Single-stage and two-stage furnaces differ in their heating output capabilities:
Single-stage furnaces operate at only one output level – full capacity. This may cause uneven heating and fluctuating bills because the temperature fluctuates more than with two-stage furnaces.
Two-stage furnaces offer two levels of heat output: a lower setting for mild weather and a higher setting for colder temperatures. They work by running at a lower speed most of the time and switching to high speed when necessary. A two-stage furnace may be a more expensive initial investment, but it can provide greater comfort and savings over time.