Abstract: The wasteful discharge of heated or cooled room air in a ventilating system to which a number of fume hoods are connected is reduced by making it possible to turn off unused hoods without unduly unbalancing the system and by making it possible, when access windows are closed, to reduce the volume of air flow to below the required open window level. Unconditioned outside air is automatically fed into the system in an amount compensating for the reduction in air available from the hoods when they are shut off or when flow from them is reduced. Switches are provided on the hood windows to change hood damper openings automatically to increase air flow when access windows are open and reduce it when windows are closed.

Abstract: An enclosed fume hood having an access window with a vertically sliding sash is provided with an additional horizontally movable sash which closes off only a fraction of the open window space and thus reduces the volume of air required to be drawn through the window in order to maintain the requisite air velocity while still permitting complete access to the interior of the hood. Provision is also made to permit the horizontally movable sash to be easily removed, when necessary, to permit insertion into the hood of apparatus wider than the residual open window space.