The ground temperature a few meters below the surface is fairly constant throughout the year and is near the average value of the air temperature. In areas in which the air temperature drops very low in the winter, the exterior unit of a heat pump designed for heating is sometimes buried underground in order to use the earth as a thermal reservoir. Why is it worthwhile to bury the heat exchanger, even if the underground unit costs more to purchase and install than one above ground?

Since the Earth contains a constant temperature range heat exchangers can be built underground without other added expenses yes they are expensive to put underground but the pay off is greater long term wise since the heat rises it can be said you could put a heatpump in a small area and it can over time increase the relative heat in that area of effect.

hope this helps look into, heat displacement thru underground heating to get a clearer picture

answer:

constructing a building's framework requires a solid foundation

explanation: