Low efficiency furnace is less than 75% efficient. However, the federal government now requires residential gas-fired furnaces to have a minimum efficiency of 80%. Medium efficiency furnaces are between 80-82% efficient. High-efficiency furnaces (also called condensing furnaces) have AFUE ratings that range from 90% to about 97%. These furnaces have a secondary heat exchanger where the moisture in the escaping flue gases is condensed. This phase change from water vapor to liquid water releases heat, improving the unit’s efficiency.