How do I connect my furnace to my gas generator?

You can just use an extension cord. Cut female end off and wire it to the furnace. Then just plug male end into gen. This is of course just one way to do it.

Now, if you want to be able to keep furnace wired to house, and be able to use gen when power goes out, just by flipping a switch, it gets a little more involved.

Here is one simple way to switch from home power to generator.

Instead of having furnace hardwired to breaker box, just place a dedicated outlet near furnace. Now wire in a cord, (as I suggested earlier, only make sure it is rated for correct amps and such). Now, you can plug the furnace into the outlet for house power, or unplug it and plug it into your generator. Make sense? Let me know what you think. I will offer more advice if you like.

There are ways to have generator backup switch over automatically, but this gets pretty involved. Please let me know what you are trying to accomplish, and I will help if my knowledge allows.

A Gen-Tran type panel is an easy way to achieve this. You can get small ones for just a well or furnace, up to ten circuit ones.

Whatever you do you MUST have some form of interlock or other way to be sure there is NO WAY to back-feed the utility lines. Simply shutting off the main breaker is NOT enough!