What combination of material will reduce the clearance of an unlisted residential fired furnace from 18" to 6"?

What combination of material will reduce the clearance of an unlisted residential fired furnace from 18" to 6"?

• A. Ceramic tile spaced 2" off combustible assembly
• B. Glass fiber insulation 1" off
• C. Galvanized sheet metal spaced 1" off combustible assembly
• D. Wood paneling 0" off

Answer: (C)

Understanding how to reduce clearance to combustibles: you can safely bring a furnace closer only if you install a listed heat shield that is noncombustible and keeps a specified air gap between the shield and the combustible surface. A galvanized sheet metal shield placed with a 1-inch air gap from the wall acts as that barrier. The metal doesn’t burn, and the air gap reduces heat transfer by both conduction and radiant heat, so the hot surface behind the shield is kept from heating the nearby combustible material as quickly. When installed per the listing instructions, this setup can allow the clearance to be reduced to 6 inches. Other materials don’t provide the same recognized protection by themselves. Ceramic tile is noncombustible but, unless part of a listed shield assembly, may not qualify for the required clearance reduction. Glass fiber insulation isn’t a shield on its own and can deteriorate or fail to meet listing requirements in this application. Wood paneling is combustible, and placing it close to a furnace would create a fire hazard, so it cannot be used to achieve a reduced clearance.

Understanding how to reduce clearance to combustibles: you can safely bring a furnace closer only if you install a listed heat shield that is noncombustible and keeps a specified air gap between the shield and the combustible surface. A galvanized sheet metal shield placed with a 1-inch air gap from the wall acts as that barrier. The metal doesn’t burn, and the air gap reduces heat transfer by both conduction and radiant heat, so the hot surface behind the shield is kept from heating the nearby combustible material as quickly. When installed per the listing instructions, this setup can allow the clearance to be reduced to 6 inches.

Other materials don’t provide the same recognized protection by themselves. Ceramic tile is noncombustible but, unless part of a listed shield assembly, may not qualify for the required clearance reduction. Glass fiber insulation isn’t a shield on its own and can deteriorate or fail to meet listing requirements in this application. Wood paneling is combustible, and placing it close to a furnace would create a fire hazard, so it cannot be used to achieve a reduced clearance.