Ductwork and system components handling combustible material and operating at less than 140°F (60°C) shall have a clearance of not less than 18 inches (457 mm) from combustible construction or a combustible material. [NFPA 91:4.6.2]
- Where the ductwork system is operating at less than 140°F (60°C) and is equipped with an approved automatic extinguishing system designed for the specific hazard, the clearance shall be permitted to be reduced to 6 inches (152 mm) from combustible materials and 1/2 of an inch (12.7 mm) from combustible construction. [NFPA 91:184.108.40.206]
- Where the combustible material and construction is protected by the use of materials or products listed for protection purposes or in accordance with Table 303.10.1.
Spacers and ties for protection materials shall be of noncombustible material and shall not be installed on the duct side of the protection system. [NFPA 91:220.127.116.11]
With clearance reduction systems using a ventilated airspace, air circulation shall be provided as described in Table 303.10.1. There shall be not less than 1 inch (25.4 mm) between the wall protector and combustible walls and ceilings for clearance, reduction systems using a ventilated space.
Mineral wool batts (blanket or board) shall have a density of not less than 8 pounds per cubic feet (lb/ft3) (128 kg/m3) and have a melting point of not less than 1500°F (816°C). [NFPA 91:18.104.22.168]
Insulation board used as a part of a clearance-reduction system shall have a thermal conductivity of 1 British thermal unit inch per hour square foot degree Fahrenheit [Btu•in/(h•ft2•°F)] [0.1 W/(m•K)] or less. Insulation board shall be formed of noncombustible material. [NFPA 91:22.214.171.124]
There shall be not less than 1 inch (25.4 mm) between the duct and the wall protector. In no case shall the clearance between the duct and the wall surface be reduced below that shown in Table 303.10.1.
Duct systems operating at elevated temperatures exceeding 140°F (60°C) shall have clearances from combustible building construction or combustible materials of not less than 18 inches (457 mm).
Air circulation shall be permitted to be provided by leaving edges of the wall protecting system open with not less than a 1 inch (25.4 mm) air gap. [NFPA 91:126.96.36.199.1]
Where the means for protection is mounted on a single flat wall away from corners, air circulation shall be permitted to be provided by one of the following:
- Leaving the top and bottom edges open to circulation by maintaining the 1 inch (25.4 mm) air gap.
- Leaving the top and both side edges open to circulation by maintaining the 1 inch (25.4 mm) air gap. [NFPA 91:188.8.131.52.2]
Thermal shielding that covers two walls in a corner shall be permitted to be open at the top and bottom edges with not less than 1 inch (25.4 mm) air gap. [NFPA 91:184.108.40.206.3]