The efficiency of a furnace is evaluated using its rating in annual fuel utilization efficiency (AFUE). This compares the quantity of heat a furnace produces to the energy it consumes. The efficiency of the unit is described by its AFUE rating over the course of a year. The AFUE rating of your furnace is listed on the EnerGuide tag which is attached to the side of your appliance (if your furnace does not have an EnerGuide logo, it is probably outdated and needs to be replaced). Depending on the furnace type and fuel source, furnace efficiencies typically range from 80 to 88 percent for mid-efficiency furnaces and from 90 to 98 percent for high-efficiency furnaces. We typically recommend homeowners to use furnaces with a minimum efficiency of 92%.