A furnace is a heating unit that works by generating and distributing heated air throughout your home. It is one of the oldest types of heating systems still in use today for both residential homes and commercial buildings. Older versions were powered by coal and wood versus today’s modern versions, which burn natural gas, propane, and electricity to produce heat. 

The various types of furnaces differ in how they work— single-stage, two-stage, variable speed, and the fuels they are powered by— gas, oil, and electric. While the basic operation remains the same, these have specific characteristics, costs, and efficiency ratings.