furnace which has been constructed in accordance with the principles of the invention it has been found through testing that for that particular construction maximum efficiencies were achieved by reducing the secondary air during periods of maximum primary air delivery to one fourth of that available during periods of minimum primary air delivery this was achieved by utilizing a pipe 56 having a three quarter inch 3 4 34 inner diameter and a damper plate 61 having a hole 62 of three eighths of an inch 3 8 34 diameter accordingly the maximum secondary air opening was 0 44 square inches and the minimum secondary air opening was 0 11 square inches the primary air duct 48 and damper vane 49 are constructed so that in the maximum open position there exists a one half inch 1 2 34 vertical opening and in the minimum open position there exists a three sixteenths inch 3 16 34 vertical opening the horizontal length of the primary air intake duct was nine and one half inches 91 2 34 so that the primary air opening at maximum was approximately four and three quarters square inches and at minimum was approximately one and three quarters square inches accordingly the ratio between maximum and minimum primary air under these conditions was substantially eight to three 8 3 the ratio of primary air opening to secondary air opening during high heat demand periods was substantially 43 to 1 while this ratio during low heat demand periods was substantially 4 to 1 this shows a relative proportioning shift of substantially 11 to 1 the improvements effected by the.