Heating system components include the heat exchanger, burners, ductwork and vent pipes, or flues. A gas-powered heating system's thermal energy efficiency is measured by a standard known as the AFUE. The AFUE indicates the efficiency by which fuel is converted to energy, and it ranges between 80 and 98 percent. The higher the AFUE number, the more efficiently the unit converts gas fuel to warm air, which means the more energy efficient the unit will be. The more energy efficient the heating system, the more money the property owner will save in energy costs over the course of a season.