Furnace ratings are efficiency ratios that show how well a furnace can convert fuel into heat energy. This efficiency rating or ratio is given by the annual fuel utilization efficiency ratio as a percentage.
The efficiency rating of a home furnace is important because it relates the amount of gas consumption to its heat production. An energy efficient furnace should have a high percent ratio so that gas fuel costs are low. By law, the furnaces used in homes today must have a minimum AFUE rating of 78 percent, but there are newer furnace types that can have an AFUE rating of 97 percent. Furnaces that are older can have an efficiency rating of 65 percent, as stated by Consumer Reports. In general, new furnaces that have higher efficiency ratings can cost more when buying them.