Assuming your boiler is 120v, you can have an electrician install a "generator switch" that lets you plug in a generator directly to the boiler; or you can just rewire the boiler to a standard, 3-prong 120v plug.

The bigger issue is how many amps the heating system draws. Assuming it's really a furnace that pumps hot water, (as opposed to a boiler that sends steam to radiators) those pumps really do draw a lot of current.