Abstract: A pool heater to be installed on a commercially available residential heat pump in order to add swimming pool heating to the basic functions of the heat pump, i.e. heating and cooling of a dwelling. This "add-on a heat pump" pool heater comprises a refrigerant-to-water heat exchanger with a water flow control and associated piping, a water temperature control, a flow detecting device and a specialized control logic which is integrated to the heat pump control. The heat exchanger is connected on the refrigerant side to the outlet of the compressor of the heat pump circuit and to the inlet of the reversing valve. On the water side, the heat exchange is connected on a diversion loop of the pool water filtration system to allow water to be heated by the refrigerant exiting the compressor. The integrated control conserves all the basic modes of the heat pump, i.e.