A heat exchanger apparatus containing a heat exchanger and a thermally actuated bypass valve is described. The heat exchanger has a plurality of plates defining a first, a second and a bypass channels. A first fluid inlet manifold is in fluid communication with the first and the bypass channels. The bypass valve is positioned in the first fluid inlet manifold, and contains a sleeve having a first slot and a second slot, that permit fluid flow from a first fluid inlet to the bypass channel and to the first fluid inlet manifold, respectively. A drum is positioned within the sleeve and is movable from a first position to a second position. The drum has an aperture permitting first fluid flow to the first slot in the first position and to the second slot in the second position. An actuator engages the drum and actuates it to move from the first position to the second position.