Abstract: A thermally powered variable air volume diffuser is provided, comprising a diffuser housing and a thermally powered actuator assembly mounted therein which varies the size of a supply air discharge opening. The actuator assembly comprises four thermal actuators and means operatively connecting the thermal actuators to a flow control disc defining the discharge opening. The volume of air supplied to a room is varied in response to changes in air supply temperature and room temperature. In the heating cycle, the first thermal actuator lowers the flow control disc, causing increased supply air output. Heated air from the air supply duct causes the second thermal actuator to engage the third thermal actuator. As room temperature rises, the third thermal actuator pushes the second thermal actuator, raising the flow control disc and reducing supply air output.