For a duct carrying heat from a furnace, what is the minimum clearance to combustibles?

• A. Per manufacturers specs
• B. 12 inches
• C. 6 inches
• D. None required

Answer: (A)

Clearances to combustibles are decided by the equipment’s own design and ratings, not by a universal rule. Ducts carrying heat from a furnace can reach different temperatures depending on the system, the duct material, insulation, and any shielding in place. The manufacturer specifies the exact minimum distance needed to keep nearby combustibles safe from heat and potential ignition. Because those conditions vary from one furnace and duct setup to another, the only reliable minimum is what the manufacturer’s installation instructions require. If you don’t have those specs, consult the manual or the installer and follow local codes.