Abstract: An actuator mechanism for automatically changing and adjusting the orientation of air discharged from the register of an air distribution system solely under the variation of the supply air temperature. The actuator mechanism includes a ram exposed to the supply air and including a cylinder filled with a mixture of alkane waxes and an operating rod. The solid to liquid phase transition of the waxes with temperature increase will push the rod outwardly while a return spring will return the rod as the temperature decreases. The operating rod is connected to the register slats through a transverse arm, a common link and spring clips releasably fitted to the slats.