A fuel-fired air heating furnace is provided with NOx reduction apparatus associated with a plurality of combustor tubes forming a portion of the furnace heat exchanger section. Inshot-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 NOx reduction members formed from a metal rod material, and a mounting plate having a row of spaced apart pairs of mounting holes therein. Each NOx reduction member has an elongated, generally U-shaped configuration, with a closed inner end, an open outer end, and a spaced pair of corrugated leg portions extending between such inner and outer ends. The NOx reduction members are installed in a parallel relationship on the mounting plate, with their closed ends facing in the same direction, by resiliently deflecting the leg portions of each member toward each other, inserting the outer leg ends in one of the mounting plate hole pairs, and then releasing the legs. With the NOx reduction members secured to the mounting plate in this manner, they are simultaneously inserted closed ends first into their associated combustor tubes, and the mounting plate suitably supported adjacent the combustor tube inlets. The burners are then positioned to inject flames into the combustor tubes with the NOx reduction members positioned therein to operatively intercept the incoming burner flames.