They don't rely on any special geological phenomena. They work like a regular air conditioner or heat pump (compressor, refrigerant, condenser, evaporator) but

They don't rely on any special geological phenomena. They work like a regular air conditioner or heat pump (compressor, refrigerant, condenser, evaporator) but instead of pumping heat to/from the outside air, they pump it to/from the ground. The advantage is that, while the outside air is almost always the wrong temperature (hot in the summer when your pump needs it to absorb heat, cold in the winter when you need to pull heat out of it), the ground is basically always the right temperature. The disadvantage is install costs because you have to put piping underground by either trenching or drilling.