The federal government sets minimum efficiency standards for heating and cooling systems. Furnace efficiency is rated via annual fuel utilization efficiency, or AFUE, with the minimum efficiency at 78 percent. However, high-efficiency furnaces go well beyond that percentage, ranging between 90 and 98 percent AFUE. The top performer nets significant savings on heating bills, wasting only 2 percent of the energy they consume, which is typically lost through the chimney or venting system.
