Abstract: A tunnel furnace composed of individual sections for the heat treatment of articles which are transported through said tunnel on an air permeable belt whereby each section is separated by shields into a main heating space and a pair of flanking outer spaces in communication with the heating space at the top thereof and below said belt, and circulation fans are disposed within the furnace for circulating air in a continuous path upwards through the conveyor belt and then downwards through the outer spaces. A heater is disposed in each outer space comprising a burner and a pipe elbow section for conducting the flame and the combustion gases, the elbow section having an S-shaped configuration so that the combustion gases flow upward during a portion of their travel within the elbow section terminating in a mouth exposed to the circulating air flow.