Abstract: An improved heat pump water heater wherein the condenser assembly of the heat pump is inserted into the water tank through an existing opening in the top of the tank, the assembly comprising a tube-in-a-tube construction with an elongated cylindrical outer body heat exchanger having a closed bottom with the superheated refrigerant that exits the compressor of the heat pump entering the top of the outer body. As the refrigerant condenses along the interior surface of the outer body, the heat from the refrigerant is transferred to the water through the outer body. The refrigerant then enters the bottom of an inner body coaxially disposed within the outer body and exits the top of the inner body into the refrigerant conduit leading into the expansion device of the heat pump. The outer body, in a second embodiment of the invention, acts not only as a heat exchanger but also as the sacrificial anode in the water tank by being constructed of a metal which is more likely to corrode than the metal of the tank.