(1) 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 duct from direct radiation from the furnace heat exchanger.
(2) 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,
(a) be equal to the required plenum clearance within a horizontal distance of 1 800 mm of the plenum, and
(b) be not less than 12 mm at a horizontal distance of 1 800 mm 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.
(3) Where the plenum clearance is more than 150 mm, the clearance between a supply duct and combustible material shall,
(a) be equal to the required plenum clearance within a horizontal distance of 1 000 mm of the plenum,
(b) be not less than 150 mm within a horizontal distance between 1 000 mm and 1 800 mm from the plenum, and
(c) be not less than 25 mm at a horizontal distance of 1 800 mm 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.
(4) 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 (1), (2) and (3).