Bypass valves and heat exchangers employing same are provided where the bypass valves cause the flow through the heat exchangers to be short-circuited under certain temperature conditions. The heat exchangers are formed of stacked plate pairs or tubes having enlarged communicating distal end portions forming inlet and outlet manifolds. The bypass valves can be plugged in where desired between the enlarged distal end portions to produce bypass flow between the inlet and outlet manifolds. The bypass valves include a housing which can be brazed in place during brazing of the heat exchanger. The housing has inlet and outlet openings in communication with the respective inlet and outlet manifolds for bypass flow therebetween. A removable temperature responsive actuator is located in the housing for blocking and unblocking the bypass flow through the valve.