Let’s start by understanding how an air conditioner works:

A heat pump is an air conditioner that is reversible. When you want heat inside the house, you take the air conditioner’s hot coils and put them inside rather than outside.

Most air conditioners and heat pumps use the outside air to blow over the coils. In the summer that means you are using 90 or 100 degree F air to cool the outside coils. In the winter you are using 20 or 30 degree F air to heat the outside coils.

With a geothermal heat pump, you are using water that has been running through the ground to heat or cool the outsde coil. Since the ground maintains a constant temperature of about 55 degrees (in the U.S.), it makes the heat pump far more efficient in both summer and winter, and especially in the summer.

See also: How Refrigerators Work