An improved heat exchanger, and flow control valve assembly associated with a heat exchanger, of the type wherein the temperature responsive flow control valve is integrated with the heat exchanger the valve assembly is well suited for mounting at the discharge port of the heat exchanger, without relying on rigid interaction with the inside wall of the discharge header or the tube ends in the discharge header. In this manner, the same basic valve assembly can be utilized in a variety of heat exchanger headers, merely by adapting the connection between the end of the valve assembly closest to the discharge port, and the housing or related structure of the heat exchanger at the discharge port. To the extent a relatively standard size valve assembly is to be used with heat exchangers having discharge conduits of different cross sectional area or shape, a baffle member can be placed around the valve assembly before insertion into the header.