# Siemens STA21 Thermal Actuator Siemens STA21 is a thermal electric actuator for heating zone valves, working with floor heating manifolds and radiator valves, AC230V power supply, 104N output thrust, 2.5mm stroke. It adopts wax thermal expansion driving principle, silent operation and maintenance-free structure, equipped with visible valve position indicator. Widely used for HVAC heating system renovation, maintenance and spare replacement. **