Abstract: The furnace includes an upstanding hopper-like member having an increasing cross-sectional area from a fuel inlet at its upper end to a hollow box-like grate releasably and removably mounted within its lower end portion at an elevation slightly below the upper extremity of a gas passageway opening laterally from such member and communicating with a manifold chamber extending horizontally therefrom. The grate has elongate slot-like openings extending horizontally of and vertically through its medial portion, and preferably includes cylindrical solid rods extending longitudinally of the upper portions of such openings. An electrically-powered blower is releasably connected to the grate, for movement therewith, and during operation conducts air to the interior of the hollow sections of the grate. The latter are provided with apertures which discharge jets of air vertically upwardly and downwardly, and also angularly upwardly and downwardly, from the grate.