A single-stage furnace and a multi-stage, variable-speed furnace may look identical, but there is a very important difference. A single- stage furnace is sized to heat your home on the coldest days of the year. That means it delivers its full heating capacity, even on mild days. As s result, it produces uncomfortable temperature swings (see light green line).
A multi-stage, variable-speed furnace reduces temperature swings because it is two furnaces in one: low fire for mild days and high fire for only the coldest days. A two-stage variable-speed furnace typically operates in low fire 80% of the time which produces much more even heating (see dark green line).
In addition, because the system runs for longer cycles, the air is filtered more often. Temperature stratification (the effect of warm air rising and cool air sinking) is also lessened because the air is circulated more often.
Temperature stratification can also be improved through constant fan. With a Genteq based system or on Evergreen replacement motor this can be done electrically efficient.