* The heat pump can use the decalescence medium (secondary refrigerant) inside the equipment to collect heat from the air or natural environment, and compress it by the compressor, and then, make the secondary refrigerant to letout heat to heat the cold water through heat exchanger. It expulses cold air simultaneity. The prepared hot water can send to the user to heating or bath through the water cycle system.

* The ratio of the output energy and input energy of Kelvin heat pump, the ratio of energy efficiency (COP), is 2 to 6 with an average above 3.

* While the ratio of energy efficiency (COP) of normal electric water heating is no more than 0.95, and that of gas or oil-burning water heating is only 0.6 - 0.8.

* Coal burning water heating is even at such a lower level ranging 0.3 - 0.7.