Back feeding power into a circuit can be dangerous. Only if you are skilled and safe with electricity would you do any wiring. I am not suggesting, or reccomending anyone do any wiring.
In theory, one could unwire the furnace from the house power, and insulate the house power wires. Connect the furnace wires to a cord and plug harvested from a three wire appliance that's being thrown out. When the power comes back, rewire the furnace to the breaker panel, and put it all back like you found it. The furnace gets powered, and much lower chance of back feeding, and killing a lineman.
You will need a bit of electrical knowledge, wire nuts, electric tape, safe working habits. Note, that I didn't provide step by step coaching. If you don't know the details, call an electrician or a friend who does. Remember, the utility power can come back on at any second, and ignorant people can be killed from electricity.
I installed furnaces for six years, and have worked with electricity for 30 years. It still scares me, and I double and triple check safety. Act with wisdom, and at your own risk.
Christopher A. Young Learn more about Jesus
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I think I could wire a plug for my furnace. As it is, from the breaker, goes to box on furnace, then hardwired to furnace circuit.
Really, thought about turning breaker off, then run reverse plug from generator to box....
Greg