In a heat exchanger employing double wall tubing comprising concentric inner and outer tubes, the inner and outer tubes are disposed within apertures of spaced tube sheets and connected thereto by roller expansion of the tubes. To maintain venting of the air space between the tubes, a split wall ferrule is located over the inner tube and within the outer tube in the region of the tube sheet to which the outer tube is connected. The ferrule, which has a length sufficient to traverse the thickness of the tube sheet, transmits expansion forces exerted on the interior of the inner tube to the outer tube so as to expand the outer tube to connect it to the tube sheet. The split in the ferrule wall forms a passageway that connects the air space between the tubes to the space between the tube sheets.