Simply Put. Two Stage vs. Single Stage vs. Communicating Furnaces.
Single Stage Furnaces
Picture someone on a treadmill. Now picture that person going from a standstill to a full sprint at every work out. This is how a single stage furance operates; there's an "off" speed and then an "on" speed. They only have one stage of heat output - high. Single stage furnaces make sense for temperate climates or for folks on a tight budget.
Two Stage Furnaces
These furnaces operate much more efficiently. The first stage operates the majority of the time in most climates running at about 65%. When the temperature outside becomes extremely cold and the first stage is not sufficient enough, two stage furnaces kick it up for additional heat. Like finishing strong during a workout. They run quieter and don't operate at full-blast constantly.
As the name suggests, the fan motor in these furnaces moves at different speeds to control the amount of heated air dispersed throughout the home as needed. Much like the effort needed for walking on a level surface, then up an incline, then up a hill and back down. Or any variation. So even when the furnace is not "on," the motor in a modulating furnace circulates air throughout the home. The result? More comfort, less energy consumption and lower utility bills every month.