A heat exchanger including a stack of flat tubes for gas and a housing for the stack of flat tubes, with the housing enclosing only part of the periphery of the flat tube stack. The tubes have wide and narrow sides and are spaced to form channels therebetween for flow of a coolant. Cross-sectional widenings along the length of the tubes assist in maintaining the flat tubes in a spaced condition along the periphery not enclosed by the housing, and close the channels along the tube stack periphery not enclosed by the housing.