The Siemens SSD81 actuator is part of the SSD Series for Pressure Independent Control (PICV) valves. It operates on 24 Vac ±20% and accepts a floating control signal, delivering a nominal force of 250 N. Run times vary between 75–150 seconds depending on valve stroke (2.5–5 mm). The SSD81 is available in fail‑in‑place and spring‑return fail‑safe variants, ensuring reliable positioning during power loss. Key features include manual override, visual position indication, direct‑coupled installation without tools, and UL/cUL listing for plenum use. Applications include chilled and hot water circuits or water‑glycol solutions up to 50% in closed loop systems. The actuator is maintenance‑free and designed for long‑term stability in HVAC environments.