Abstract: A furnace consists of at least one wall-mounted burner tilted downward for feeding air-fuel mixture and of a prism-shaped ash hopper. Slopes of the walls of lower combustion chamber region define the slotted mouth of ash hopper. The bottom airflow device is located below the ash hopper mouth the wall opposite to the airflow nozzle of the bottom airflow device has a concave form. In accordance with said invention the bottom airflow device spans along the entire width of the ash hopper mouth. The concave wall in its upper region meets the ash hopper mouth and an imaginary plane being a continuation of said concave wall crosses the opposite slopes of the wall of ash hopper in its middle region. The axis of bottom airflow nozzle passes along the lower region of said concave wall and is directed to form a 0-to-45 degree angle between said axis and lower region of said concave wall. The concave wall may be tilted relative to horizon at the angle of 20-45 degrees.