When thinking about making a furnace upgrade, there are few considerations you’ll want to weigh more heavily than fuel efficiency and as a direct result, lifetime costs of the upgrade. For comparing the efficiency of different furnaces, the best tool available is the AFUE, or “annual fuel utilization efficiency” rating. What this rating does is measure how well a heating system converts energy into heat. For example, a furnace with a 75 percent AFUE rating is able of converting 75 percent of the energy that it uses into heat, essentially letting the other 25 percent go to waste.