AFUE stands for Annual Fuel Utilization Efficiency. The AFUE measures a gas furnace's efficiency in converting fuel to energy. A furnace that has an 80 percent AFUE rating can turn 80 percent of the energy it consumes into heat. The other 20 percent is used during the heating process.
A higher AFUE rating means greater energy efficiency. The minimum standard AFUE for new furnaces is 78 percent. A mid-efficiency furnace usually has an AFUE rating between 80 and 85 percent, while a high-efficiency unit has a rating between 90 and 97 percent.