Gas furnaces are rated for efficiency with an Annual Fuel Utilization Efficiency number, or an AFUE. According to the state's Energy Efficiency Standards, Title 24, the minimum AFUE for central furnace systems now sold in California is 0.78, which means that 78 percent of the fuel used by the furnace actually reaches your home's duct work as heat.
The higher the AFUE, the more efficient the furnace. AFUE numbers in today's furnaces range from 0.78 to around 0.90. If you are thinking about purchasing a new central furnace, please check out our Appliance Database that lists the most energy-efficient models.
Source: U.S. DOE/Energy Star