A gas manifold for a convection conveyor oven includes an elongated housing having a wall that at least partially encloses an interior volume; a gas inlet in fluid communication with the interior volume and extending through the wall; a plurality of gas outlets in fluid communication with the interior volume and extending through the wall; a plurality of seat inserts removably coupled with a plurality of gas outlets; a plurality of valve openings formed in the wall and aligned opposite the plurality of gas outlets; and at least one valve removably coupled to the wall and aligned with a first valve opening, wherein the valve is aligned with a first gas outlet and has a first position in which a first seat insert is not in fluid communication with the gas inlet.