There are several types of gas furnaces available today. They vary not only in efficiency but also in the way heat and air is delivered through your system.
A single stage furnace has the simplest air delivery system and is often the least efficient, although some single stage furnaces deliver high efficiency, without the benefits of a two stage or variable speed blower fan. The single stage furnace runs on a single stage motor and when the system comes on, or cycles, the speed and air delivery does not change. It simply turns on and turns off once temperature set point has been achieved.
A two stage furnace offers heating in two stages. This operates efficiently as the furnace will not bring on the second stage unless the demand is there. Very similar to a multi speed fan, a two stage furnace has two firing rates; full firing for the coldest exterior temperatures and a lower level for days when less heat is needed. This increases fuel efficiency somewhat and especially improves comfort by maintaining constant temperature.
A variable speed furnace offers the ultimate in comfort as it operates with a fluctuating fan speed according to temperature demand and in the cooling cycle also offers the ability to enhance humidity removal in the conditioned area when paired with a thermostat that also monitors and controls humidity levels. The fan is set to deliver a certain airflow and the circuit board controller calculates the appropriate speed for the fan to attain the proper airflow, based on the static pressure (air resistance) in the rest of the duct system, which can significantly increase fan efficiency at low speeds, when compared with a non variable motor.