The Federal Trade Commission requires new gas furnaces to display their annual fuel utilization efficiency (AFUE) so that consumers can compare heating efficiencies of various models. AFUE is a measure of how efficient the appliance is in converting the energy in its fuel to heat over the course of a typical year.
Specifically, AFUE is the ratio of annual heat output of the furnace compared to the total annual fossil fuel energy consumed by a furnace. An AFUE of 90 percent means that 90 percent of the energy in the fuel becomes heat for the home and the other 10 percent escapes up the chimney and elsewhere. AFUE doesn't include the heat losses of the duct system or piping, which can be as much as 35 percent of the energy for output of the furnace when ducts are in the attic, garage, or other partially conditioned or unconditioned space.
Although older furnaces have efficiencies in the range of 56 to 70 percent, modern conventional heating systems can achieve efficiencies as high as 98.5 percent, converting nearly all the fuel to useful heat for your home. Energy efficiency upgrades and a new high-efficiency heating system can often cut your fuel bills and the furnace pollution output in half. Upgrading your furnace or boiler from 56 to 90 percent efficiency in an average cold-climate house will save 1.5 tons of carbon dioxide emissions each year if you heat with natural gas, or 2.5 tons if you heat with oil.
Source: Energy.gov
