[matthiasl]

One way to classify heat pumps is by the source and destination of the heat.

  - air/air
  - air/water   (i.e. an "air pump that heats a liquid in pipes")
  - ground/water

are all commercially available, at widely varying prices:

https://www.nibe.eu/en-eu/products/heat-pumps

The efficiency depends on the pump, the house and the weather. At +10 (Celsius) ambient temperature, the air source pumps will generally come out ahead, at -20 the ground-source pump is going to win.