An apparatus and method for cooking, heating, or drying in which, as items or materials are conveyed through a heating chamber by a linear, spiral, or other type of conveyor, hot combustion product gases are delivered upwardly through an upwardly extending first annulus or other flow gap which is closest to the conveyor. The hot combustion product gases are then delivered downwardly through the heating chamber into contact with the items or materials on the conveyor. Next the combustion product gases are delivered from the lower portion of the heating chamber into the lower portion of a second annulus or other flow gap or passage. The second flow gap or passage is separated from the heating chamber by the first annulus or other flow gap. Subsequently, the combustion product gases are delivered upwardly through the second flow gap or passage.