Abstract: A thermally powered change over diffuser for controlling the flow orientation of air discharged from a ceiling mounted duct outlet includes a pair of vanes pivotably mounted on a platform for scissor-wise movement between an open and a closed position, and a thermally responsive actuator. Discharge of cooled air from the duct causes the actuator to move the vanes together into the closed position, blocking the center of the flow path, and forcing the discharge air to exit in a substantially horizontal orientation. Discharge of heated air causes the actuator to move the vanes apart, into the open position, allowing warm air to flow vertically downwardly into a building space.