Abstract: A series of primary-air orifices (4) extend into the combustion chamber surrounded by a housing (1). A sloping bottom (6) and a grate (12), likewise inclined at an angle (.alpha.), are located underneath. The grate consists of a row of parallel pipes (15a to 15c) which are arranged at a distance from one another and which are provided over their entire length with air outflow orifices (16a, 16b). A covering slide (13) sliding in grooves (17) on the grate makes it possible to cover to a greater or lesser extent both the gaps between the grate pipes (15a to 15c) and, if appropriate, some of the air outflow orifices provided in the grate pipes. The covering slide (13) can be actuated by hand or by means of a motor via a rack (20) connected rigidly to it and via a pinion (19).