Abstract: A power driven valve actuator includes a thermostatic device which is movably supported in a housing to accommodate overtravel of a thrust member against the stem of the valve resulting from excess expansion of a thermostatic element enclosed in the thermostatic device and heated by a heating device. Movement of the thermostatic device trips a dust-tight switch to interrupt power to the heating device, and a spring engaging the thermostatic device returns the thermostatic device to a starting position, so that whenever heat is called for by the thermostat, the thermostatic device cycles in a reciprocating motion within the housing after the valve has been fully opened. The heating device comprises an electric resistance heating element placed in contact with a thermal enclosure of the thermostatic element and covered by an insulating material.