Abstract: Back draft is prevented in a conventional furnace or woodstove system in which the combustion gases are generally sufficient to generate the positive draft for combustion but occasionally fails due to pressure differences, combustion conditions or outside temperature conditions. The back draft device comprises a nozzle mounted in the flue which is connected to a duct supplying pressurized air from a fan which is passed over a heater so that heated air is injected into the flue. A flow control device comprises a housing which is divided by a transverse sheet. In a front face on one side of the divider is provided a flap valve controlling air inlet into the duct of the fan. On the other side of the divider is provided a flap valve controlling an air outlet from the outside of the building. The flow control device automatically controls whether the air for the nozzle is drawn from the interior or the exterior of the building depending upon the pressure differences.