It is commonplace for a combi boiler to fire up now and again even when the central heating is switched off. The boiler can tend to do this even when heat or hot water is not being demanded by or distributed to your home radiators or alternate heating devices.
This method of pre-heating enables your boiler to guarantee hot water is available as soon as it is turned back on. Otherwise, you might need to run your taps for a while until the hot water supply returns, which could cause your energy use and bills to spiral in turn.
To find out more, visit our related blog, Why are my radiators getting hot when the heating is off?