Abstract: A shower or car-wash is provided with a heat-exchanger element of thin sheet metal mounted under or in the floor to pre-heat incoming cold water using heat recovered from the spent sprayed water. The heat exchanger element has a rectangle wave shaped cross section thereby providing greater heat transfer area and being able to withstand heavy vertical loads. The vertical portions of the rectangle wave may advantageously be provided with vertical reinforcing ribs in the form of depressions stamped in the metal.