Furnaces are rated by the Annual Fuel Utilization (AFUE) ratio, which is a percent of heat produced for every dollar of fuel consumed. Like the miles per gallon rating on your car, the higher the AFUE rating, the lower your fuel costs. All furnaces manufactured must meet at least 78 percent AFUE.

AFUE tells you how much of your fuel is used to heat your home and how much fuel is wasted. Example: If your furnace has a rating of 80%, the furnace converts 80% of the fuel that you supply to heat and 20% is lost out of the chimney.