A heat exchanger including upper and lower header tanks, a vertical baffle in the lower header tank defining first and second longitudinal chambers, a check valve, two tube rows, and a coolant inlet to the first chamber and outlet from the second chamber. The baffle includes a by-pass hole therethrough. One tube row includes a plurality of vertical tubes extending between the upper header tank and the first chamber of the lower header tank, and the other tube row includes a plurality of vertical tubes extending between the upper header tank and the second chamber of the lower header tank. The check valve creates a leak-proof seal during standard operating mode to prevent pressure release from the header tank and creates an air path between the header tank and the environment during draining mode to allow vacuum relief from the header tank.