by the heat energy extracted from the waste air from the trough mangle 10 the heat exchanger 12 has a multiplicity of preferably identical heat exchanger plates 13 which are arranged in a common elongate cuboidal housing 14 the heat exchanger 12 through which both the humid waste air from the trough mangle 10 and the water which is to be heated flow with co current or countercurrent action in the longitudinal direction 15 runs in an inclined state in the longitudinal direction 15 to be precise is directed downwards from the inlet end 11 to the outlet end 16 the outlet end 16 of the heat exchanger 12 is thus located on a lower level than the inlet end 11 of the heat exchanger 12 the housing 14 of the heat exchanger 12 is enclosed by a three dimensional frame 17 in the exemplary embodiment shown the heat exchanger 12 is suspended by the frame 17 in an obliquely downwardly directed manner in the longitudinal direction 15 as seen in the flow direction of the humid air on preferably horizontal carriers 18 the carriers 18 are located beneath a building roof or form a constituent part of the building roof if appropriate the carriers 18 like the heat exchanger 12 may also be directed obliquely in the exemplary embodiment shown the frame 17 also bears connections fittings and parts of conduits in order for the water which.