A two stage gas furnace differs from a one-stage unit with its capability to operate a high or low stages of heating. The result is that the system saves energy by using less fuel during the low stage of heating.
Two stage gas furnaces kick into a high stage of heating in the morning when outside temperatures tend to be lower to heat up a building quickly. Once the building achieves the thermostat temperature setting, the burner dips to the lower heating stage, which uses less energy.
As the day progresses, the two stage furnace communicates with the thermostat, which continues to allow the furnace to run at the low stage of heating, thereby regulating the temperature within a degree or two of the thermostat setting. The average two stage gas furnace operates at their low heating stage about 90 percent of a 24-hour period.
Many homeowners further capitalize on the energy savings a two stage gas furnace provides by installing variable speed blowers, which use fans to force heated air through a building's ducts into all parts of its interior. Variable speed blowers adjust fan speed to coordinate with the furnace speed, ensuring delivery of heat in the desired amounts at the desired times. The blowers in standard one stage furnaces, on the other hand, run at one speed all the time.