Given that it sounds like the poster is asking about installing a furnace in a pre-existing structure that doesn't already have one, and may not be vented and/or ducted for a furnace, I would suggest NOT going with the low bid - a poorly installed furnace can cause all sorts of problems, in terms of both money and health. Call the Better Business Bureau and ask for recommendations; check resources such as Angie's List for reviews of companies and contractors; get multiple bids and ask a lot of questions about size, installation costs, upkeep, etc. Check with the local power company (also a possible source of recommendations - but be cautious about possible proprietary recommendations) to decide if gas, propane, or electric will be most efficient/cost effective, and how much it would cost to add the piping necessary for gas or propane.
