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Supply air ducts connecting to unlisted central heating furnaces equipped with temperature limit controls with a maximum setting of 250°F (121°C) shall have a minimum clearance to combustibles of 6 inches (152 mm) for a distance of not less than 6 feet (1829 mm) from the furnace supply plenum. Clearance shall not be required beyond the 6 feet (1829 mm) distance. [NFPA 54:10.3.2.8]

Related Code Sections

905.0 Installation of Specific Appliances, Duct Furnaces
shall the clearance be such as to interfere with combustion air and accessibility. Unlisted duct furnaces shall be installed with clearances ...
904.2.2 Installation of Specific Appliances, Unlisted Units
Unlisted central-heating furnaces and low pressure boilers shall be installed with clearances from combustible material not less than those specified ...
905.5 Installation of Specific Appliances, Circulating Air
Where a duct furnace is installed so that supply ducts carry air circulated by the furnace to areas outside the space containing the furnace ...