Abstract: A furnace in which burners are incorporated into water-cooled tube panels without the need for heavy steel castings, the furnace including one or more walls formed of upright tubes having fins connecting adjacent tubes, some of said tubes and associated fins in said wall or walls for at least part of their length being outwardly, with respect to the interior of the furnace, out of line with other tubes in said wall to define upright recesses, fuel nozzles and openings for the entry of combustion air being provided through the portions of said fins which define in part the upright recesses, the arrangement of said fuel nozzles and said openings being adapted such that fuel supplied from said fuel nozzles and combustion air supplied from said openings enter the recesses in converging streams.