2-stage cooling is a method of cooling that can better manage the cooling of your home while outdoor temperatures are changing. It can also better maximize indoor comfort and energy efficiency. 2-stage cooling systems are typically the most energy efficient systems out there.
2-stage cooling can be done by having one unit with two compressors, one small and one large. The small one is typically capable of putting out about 50% the capacity of the larger one. The second way to have 2-stage cooling is to have a single scroll unloading compressor. This compressor can unload its capacity down to about 66% of maximum capacity. Both these methods of getting 2-stage cooling allow the unit to run at a lower stage when the high stage is not needed. By doing this, energy is saved.
2-stage heating has the same principle as 2-stage cooling. It allows the unit to adjust itself based on the amount of heating that is necessary. 2-stage heating is a good way to save on heating costs.
A heat pump system would have two different sized compressors, a small and a large one. The compressor that is used at a given time would depend upon the need. If only a small amount of heating is necessary, the smaller compressor would be used to save energy. The large compressor would only be used when a large amount of heating was needed.
A gas furnace that is 2-stage has a modulating gas valve that regulates gas flow depending upon the need.