A low cost hybrid horizontal drainpipe heat exchanger using a mixture of plastic and copper to reduce cost, the heat exchanger including a conduit which has upper and lower tube segments sealed together along respective longitudinal edges so as to create a tube or a pipe wherein the lower segment is thermally conductive and the upper segment is thermally insulative, the arrangement being such that any drainwater flowing through the drainpipe will flow interiorly on the lower segment for heat exchange purposes.