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") -

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 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.