16”L X 8” Automatic Rectangular Damper The ZD is a power close and spring open damper. It has a 24-volt motor used to control circulating air in heating, cooling and ventilating systems. It is used when a normally-open damper is required. The ZD is typically used with the TrueZONE systems and other electronic zoning systems. Works good previously was used in gas hot air duct. Used in duct for a hot air furnace.