Abstract: A housing is provided defining a closed lower combustion chamber and a closed upper afterburn chamber disposed immediately above the combustion chamber with one marginal portion of the combustion chamber projecting horizontally outwardly beyond the corresponding marginal portion of the afterburn chamber. A combined flue gas and bypass chamber is disposed above the one marginal portion of the combustion chamber and is horizontally registered with the afterburn chamber. Partition structure divides the combined flue gas and bypass chamber into central and remote end portions spaced along the aforementioned one marginal portion and a pair of first ports communicate the remote portions with adjacent underlying upper portions of the combustion chamber, a pair of second ports communicate the remote portions with adjacent portions of the afterburn chamber and third port structure communicates the central portion of the afterburn chamber with the central portion of the combined chamber.