Sorry, I didn't mean to hijack the thread.

To power the main heating system I would surely need to install a bypass switch, as the power line of the furnace goes directly in the wall and not in a standard socket.

It's a job I'd need to leave to a licensed electrician for technical and legal reasons.

But there's another concern with powering the main heating with a generator.

I just realized that my furnace It's really what you English speakers call a gas boiler, one that use heated water that pass through radiators, and not warm air, to heat the rooms. Sorry for the misundertanding. So, for my heating system to work, I also need water and constant water pressure, something I would not take for granted during an hypothetical, widespread power outage.