If you have it may be set wrong and is bringing on the system at too high a temperature.
The idea is to switch on the system, overriding the room stat, to prevent the pipes in the loft freezing up at times when the heating is otherwise shut down.
A stuck zone valve actuator is a possible reason - hasn't returned to the off position (usually a mechanical process) and therefore the boiler circuit is still energised. Try finding a manual override lever on the zone valve and move it to the "off" or "closed" position.