Efficiency, size and number of stages are the most important things to consider when shopping for a furnace. The processes used by humidifiers and thermostats and the quality of the filters and disinfecting system are also important.
A rating that is helpful in evaluating the efficiency of a furnace is its Annual Fuel Utilization Efficiency. This number shows the percentage of energy that the furnace uses that actually goes toward the heating of the house. The remainder of the energy heads back up through the chimney. This means that a 75 percent AFUE furnace converts 75 percent of the energy that it receives into heat. However, it wastes the other 25 percent. A furnace that is 20 years old is likely to have an AFUE rating as low as 65 to 70 percent.
The largest contributor to utility costs is the heating bill during the cold months. People who plan to live in their homes for at least five years beyond the replacement of the heater should pay more for a unit with a higher AFUE rating as the energy savings make the higher expense pay off over time.
A heating and air consultant can make a recommendation for the size of a heating unit with respect to the square footage being heated.
Most consumers buy single- or dual-stage furnaces. Single-stage furnaces are cheaper up front. Two-stage furnaces, however, cost less to operate because their design allows the heater to hold to a narrower temperature range, meaning that it runs at full power for a shorter time.