Abstract: Flow of gas through a conduit of a gas-handling system, particularly the exhaust system of a material processing furnace operating at high temperatures, is controlled by directing a control gas (usually air) into the conduit in opposition to the normal flow of gas therethrough at a location such that it can comprehend substantially the entire cross-sectional area of the conduit substantially without diminution of such area or interference with fluid flow at such location when full normal flow is desired. The apparatus includes a duct leading from a pressure source of control gas to nozzle means disposed substantially entirely outside the conduit but with the discharge therefrom directed into the conduit from at least one side thereof and in opposition to normal gas flow through the conduit.