A single-stage heat pump has one compressor that is used to both heat and cool your home. A two-stage heat pump has two compressors – one for heating and one for cooling. The main difference between the two is that a two-stage heat pump can operate at lower speeds most of the time, which results in quieter operation, improved humidity control, and increased efficiency. In the cooling mode, a two-stage heat pump will run at low speed for most of the time, only ramping up to high speed when more cooling is needed. This results in fewer temperature swings and increased comfort. In the heating mode, a two-stage heat pump will also run at low speed for most of the time, only switching to high speed when it is very cold outside and more heat is needed. Two-stage heat pumps cost more than single-stage heat pumps, but they can save you money in the long run due to their increased efficiency.