iO HVAC Controls HD-1806-BM The HD Series dampers are extruded aluminum parallel blade dampers complete with a 24 volt spring return actuator. The HD Series dampers can be installed in any position and are compatible with most 24V zoning and ventilation controls. They are rated for duct systems less than 1.0O WC. These dampers have a simple adjustment for limited position. They are a highly universal option for both zoning an ventilation applications.