A supplemental heat-providing system is provided for heating a refrigerant. The system includes an outdoor unit that can act as a heat pump located outside a building and configured to use the refrigerant to condition an indoor space inside the building; a hot water heater located in the building and connected to one or more points of use in the building, the hot water heater being configured to heat water and provide the water to the one or more points of use; a water-refrigerant heat exchanger containing a refrigerant pathway and a water pathway, configured to pass the refrigerant from the outdoor unit through the refrigerant pathway, to pass the water from the hot water heater through the water pathway, and to exchange heat from the water to the refrigerant; and a water pump configured to selectively pump the water from the hot water heater through water pathway.