A casing (1) is dedicated to housing a heat exchanger (3) in a heating, ventilation and/or air conditioning installation, for example of a motor vehicle. The heat exchanger (3) comprises an air inlet face (12), an air outlet face (13), a rear end face (15), a front end face (14), and at least one side face (16, 17) capped with a radiator tank (4, 5). The casing (1) comprises side walls (18, 19) defining at least one housing to receive at least one part of the radiator tank (4, 5). Furthermore, each side wall (18, 19), defining a housing, comprises first (21, 22) and/or second (23, 24) primary parts intended to be respectively placed opposite radiator tank side faces, substantially parallel to the inlet face (12) and/or outlet face (13), and each radiator tank (4, 5) comprises on its side faces first (25, 26) and second (27, 28) secondary protruding parts, the first and/or second primary parts and the corresponding first and/or second secondary parts being respectively shaped so as to define together a duct (29, 30; 31, 32) constraining an air flow to change direction at least three times, so that the circulation of the air flow is limited in the whole of the duct (29, 30; 31, 32).