BAS-CVS Ceiling Dampers are either tested to UL555C (for non-wood truss/joist construction) or to UL 263 (for wood truss/joist construction) and intended to function as a heat barrier in air-handling openings penetrating the ceiling membrane of fire-resistive floor-ceiling designs or roof-ceiling designs for which they have been investigated. Some ceiling dampers are intended for use in lieu of hinged-door-type dampers in floor-ceiling and/or roof-ceiling designs that contain air ducts and specify the use of a hinged-door-type damper over each duct outlet. Some ceiling dampers are intended for use in specific floor-ceiling and/or roof-ceiling designs as specified in the designs and marked on the damper. Ceiling dampers are intended for installation in a duct outlet in lieu of hinged-door-type dampers, the location of the ceiling damper in the duct outlet relative to the ceiling level is specified in the installation instructions. This location must be followed during installation in order to obtain the hourly fire-rated performance of the design.