Abstract: A fan assembly for a ventilator system. The assembly comprises a fan housing a inlet end and an outlet end. One or more blower wheels are secured for rotation in the fan housing to direct air from the inlet end to the outlet end. Each of the blower wheels has a plurality of inclined peripheral fan blades. Means is provided to support and impart a rotational drive to the blower wheels. An adjustable cylindrical damper is movably supported in closely spaced relationship to the periphery of the blower wheels to vary the quantity of fluid directed to the outlet end of the fan housing.