More than 65% of U.S. homes are heated with central forced-air furnaces. Most furnaces heat air by burning gas, oil, or propane. Some use electricity to heat an element. The furnace burner is located in the air handler box along with a fan. The fan blows the heated air into supply ducts, for distribution to registers located throughout the house. Air from individual rooms or common areas is drawn into return registers and brought back through return ducts to the furnace air handler where it is reheated and redistributed.
The efficiency of a furnace or boiler is measured by annual fuel utilization efficiency (AFUE). An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes out the vent pipe and through the furnace jacket. AFUE doesn’t include the power consumption of the fan and controls. Nor does it include heat losses of the duct system, which can be up to 30% or more when ducts are leaky, poorly insulated, and located in an uninsulated attic or crawlspace. If not properly maintained, the actual efficiency of the furnace can be significantly lower than the rated efficiency.
Low-Efficiency Furnaces (Less than 78%)
While nearly all furnaces sold today have an AFUE of at least 80%, many older, less efficient furnaces in the 56% to 72% range are still in operation, including even old coal burners that have been switched over to oil or gas. If a furnace has a pilot light, it was probably installed prior to 1992 and likely has an efficiency of less than 72%. Old gas furnaces may lack a vent damper (which saves energy by limiting the flow of heated air up the chimney when the heating system is off).
Older style furnaces can be called draft-hood furnaces because they draw combustion and dilution air from the room in which they are located; the dilution air enters through a draft hood (also called a draft diverter). Oil furnaces use a barometric damper instead of a draft diverter for dilution air in the chimney or vent system to decouple the burner from the chimney, thus avoiding poor combustion.
Older style furnaces also rely on natural draft to carry exhaust gases out of the home. The combustion gases exit the home through the chimney using only their buoyancy, combined with the stack effect of the chimney’s height. Naturally drafting chimneys can have problems exhausting the combustion gases because of chimney blockage, wind, or depressurization of the home, which can overcome the buoyancy of the gases. Depressurization is a situation that can occur if more air is being pulled out of the home than is being drawn into the home through air leaks or intentional ventilation. Certain changes in the home can lead to depressurization, for example, if a new kitchen range is installed with a high-powered exhaust fan or if a significant amount of air sealing is done. These changes could possibly increase the risk of backdrafting, where exhaust products spill back into the house rather than going up the flue. A certified energy contractor will conduct combustion safety testing as part of any combustion appliance repair or replacement or air sealing project. The testing will ensure that the furnace and combustion appliances have adequate combustion air and are venting properly.
Mid-Efficiency Furnaces (80% to 82%)
Mid-efficiency (80% to 82%) furnaces have no draft hood. Instead, they use fan-assisted draft with a fan located at the outlet (draft inducer) of the heat exchanger to create a regulated flow of combustion air. This design change increased efficiency from 65% AFUE in atmospheric draft furnaces to over 80% in fan-assisted furnaces. Fan-assisted draft minimizes the risk of backdrafting.
For furnaces with an oil burner, one way to determine the age and efficiency is to look at the nameplate. If the oil furnace has a nameplate motor speed of 1,725 rpm, it is most likely an older model with a less efficient burner. Models with a nameplate motor speed of 3,450 are newer than 20 years old and have a flame retention burner that wastes less heat; they have a steady-state efficiency of 80% or more.
Bigger isn’t Always Better
Many older furnaces and central air conditioning systems are oversized. An energy performance contractor can run energy analysis software to help you determine your heating and cooling needs after air sealing and insulation improvements are made.
In the past, many HVAC contractors used “rules of thumb” to determine HVAC sizing and, as a result, many installed oversized systems. Today’s trained HVAC contractor should determine your heating and cooling loads, and the right size for your HVAC equipment, based on calculations developed by the Air Conditioning Contractors of America (ACCA 2005; 1995; 2009).
High-Efficiency Furnaces (90% to 98%)
The most efficient furnaces available today are sealed-combustion (direct-vent) condensing furnaces, which have efficiencies of 90% to 98%. About 12% of the heat from the combustion process is captured in water vapor as latent heat. A condensing furnace condenses this water vapor through a secondary heat exchanger to capture the heat rather than letting it escape up the flue. This technological advance enabled gas furnaces to jump from 82% AFUE to 90% AFUE or higher (ACEEE 2011). There are no gas furnaces with AFUE ratings between 83% and 89% because of problems arising from condensation in the heat exchangers that occur within this range.
High-efficiency condensing furnaces avoid back-drafting issues by having sealed combustion. These furnaces are designed to vent exhaust gases (combustion products) directly to the outside through a dedicated vent pipe. They should also be installed with a second vent pipe to bring outside air directly into the combustion chamber.
Although a condensing unit costs more than a non-condensing unit, the condensing unit will save money in fuel costs over its 15- to 20-year life and is a particularly wise investment in cold climates.
Both mid- and high–efficiency furnaces are available with two-stage gas valves, two-speed draft fans, and variable-speed blower fans, which reduce their electricity usage by better matching air flow rate to the heating needs of the home.
Nearly all combustion furnaces sold today meet or exceed 80% AFUE. About one-third of current sales on a national basis are 90% AFUE or better. In just the past 10 years alone, about 7.5 million condensing furnaces went into replacement installations in the United States (ACEEE 2011).
The AFUE rating for an all-electric resistance furnace or boiler is between 95% and 100%; because there is no combustion, no heat loss occurs up the flue, but there may be some heat loss through the furnace housing. However in some parts of the country, electricity is expensive to purchase and it can be expensive to produce from an environmental standpoint as well, making electric heat a less cost- effective option.