A heat exchanger (41) comprising a heat-exchanger housing (VH) that has an upper housing part (42) and a lower housing part (43) wherein the housing completely encloses a heat exchanger pack (VP) having a primary medium inlet (48) and a primary medium outlet (49) and also a secondary medium inlet (46) and a secondary medium outlet (47), wherein the heat exchanger pack (VP) has N number, where preferably N=3, of sections of heat exchange elements (51, 52, 53), wherein each section consists of elongate mutually parallel flat primary medium channels (L) whose ends are each coupled together by means of a transfer channel (54) for connection to adjacent sections or connection to the inlet/outlet (48, 49) for the supply and the exit of primary medium to/from the heat exchanger pack (VP), wherein the upper housing part (42) of the heat-exchanger housing (VH) has over each section of said heat exchange elements (51, 52, 53) a number of waste water channels (50) arranged in sections (S 1 , S 2 , S 3 ) where the first end of each channel (50) start at the secondary medium inlet (46) and run along the heat exchange elements to form said sections (S 1 , S 2 , S 3 ) in a meander form along in the longitudinal direction of the heat-exchanger housing (VH) and that the waste water channels (50) ends at the secondary medium outlet (47).