The invention relates to a heat exchanger, particularly a radiator for a heating or air conditioning unit in motor vehicles, which cools a coolant. Said heat exchanger (1) is penetrated by air, comprises collector pipes (S1, S2, S2, S4) and several essentially horizontally disposed pipes (5), and is divided into several partial blocks (T1, T2, T3, T4). The surfaces of the inventive partial blocks are selected according to the dimensions of structural space-related zones having different air temperatures inside the assembly space of the heat exchanger, the partial block which is first penetrated by the coolant being arranged within a structural space-related zone having a higher air temperature, preferably within the zone having the highest air temperature.