Abstract: There is disclosed an improved flue box assembly which consists of a housing formed by a continuous sidewall, a top wall and a bottom wall, each of the top wall and bottom wall having an aperture disposed therein, the interior diametric sizing of the housing being substantially greater than both the top wall and bottom wall apertures, and the housing including a damper plate pivotally mounted interioraly thereof and spaced between the top and bottom wall respectively, the damper plate further including a flue opening having a diametric sizing substantially smaller than the top and bottom wall apertures, and the damper plate being constructed and accommodated to be movable between an open position and a closed position, such that when the damper plate is in the closed position, the flue opening is interposed between the top wall aperture and the bottom wall aperture and insures a maximum burning of the fuel in the corresponding furnace, while alternatively, under back pressures or an explosion condition, the d