Zone Damper (ZD), 18-inch X 18-inch parallel blade damper, is a power-close and spring-open damper with 24-volt motor used to control circulating air in HVAC systems and used when a normally-open damper is required. ZD is typically used with TrueZONE and other electronic zoning systems. Mechanical visual position-indicator for damper status. Solid construction using extruded aluminum frame and blades. Parallel blade design for low leakage. Easy, two-wire installation. Reliable, strong, 24-volt spring-return motor. Fail-safe, normally open operation. Ordering instructions: Order ZD (dimension 1) x (dimension 2). (Motor is always mounted on dimension 2 side.) For example, ZD10x8 is 10-inch x 8-inch damper with motor on 8-inch side; ZD8x10 is 8-inch x 10-inch damper with motor on 10-inch side. Damper can be installed in any orientation (mounting side can be either on bottom or side of duct).