3.6.5.6.Clearance of Ducts and Plenums

(1) The clearance of furnace plenums from combustible material shall conform to the requirements of the appropriate standards referenced in Sentence 6.2.1.5.(1). (2) If the plenum clearance required in accordance with Sentence (1) is not more than 75 mm, the clearance between a supply duct and combustible material shall be not less than (a) the required plenum clearance within a horizontal distance of 450 mm from the plenum, and (b) 12 mm at a horizontal distance of 450 mm or more from the plenum, except that this clearance is permitted to be reduced to zero beyond a bend or offset in the duct sufficiently large to shield the remainder of the supply duct from direct radiation from the furnace heat exchanger. (See Note A-3.6.5.6.(2)) (3) If the plenum clearance required in accordance with Sentence (1) is more than 75 mm but not more than 150 mm, the clearance between a supply duct and combustible material shall be not less than (a) the required plenum clearance within a horizontal distance of 1 800 mm from the plenum, and (b) 12 mm at a horizontal distance of 1 800 mm or more from the plenum, except that this distance is permitted to be reduced to zero beyond a bend or offset in the duct sufficiently large to shield the remainder of the supply duct from direct radiation from the furnace heat exchanger. (See Note A-3.6.5.6.(3)) (4) If the plenum clearance required in accordance with Sentence (1) is more than 150 mm, the clearance between a supply duct and combustible material shall be not less than (a) the required plenum clearance within a horizontal distance of 1 000 mm from the plenum, (b) 150 mm within a horizontal distance between 1 000 mm and 1 800 mm from the plenum, and (c) 25 mm at a horizontal distance of 1 800 mm or more from the plenum, except that this distance is permitted to be reduced to 8 mm beyond a bend or offset in the duct sufficiently large to shield the remainder of the supply duct from direct radiation from the furnace heat exchanger. (See Note A-3.6.5.6.(4)) (5) If a register is installed in a floor directly over a pipeless furnace, a double-walled register box with not less than 100 mm between walls, or a register box with the warm-air passage completely surrounded by the cold-air passage, shall be permitted instead of the clearances listed in Sentences (2) to (4).