The vessel has a casing (1) with a water feed and with radial longitudinally extending ribs (2) defining a draught duct (III) with the casing wall. A second draught duct (II) is defined by the rear of a wall of the combustion chamber casing (5) and forms a channel which is part of he combustion chamber shell. The third (III) is defined by a burner side overflow chamber (6) and a fume collector (7) and extends across the combustion chamber.
