Many older furnaces are single-stage (or single-speed) heating systems. This means that they are either on or off, and when they are on they put out heat at a constant level. To keep the house at a constant temperature, a single-speed furnace must cycle on and off frequently. Newer furnaces, by contrast, are generally multistage or multispeed furnaces that can produce heat at two or more different rates. A two-speed furnace, for instance, has a high setting and a low setting. When less heat is required, it can run steadily on low rather than cycling repeatedly from high to off. This feature helps keep the house comfortable by reducing drafts and fluctuations in temperature. It also reduces noise, because the low setting is quieter than the high setting.
Multispeed furnaces are also more efficient than single-stage furnaces. HGTVPro.com likens a single-stage furnace to a car being driven in stop-and-go traffic. Because it must constantly speed up and slow down again, it runs less efficiently and gets fewer miles to the gallon. The same car driven at a steady speed will get better mileage. In the same way, a multistage furnace will use less fuel because it runs at a lower, steady rate instead of cycling on and off.
Even more efficient than a multispeed furnace is a true variable-speed furnace. Contractor Blake Ballard, one of the HVAC experts on AllExperts.com, says, "Virtually every natural gas furnace on the market is a multispeed, but very few are variable speed." A variable-speed system pairs a variable-speed blower with a computer-controlled thermostat. These systems can run at several different levels, adjusting automatically to maintain a constant temperature. Because they adjust energy use based on need, they're the most efficient type of furnace, as well as the quietest. Many models also include a fan that runs during the summer to reduce your home's cooling needs, and some also have a dehumidifier function.
In addition to saving fuel, variable-speed furnaces use less electricity to power the fan motor. According to the American Council for an Energy-Efficient Economy, the amount of electricity used to run a furnace motor can be significant. Thus, a more efficient, variable-speed fan motor can produce significant savings. Chad Jones, a heating and cooling service technician in Michigan who posts frequently to the heating and cooling forum at ConsumerReports.org, says, "A two-stage variable-speed furnace no matter what brand will save you $200 to $300 per year on just your electric bill. The variable-speed blower uses only the equivalent of a light bulb's electricity and a standard furnace uses about 600 watts of electricity when it is running. You will make your $600 back in about two to three years." AllExperts.com's Ballard agrees that a variable-speed system is a good investment. Taking all their benefits into account, Ballard says variable-speed furnaces are "really worth it," and he uses one in his own home.