Abstract: The invention concerns a furnace for biological fuels in which the combustion is effected downwards in order to allow firing down to very low output effect. In accordance with the invention the furnace has a fuel storage vessel (1) which also serves as a gas collection vessel and which during operation is pressurized to a slight atmospheric overpressure. The bottom part of the storage vessel is formed with sloping walls (4) the lower edges of which form a burner opening and above which opening is provided a draft-air supply means which extends closely alongside the sloping bottom along the portions thereof positioned closest to the burner opening.