Furnaces are the most common type of home heating equipment in the United States. Furnaces burn natural gas, propane, or oil for heat and use a fan to distribute the heat through a duct system. They are often installed in conjunction with a central air conditioner, using the same air distribution system. Non-weatherized gas furnaces, which are installed in basements, utility rooms or other indoor spaces, are by far the most common type. Weatherized furnaces for outdoor installation, such as on rooftops, are sometimes used in warmer climates. Furnace efficiency is measured in annual fuel utilization efficiency (AFUE). Furnaces with 90% or greater AFUE are known as "condensing" products because they use technology that condenses water out of flue gases to recoup heat to warm the home that would otherwise be vented up the chimney. The maximum efficiency for non-condensing non-weatherized gas furnaces is 80%.
DOE finalized the current standards for non-weatherized gas furnaces in 2007 and they took effect in 2015. The standard was set at 80% AFUE, a level already met in 2007 by 99% of furnaces sold.
In 2011, DOE issued new standards based on a joint recommendation provided by representatives of manufacturers, states, consumer groups and efficiency advocates. The 2011 rule established regional standards for non-weatherized gas furnaces, adopting 90% AFUE in the North but leaving the standard at 80% AFUE in the South. 81% AFUE was set for weatherized gas furnaces and 83% AFUE for oil furnaces. However, in response to a lawsuit, DOE agreed to withdraw the non-weatherized gas furnace standard and conduct a new rulemaking for this equipment with a revised standard due in 2016. DOE issued a supplemental proposed rule in September 2016 but has not yet issued a final rule.
The revised standards for non-weatherized oil –fired furnaces went into effect in 2013 and those for weatherized gas furnaces in 2015.
Space heating accounts for about 25% of total residential energy consumption. About 42% of U.S. households use gas furnaces (the most common equipment and fuel used for space heating) and about 3% use oil furnaces. Non-weatherized, condensing furnaces are typically the most efficient (90% and above). Gas furnaces with AFUE ratings between 80 and 90% are rare to non-existent because they can lead to safety problems.