AFUE stands for Annual Fuel Utilization Efficiency. It is a measure of how effectively a furnace converts fuel to heat. . It's a percentage reflecting how much of the energy in the fuel becomes heat for your space. For instance, a furnace with a 95% AFUE rating converts 95% of the fuel's energy into usable heat, and only 5% is wasted. Always choose a furnace with the highest AFUE rating you can afford. The minimum AFUE rating for new furnaces is 80%, but high-efficiency models can have AFUE ratings of 90% to 98% or more.