Abstract: A firebox including front, rear and opposite sides interconnected by means of a bottom side at their lower ends and by a top side at their upper ends is provided. Outer rear and bottom walls extend between the rear marginal portions of the opposite sides rearwardly of the rear side and between the lower marginal portions of the opposite sides and the lower marginal edges of the rear wall and the front side below the bottom, respectively, defining a rear to front extending flow passage between the bottom side and wall and a downwardly extending flow passage between the rear side and rear wall. The interior of the firebox includes hollow upper opposite side front to rear extending side manifolds and an upper front transverse manifold extending between and communicated with the forward ends of the side manifolds.