The installation of duct furnaces shall comply with the following clearance requirements:
1. Listed duct furnaces shall be installed with clearances of at least 6 inches (150 mm) between adjacent walls, ceilings, and floors of combustible material and the furnace draft hood. Furnaces listed for installation at lesser clearances shall be installed in accordance with their listings. In no case shall the clearance be such as to interfere with combustion air and accessibility.