The major manufacturers of gas furnaces include American Standard/Trane, Carrier/Bryant, Lennox and Rheem/Ruud. All of these brands make multistage furnaces, and all make at least some models that have an AFUE of 90 percent or more. A survey conducted by ConsumerReports.org found no differences among these major brands in terms of their overall reliability. They do vary somewhat in price, from about $2,300 for the average Rheem furnace to about $3,000 for the average Trane or American Standard furnace. However, these prices are only averages. The final bill will depend largely on installation costs, and the only way to estimate these is to get quotes from individual contractors, as discussed earlier.
Although there may not be so much variation in overall quality between furnace brands, specific models certainly do vary in terms of their features. Many older furnaces lack basic energy-saving features that are standard on newer models. For example, newer furnaces typically have vent dampers or an induced-draft fan to prevent heated air from escaping up the chimney when the heating system is turned off. Older furnaces that lack this feature are so inefficient that the American Council for an Energy-Efficient Economy recommends replacing them. Also, many older furnaces have pilot lights that run continuously, ready to light the burners whenever the furnace turns on. Newer furnaces generally use electronic spark ignition, which strikes a spark to kindle a flame only when it is needed. An even newer method is hot-surface ignition, which uses a white-hot coil to ignite the gas. The advantage of this, according to HomeTips.com, is that the ignition coil will not "corrode or be fouled by dirt or bugs" the way a spark plug can. Another feature typical of new furnaces is multistage (or multispeed) operation, discussed earlier.
Other new features are specific to certain furnace models. For example, every gas furnace includes a heat exchanger, the part of the furnace that extracts heat from the gas as it burns. Condensing furnaces, however, include a second heat exchanger, which recovers the heat from a furnace's exhaust gases. This process leaves behind cooled gas (which may be vented out through a wall with inexpensive PVC pipe, rather than sent up the flue) and water, which must be drained or pumped away. All furnaces with an AFUE of 90 percent or more are condensing furnaces. Because the exhaust gas may be corrosive, the second heat exchanger must be made of some material that will resist corrosion, such as stainless steel.
Another feature that experts recommend is sealed combustion. With this feature, says Karen Youso of The Seattle Times, "The furnace's burner is sealed from household air, protecting you from carbon monoxide." A sealed-combustion furnace can improve efficiency as well. HomeTips.com describes a furnace that brings in air from outdoors, "mixing it with the fuel at a controlled rate to maximize heat from the fuel it consumes."
The valves on a gas furnace may affect its efficiency as well. Two-stage gas valves open up at full throttle when the furnace is first turned on, then cut back the flow of gas to save fuel. In addition to their energy-saving benefits, two-stage valves tend to be better built than standard gas valves, according to Chad Jones, an HVAC technician from Michigan who has contributed regularly to the heating and cooling forum at ConsumerReports.org.
A final energy-saving feature that may be of interest is zoned heating. With this feature, you can send different amounts of heat to different rooms or areas in your home. Programmable furnace thermostats, combined with a central controller and a series of dampers to control airflow, adjust the amount of heat each room receives at different times of day. This feature can be very useful in larger homes, especially if different parts of the home have different heating or cooling needs (for example, because one area has many more windows). However, the editors of ConsumerReports.org warn that contractors they interviewed say zoned heating systems require repairs more often. Zoned heating works best with a variable-output furnace, according to HomeTips.com.
In addition to the features of the furnace, you should consider its warranty. ConsumerReports.org says that more efficient (and expensive) furnaces tend to have longer warranties than basic models. With a condensing furnace, it is especially important to have a long-term warranty on the heat exchanger, according to HomeTips.com. The best heat exchangers will resist corrosion and chemical buildup during the entire life of the home.