I saw a writeup where a gas furnace was wired with a piece of an extension cord with a plug (properly sized) that was then plugged into an outlet (dedicated circuit). There was only one outlet so only one device could be plugged at a time. This would allow the furnace to be easily switched to a generator in case of a power failure. This was said to be NEC compliant as the circuit was dedicated for the gas appliance and the wiring/outlet was sized properly. This seems like a much simpler and cheaper approach than a transfer switch but before I go and do this I wanted to be sure this was a sound idea.
I have to admit, I do not know the NEC regulations regarding this type of setup and all furnaces I've seen are hardwired.
I have seen dedicated outlets setup to be used for misc. utility and/or used to back feed the breaker panel in order to run the furnace and/or other select items.
Sorry I can't be of more help to you but, Speedy may be able to provide NEC compliant input for you.
To be clear, I have NO intention of backfeeding the panel, just want to plug the furnace into a generator. As an additional point of clarification, the power supply for the thermostat would also be powered with this setup.