A heat exchanger includes: a bypass valve; a stack of standard tubes; and, in the stack, a pair of stacked-together adapter tubes. Each standard tube end has spaced-apart walls including openings. Each adapter tube end has: a wall including an opening; and a passage. At one end of the pair, the passages communicate with one another and the openings communicate with the openings in adjacent standard tubes; at the other end, the openings communicate with the openings in adjacent standard tubes. The valve includes a plug with opposed plug walls, one plug wall having one of an inlet and outlet, the plug being disposed with its walls between and at said other end of the pair and with the inlet and outlet communicating with the passages. An actuator is adjacent to the plug and has a reciprocating plunger for selectively blocking at least said one of the inlet and outlet.