTSIDE OF THE PLANNED "ON" TIMES, the green light has come on and the boiler has fired up.
Am I right in thinking that irrespective of what the thermostat is set at, if my boiler is set to ceratin hours, then it should stay at these certain hours. Or I have I got a phantom boiler which now seems to light up of its own accord?
It is a Suprima 60 type, which I am aware from these pages, often "locks out" when the boiler has reached it required temperature ............but now it has developed a mind of its own and ignores the on/off settings that I have imposed on it.
Any ideas?

 

[ChrisR]

The usual culprit isa faulty switch in a motorised valve. If you have two, they'll be like this:

notice the boiler supply comes from the valve, not the programmer

 

[Paul Barker]

Additional to what Chris said, Suprima pcb is a wild card. But my first port of call would be the thermister. If it is out of range the boiler might think it is frozen to death and go into a protective antifrost cycle.