Geothermal heat pumps are much more efficient than air source heat pumps because earth temperatures are much more uniform through the year than air temperatures. Not only are earth temperatures more constant, but also the range of temperatures in ground water is rather small in the United States, varying from upper 40s to upper 70s nationwide.
To further improve efficiency, many manufacturers use variable-speed, electronically-controlled motors on the duct system fans. Depending on unit size, manufacturers may opt for reciprocating, inertia, rotary, or scroll compressors -- all of which are hermetically sealed and mounted in the indoor cabinet. Some advanced heat pumps feature two-speed or variable speed operation while others feature dual compressors to vary output capacity and match loads.