(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).