AFUE stands for Annual Fuel Utilization Efficiency. It is a measure of the efficiency of a furnace or boiler in converting fuel into energy. The AFUE rating is expressed as a percentage, and it represents the amount of the fuel that is used to heat the home, compared to the amount of fuel that is consumed by the furnace or boiler. A higher AFUE rating means that the furnace or boiler is more efficient, which can result in lower energy bills and reduced greenhouse gas emissions.