Make sure the company you’re relying on has a good, dependable service department. Not much is worse than you’re furnace going out on a freezing cold day and the service department can’t fit you in their schedule for two weeks. Also, if something does happen to your furnace, it is best to get it repaired correctly the first time.
Make sure to get 2 to 3 estimates, see who’s being fair and who is not. Buying a furnace is like buying a car; you have to shop around a little.
Ask your friends and neighbors about who they use. They will be one of the most honest review sources. Neighbors will tell you the honest truth. If they have had a bad experience they will warn you. On the flip side, if they have had a great experience with a company then they will pass that information along too.