A single-stage heat pump has a compressor that works at one speed while a two-stage unit works at a high and a low speed. The low-speed provides sufficient heat for the home most of the time but operates at a higher speed when necessary, reducing the need for auxiliary heat.
Both types of heat pumps are energy efficient; however, the two-speed heat pump offers greater efficiency. Initial installation fees for a two-stage heat pump are an average of $250 more than those for the single-speed device as of 2015. However, the payback continues throughout the life of the furnace. When operating at low speed, the two-stage system is 40 percent more efficient than a similarly sized single-phase unit.
The two-stage system offers more even heating than the single-stage system. Single-stage systems are set to heat the air several degrees above the temperature setting and then allow it to cool so it is several degrees below the setting. These variations in temperature make the home less comfortable than does a two-stage system that provides tighter temperature control.
In the cooling mode, the two-stage heat pump also provides better humidity control. Reducing the humidity in a home helps the occupants to feel more comfortable, even if the temperature is set a few degrees higher.