Two-Stage Variable-Speed Furnace When a two-stage variable-speed furnace operates it can operate in either a high stage or a low stage mode. Ninety percent of the time, a two-stage variable-speed furnace will operate in the low stage mode. Operating in the low stage mode has many benefits that include a more constant temperature, increased efficiency and lower noise. The sound clip below demonstrates the typical sound made by a two-stage variable-speed furnace when it is in operation. Single-Stage Variable-Speed Furnace A Single-stage furnace has only one operational speed. Regardless of how fast the furnace needs to run it can only operate at a single speed. This has several drawbacks. Efficiency, temperature regulation and noise all suffer from this approach. The sound clip below demonstrates the typical sound made by a one stage furnace when it is in operation.