Model TF Therma-Fuser™ diffusers are 24in / 600mm square ceiling VAV diffusers with built-in temperature controls for VAV cooling and VAV heating. Four blade dampers move up to close and down to open, metering air flow (warm or cool) into the room in response to room temperature. The dampers are mechanically positioned by a thermostat / actuator which is both a room thermostat and damper motor. Room air is positively induced over the thermostat / actuator for an accurate measurement of room temperature in both cooling and heating.
The thermostat / actuator is not electric nor pneumatic. It is a large brass cylinder containing a petroleum-based wax. The wax melts and expands when heated, pushing the piston outward. A spring pulls the piston inward when the wax cools.