the front and rear walls 30 32 under certain circumstances the at least one ofa port 200 on each sidewall 35 may be positioned at approximately the same elevation as the elevation of the top row of burners 16 as schematically and alternately shown in fig6 by 200 a or even at a lower elevation approximately corresponding to a center c of the burner zone i e at the elevation of the middle row of burners 14 in a three level burner arrangement as schematically and alternately shown in fig6 by 200 b fig7 displays an alternate configuration of the ofa ports for use with a single wall fired furnace in which burners 12 14 16 are provided only on the front wall 30 of the furnace enclosure 10 in this type of furnace the flame paths are initially affected primarily by the presence of the rear wall 32 the flame paths 13 15 17 of the bottom second and third level burners 12 14 16 respectively are indicated by the lines as shown ofa ports 200 202 204 and 206 are provided through enclosure sidewalls 35 to inject ofa ofa ports 200 202 and 204 are arranged to inject over fire air at the flame path of burners 12 14 16 respectively ofa port 206 provides additional air nearest to the front wall 30 to ensure complete combustion of the fuel the particular number of.