How do you mean on/off. that is what it should do. If you mean that it is doing it very quick, I would think some obstruction to heating cct, so it is only heating what is in the boiler, check all valves are open, check 3 way valve, check pump
SD
November 2011
more questions.
is the boiler over heating like a kettle and sounds like a kettle boiling, this would point to the pump not working?
Do you get any heat coming out of the heating pipes below the boiler ?