It even works when the outside temperature is below freezing!

A heat pump converts large amounts of so-called "low grade energy" in the environment (even very low temperatures) to a smaller amount of high grade energy at a more usable temperature            (e.g. 35°C). Heat pumps can keep working effectively down to outside temperatures of -20°C. The environment benefits as well, since no emissions are created locally unlike a wood burning stove or conventional boiler for example.