A manufacturing method and structure of a heat exchanger for a bathing shower involves two plates welded together to form a passage for cold water. Hot water from the shower drips onto the upper plate and transfer heat to cold water flowing through the passage between the plates. The upper plate may be spot welded to the lower plate at bottoms of frustoconical indentations in the lower plate, the plates may sandwich an adiabatic layer, and/or the passage between the plates may be formed by pipes situated between the plates.