9.33.6.8.Clearances of Ducts and Plenums

(1) Reserved. (2) Where the plenum clearance is 75 mm or less, the clearance between a supply duct and combustible material shall (a) be equal to the required plenum clearance within 450 mm of the plenum, and (b) be not less than 12 mm at a distance of 450 mm or more from the plenum, except that this clearance may 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) Where the plenum clearance is more than 75 mm but not more than 150 mm, the clearance between a supply duct and combustible material shall be (a) equal to the required plenum clearance within a horizontal distance of 1.8 m of the plenum, and (b) not less than 12 mm at a horizontal distance of 1.8 m or more from the plenum, except that this distance may be reduced to zero beyond a bend or offset in the duct sufficiently large to shield the remainder of the duct from direct radiation from the furnace heat exchanger. (See Note A-3.6.5.6.(3)) (4) Where the plenum clearance is more than 150 mm, the clearance between a supply duct and combustible material shall be (a) equal to the required plenum clearance within a horizontal distance of 1 m of the plenum, (b) not less than 150 mm within a horizontal distance between 1 m and 1.8 m from the plenum, and (c) not less than 25 mm at a horizontal distance of 1.8 m or more from the plenum, except that this distance may 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) Where 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 in lieu of the clearances listed in Sentences (2) to (4).