Abstract: An apparatus for supplying heated air to an air stream including a first air flow duct having an inlet and an outlet, a blower for forcing air through the duct to the first duct outlet, and a second air flow duct having an inlet, an outlet, and a burner chamber between the inlet and outlet, with the second duct outlet connected to the first duct downstream of the blower for drawing atmospheric air through the second duct by air flow through the first duct. A gas burner in the burner chamber, an apertured refractory wall positioned across the second duct downstream of the burner, and a refractory liner for the inner surface of the burner chamber between the burner means and the apertured wall.