Powermax 155x weird problem?

I heard these boilers are rather unreliable but this is totally illogical - I keep it on hot water only (central heating off), as it is that time of the year, however the radiators still get hot unless I turn off the valves/thermostats on them. I thought the hot water and and the radiators are supposed to be completely different. I am not too bothred by this, but I am concerned that it burns gas to heat the radiators. Anyone has any ideas of what could be wrong with this boiler? Many thanks.