The heat pump unit is equipped with a special heat absorbing medium - refrigerant, which is lower than minus 20 ℃ in the state of liquefaction and has a temperature difference with the outside temperature. Therefore, the refrigerant can absorb the external heat energy, evaporate and vaporize inside the evaporator, raise the temperature of the refrigerant through the operation of the compressor in the heat pump unit, and then convert the refrigerant from the vaporized state to the liquefied state through the condenser. During the conversion, a large amount of heat is released, which is delivered to the reserve water in the water tank, so that the water temperature can rise to reach the purpose of hot water production. This is the uniqueness of this product and the reason for the market potential.