With the common forced air heating in my area, with either natural gas or propane, if electrical power is down the fans to distribute the warm air of course won't run, but in addition the valve that turns on the main burner won't turn on, so effectively there's no heat at all. The "pilot light," where one is used, may stay lit, but most furnaces now use an "ignitor" that only lights the pilot when the thermostat asks for heat. (The constantly burning pilot light doesn't provide any useful heat, but if the flame is "blown out" it can dump a whole lot of gas into the house before it finds a spark to ignite the accumulation and blow the roof off.)

I do have an aux generator that could run our gas heating system if there's gas but no sparks; but haven't gotten around to making a connection for the furnace that's isolated from the house wiring and that's absolutely necessary before I try use the generator to run the heating furnace.

I do probably have enough small "cube heaters" that I could plug directly into the gen, and could keep "survivable" temps in enough of the house if really necessary. Our needs for camping (which we don't expect to do much any more) have resulted in my having 350 or 400 ft of AWG10 extension cords and "lots of" AWG12, so I could sneak a tap off a neighbor's generator a block and a half down the street if really in a bind. (But I hate to paint the extension cords for proper camoflauge if I don't need to.)

John