The ground temperature a few meters below the surface is fairlyconstant throughout the year, and is near the average value of theair temperature. In areas in which the air temperature drops verylow in the winter, the exterior unit of a heat pump designed forheating is sometimes buried underground in order to use the earthas a thermal reservoir. Why is it worthwhile to bury the heatexchanger, even if the underground unit costs more to purchase andinstall than one above ground?
