I have the following:

Dedicated circuit breaker for our natural gas furnace.
The wire from the circuit breaker goes directly to the furnace and into the off/on switch box on the side of the furnace.

I would like to do the following:
Be able to plug in an extension cord from my generator to power the furnace.
I do not have, nor want, a transfer switch.

What is the best way to go about making the changes to allow the extension cord from the generator to power the fan on the furnace?

Thanks in advance,
Rick

I have studied this problem, but done nothing myself. One thing you could do is put your furnace on a cord. Terminate the NM or what ever feeds you furnace in an outlet. Connect a length of 12-2 flex with ground to the furnace. Cheapest and easiest thing might be to buy an extension cord meant for an air conditioner and cut the end off. Plug the furnace into the outlet. Then when you need the generator, move the furnace plug to an extension cord plugged into it. Use a cord heavy enough to supply what your furnace needs.

I have seen several reports of furnaces refusing to run on a generator. I suspect improper ground. Others have suggested the generator doesn't regulate the voltage well enough. I intend to test mine, but not until the weather warms up. I did try running my computer off the generator, along with the cable modem, and router. They all did fine.

what I did was to get a double pole switch with center off. I then added a male receptacle near the furnace. It is wired with the furnace to the common or center terminals on the switch and city power on one side with the receptacle an the other side. The switch I used is actually a double pole double pole with center off. The advantage of this is that it also transfers the neutral lead as well as the hot lead. I have even run my furnace on a inverter using this set up. I have not had any problems with running it on generator power either. The furnace is a carrier mvp series which is a variable speed 90+ efficiency.