Abstract: Presented is a method and apparatus for incorporating an incandescent filter for products of combustion in the combustion chamber of a furnace including a grate and an exit slot for products of combustion located above the level of the grate. Incandescent coals from the fuel being burned obstruct the exit slot, and products of combustion must pass through the incandescent coals. In one aspect, water tubes line the walls of the combustion chamber, primary combustion air is admitted between the tubes, and the admission of secondary air is controlled to maintain an incandescent bed of coals through which products of combustion must be drawn to exit the furnace. In another aspect, a reciprocating grate is provided, and primary combustion air penetrates the bed of fuel from below. Products of combustion pass back through a portion of the burning fuel maintained at incandescence by the admission of secondary air upwardly through the reciprocating grate.

Abstract: A furnace has a solid fuel hopper above a combustion chamber, an ash outlet below said combustion chamber, a combustion gas outlet at or below the level of the combustion chamber, and an auger in an ash chamber below the ash opening to regulate the size of an ash pile below the combustion chamber and hence control the escape of ash or fuel or non-combustibles through the ash opening. This enables the combustion gas outlet to be covered by hot coals within the combustion chamber so that any combustion gas from the combustion chamber or any pre-combustion zone must pass through a filter of hot coals before reaching the combustion gas outlet.

Abstract: An improved downdraft furnace wherein two combustion grates are used, with the lower one being maintained completely covered with glowing coals, which totally oxidize combustion gases and particles passing through, with the result that the exhaust is free of smoke.