DuraStack® ProModels DIS and DAS. Modular Double-Wall Positive Pressure Chimney for Boiler Breeching, Stack and Engine Exhaust, and Grease Duct. DuraStack®A modular double-wall positive pressure chimney system. Factory built for boiler breeching, engine exhaust, and chimney stack. Grease duct (DIS only). Click to download the DuraVent Commercial Line Card