Single-stage and two-stage furnaces have operational differences.
Single-stage furnaces have only one setting, they either operate at 100% or are off. This can lead to uneven heating and fluctuating bills because the temperature fluctuates more than with two-stage furnaces.
Two-stage furnaces can run at both low and high speed. 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.