The Belimo ZONE24NO 24V Zone Actuator Normally Open is a 24-volt zone actuator made primarily for HVAC systems. It works as an automatic damper controller to control the system's airflow. When no power is given, the damper stays open in its "normally open" position, permitting unhindered airflow. The actuator closes the damper by turning a shaft in response to a 24-volt signal. https://www.partshnc.com/24v-zone-act-normally-open-3