Mike,

I have done it successfully.

In theory, not hard, especially if you know what you are doing and don't mind a few minor violations of local codes.

A typical gas fired forced hot air heating system doesn't take much power to run. But every installation is different and the details get complex. Here is how I did it.

First, the system needs to be off line from the main breakers. Flip the breaker(s) that power the system to off.

Second, find an alternate way to get the power into the circuits you just isolated. In my local jurisdiction, the code calls for one standard outlet at the furnace, so this is easy.

Make a "make to male" adapter. Make sure and triple check that you do this correctly or there may be fireworks or damage to the system.

Run an extension cord to the furnace from the generator, plug one end of the male to male into the extension cord and the other into the outlet on the isolated furnace circuit. Your furnace, fans and controls should have power, and, after a brief "reset" time should come up and run and provide heat.

I have only done this in extreme conditions, when indoor temp got below 50 f., as there are conditions when the power you are injecting can leak our through the grounding system (which is not disconnected by throwing the breaker) and endanger the folks working to restore power.

I can't stress enough that every installation is different, and, depending on your skill/knowledge, the help of a friendly electrician on a non-attribution basis may be required.