A fuel fired, forced air, draft induced heating furnace is provided with NOx reduction apparatus associated with a plurality of combustor tubes forming a portion of its heat exchanger structure. In-shot type fuel burners are spaced apart from and face the open inlet ends of horizontal combustion sections of the combustor tubes. The NOx reduction apparatus includes a plurality of insert members received in inlet end portions of the combustor tubes. Each insert member is formed from a single sheet of metal mesh material and has a hollow, open-ended body portion coaxially received in its associated combustor tube and spaced laterally inwardly from its interior side surface, and a circumferentially spaced plurality of integral, laterally outwardly projecting support rib portions that slidably engage the interior side surface of the combustor tube, thereby automatically maintaining the insert member in a coaxial relationship with the combustor tube regardless of the positional orientation of the tube. To axially retain the insert within the combustor tube, an end portion of one of the centering ribs has a lateral notch that receives a rod anchored to and extending transversely across the inlet end portion of the combustor tube.