Also known as a heat pump. Depending on the temperature lift, these can be very efficient. Low temperature district heating networks are an interesting thing to look into: Low temperature means the heat losses in the network are smaller (heat losses, if I remember correctly, are proportional to the temperature difference) and at the consumer side, a heat pump can lift it back up to the temperature required for a floor (or ceiling) heating. This is _way_ more efficient than an air-to-air heat pump or even an air-to-water heat pump. [Citation Required]