Abstract: A heat exchanger is disclosed which is built up of a series of segments, which have each a fore-run and a return collector, which are interconnected by tubular or plate-shaped, substantially parallel, heat exchanging elements, in contact with cistern water through which flows the cooling medium of the heat pump. Adjacent heat exchanging segments have flow through in a reversed direction, and the fore-run and return collectors of adjacent segments are directly side by side. An electronic control unit which controls the pump and, optional valves, as a function of signals of temperature sensors on the roof surface, is included in the cistern and/or in conduits, so that during normal operation water is fed from the cistern onto the roof, is heated, and may be returned to the cistern.