NFPA 90B-1999 Standard for the Installation of Warm Air Heating and Air-Conditioning Systems

NFPA 90B-1999 Standard for the Installation of Warm Air Heating and Air-Conditioning Systems. 3-1 Clearances to Combustible Material. 3-1.1 General. Where ducts are adjacent to plaster on metal lath or to some other noncombustible finish attached to a combustible material, the clearance shall be measured to the combustible material. Exception No. 1: The clearance shall be measured to the surface of the plaster or other noncombustible finish where a clearance of 2 in. (51 mm) or less is specified above a bonnet or plenum chamber or above supply ducts. This shall not be construed to prohibit the closure of open- ings with noncombustible material where ducts pass through walls and partitions, as provided in 3-1.2. Exception No. 2: Where an appliance, ductwork, or chimney or vent con- nection is listed for different clearances, the listed clearances shall apply. 3-1.2 Clearances from Horizontal Supply Ducts. Minimum clearances from horizontal supply ducts to combustible materials shall be as follows: (a) Within a distance of 3 ft (0.91 m) of the plenum of a system classified under A, C, or G of Table 3-1.3.1, the clear- ance shall be not less than that specified above the bonnet or plenum. (b) Within a distance of 6 ft (1.83 m) of the plenum of a system classified under B or D of Table 3-1.3.1, the clearance shall be not less than 6 in. (152 mm). From ducts of furnaces classified under D, the clearance shall be not less than 1 in. (25.4 mm) beyond 6 ft (1.83 m) from the plenum to a point where there is a change in direction equivalent to 90 degrees or more. (c) From ducts of furnaces classified under item F of Table 3-1.3.1, the clearance shall be not less than 18 in. (457 mm) up to 3 ft (0.91 m) from the bonnet or plenum, not less than 6 in. (152 mm) for 3 ft to 6 ft (0.91 m to 1.83 m), and not less than 1 in. (25.4 mm) beyond 6 ft (1.83 m). (d) Beyond the distances from the plenum or change in direction specified in 3-1.2(a) and (b), no clearance shall be required. (e) Where a horizontal supply duct passes through or pierces a partition or enclosure constructed of combustible material, within the distances or point of change in direction specified in 3-1.2(a), (b), and (c), the clearance shall be not less than that specified...