A first conduit having a cylindrical upper entrance region with the axis of the cylindrical region oriented vertically, which accepts incoming downwardly flowing hot drain water, a central region of said first conduit below the said entrance region having a conical shape of increasing diameter, reaching a maximum diameter 2-7 times larger than the upper region, the shape then transitioning to a decreasing diameter area of a conical shape, the shape transitioning to a cylindrical lower region with a diameter similar to the upper region diameter; a second conduit for the flow of the shower cold water supply, with a diameter 10-40 times smaller than the maximum diameter of the drain water conduit, and a length 10-40 times longer than the vertical length of the drain water conduit, the second conduit tightly wrapped around the first conduit and in close thermal contact with the first conduit.