Abstract: An incoming air duct (9) and an exhaust air duct (12) which connect an incoming air inlet (14) and an exhaust air outlet (10), respectively, in an outside side-wall (1b) to an incoming air outlet (13) and an exhaust air inlet (11) in a room-facing side wall (1a) are arranged side by side, separated only by partitions (8) of aluminium sheet, to permit heat exchange between the exhaust air flow and the opposite incoming air flow over a plurality of straight sections (6a, 6b, 6c, 6d, 6e) of a double duct which are connected by deflections (7a, 7b, 7c, 7d) of in each case 180°.