When shopping for furnaces, you'll often see numbers like 80 percent or 95 percent. These numbers refer to the AFUE that we discussed above. For every dollar spent on heating costs, an 80 percent high efficiency furnace uses eight-tenths of every BTU to heat a home. A furnace with a 80 to 83 percent AFUE efficiency rating is considered a mid-range efficiency furnace with 80 percent being the minimum allowed by the Department of Energy. High efficiency furnaces are typically between a 90 and 98 percent AFUE rating and include features like two heat exchangers, variable speed fans, sealed combustion chamber, and smart controls.