Abstract: A method of installing a drain water heat recovery exchanger for pre-heating water to be heated using a solar, electric or combustion energy hot water heater or to be sent to the cold-water line of a hot-water-consuming apparatus. It includes providing a drain water heat recovery exchanger including a copper pipe inlet fitting and a copper pipe outlet fitting and a core waste water drain tube, the fittings being fashioned to include barbs for a male plastic pipe connection; inserting the drain water heat recovery exchanger into a drain water pipe, wherein the core waste water drain tube is connected to the drain water pipe; and connecting a first one of the fittings to a cold-water supply and a second one of the fittings to an inlet of a hot water heater, a hot water tank or a cold-water line of a hot-water-consuming apparatus.