What can you tell me about AFUE?

Furnace Efficiency is rated by its Annual Fuel Utilization Efficiency (AFUE). AFUE is the ratio of heat output divided by the heat input. This measures how completely the fuel, either gas or oil, is consumed to heat as the furnace heats your home. The higher the AFUE rating, the lower the fuel cost. Furnaces in the United States must have at least 78% AFUE.