What is the minimum clearance from the duct to a combustible interior surface?

• A. 18 inches
• B. 12 inches
• C. 24 inches
• D. 6 inches

Answer: (A)

Heat from ductwork can radiate and transfer enough energy to ignite nearby combustible interior surfaces. Keeping a safe buffer between the duct and such surfaces reduces the ignition risk and makes maintenance and inspection easier. The minimum distance established for this clearance is eighteen inches, providing an adequate safety margin under normal conditions. Going closer increases the fire risk, while a larger gap is acceptable but simply exceeds the minimum requirement. In some situations—like exceptionally hot ducts, special insulation, or specific local codes—a greater clearance may be required, so always refer to the applicable code for the exact scenario.