Size. The size, or heating capacity, of a residential furnace is quantified in terms of British thermal units per hour (Btu/h) of gas input. A Btu is equal to the amount of energy it takes to raise 1 degree of water 1 degree Fahrenheit, but to put it in more practical terms, it is about the amount of heat given off by completely burning a single kitchen match. With a few exceptions, residential furnaces are available with inputs that range from 40,000 Btu/h to 150,000 Btu/h.
Efficiency. The efficiency of residential furnaces is expressed as annual fuel utilization efficiency (AFUE), which accounts for actual operating conditions. In addition to steady state efficiency, this factor also includes on-and-off cycling, the energy embodied in combustion air, and jacket losses. The minimum AFUE available is set by federal law for most furnace types at 78 percent. The highest AFUE units available are slightly less than 98 percent efficient. To obtain the AFUE of any given furnace, look at the yellow-and-black "EnergyGuide" label on the furnace (Figure 2), check the Directory of Certified Product Performance from the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), or consult the manufacturer's literature.
Figure 2: Furnace efficiency label
The U.S. Federal Trade Commission requires that gas furnaces listed by the Gas Appliance Manufacturers Association carry this label. In addition to showing the unit's annual fuel utilization efficiency (AFUE), it also shows how that furnace compares to other brands in the same input range.
Combustion air source. Furnaces may draw the air they require for combustion from either inside the heated space or directly from outside. Drawing air directly from outside, typically through a plastic pipe that protrudes through an outside wall, is both more efficient and safer. Sometimes this method is referred to as sealed combustion, because the gas is burned in a chamber that is closed to occupied space. This arrangement virtually eliminates any risk that combustion gasses could leak into occupied space. It does, however, require some complicated installation techniques, so check the manufacturer's installation instructions carefully.
Blower speed control. Most furnace blowers operate at a constant speed. Although that speed may be adjusted by changing wiring configurations, during furnace operation the speed never varies. Premium-efficiency furnaces, however, feature blower motors that sense how much airflow is required at any given moment and modify fan speed accordingly. Not only do these furnaces save electricity, but they are also much quieter than their constant-speed counterparts.