6.3 Masonry and Factory-Built Chimneys

6.3 Masonry and Factory-Built Chimneys

Quick Answer: Masonry chimneys must have an approved clay flue tile or listed metal liner, with a cleanout located at least 6 inches below the lowest inlet. Clearances to combustibles are 2 inches for interior and 1 inch for exterior masonry chimneys. Factory-built chimneys (UL 103 Type HT for solid fuel) must be installed per manufacturer listing and require a No. 26 gage steel attic insulation shield extending at least 2 inches above the insulation.

Chimneys are vertical, structural flue pathways designed to vent high-temperature combustion products. They differ from standard vents in their construction, temperature rating, and clearance requirements. Inspectors must understand the code provisions for both masonry and factory-built chimneys.

Masonry Chimneys: Construction, Linings, and Clearances

Masonry chimneys are heavy, permanent structures built of brick, stone, or concrete. Under IRC Section R1003 and Section M1805, they must be constructed with the following safety parameters:

• Flue Liners: A masonry chimney must never be unlined. Unlined chimneys allow heat and corrosive combustion acids to degrade the masonry and mortar joints, creating fire paths to adjacent framing. Approved liners include:
  • Clay Flue Tiles: ASTM C315 clay tiles must be installed with medium-duty refractory mortar, with a minimum wall thickness of 5/8 inch.
  • Listed Metal Liners: Listed to UL 1777, these flexible or rigid metal liners are common when retrofitting older chimneys or sizing them for modern gas appliances.
• Clearance to Combustibles:
  • Interior Chimneys: Masonry chimneys located inside the building thermal envelope must maintain a minimum of 2 inches of airspace clearance to all combustible materials (framing, subfloors, drywall).
  • Exterior Chimneys: Masonry chimneys located on the exterior of the building must maintain a minimum of 1 inch of airspace clearance to combustibles.
  • Fireblocking: The airspace clearance must remain open and must only be filled at floor and ceiling penetrations with approved noncombustible fireblocking materials (such as sheet metal or mineral wool) to prevent the spread of fire between floors.
• Cleanout Openings: Every masonry chimney flue must have a cleanout opening at its base to allow for the removal of soot, creosote, and debris. The cleanout must be equipped with a tight-fitting, noncombustible door (such as cast iron or steel). The cleanout opening must be located at least 6 inches below the lowest connector inlet.

Sizing Gas Appliances in Masonry Chimneys

When venting a gas appliance into a masonry chimney, inspectors must verify the flue size. If a flue is too large, the low-volume flue gas from a small appliance will expand, cool rapidly, and fail to draft, leading to severe condensation.

• The 7-Times Rule (G2427.5.5): For a single gas appliance vented into a masonry chimney, the flow area of the chimney flue must not be less than the area of the appliance draft hood outlet, and must not exceed 7 times the area of the draft hood outlet.
• Common Venting: If multiple appliances share the chimney, the flue area must not exceed 7 times the combined area of the appliance draft hood outlets, or must follow the sizing tables in G2428. If the masonry chimney flue is oversized, it must be relined with a listed liner (such as a Type B vent or a UL 1777 metal liner) sized to match the appliances.

Factory-Built Chimneys: Listing and Attic Shields

Factory-built chimneys are pre-fabricated, double-wall or triple-wall insulated assemblies. Under IRC Section R1005 and Section M1805, they are listing-controlled systems:

• Listing Standard: Chimneys serving solid-fuel-burning appliances in dwellings must comply with the Type HT requirements of UL 103 and be marked "Type HT" and "Residential Type and Building Heating Appliance Chimney." This standard requires the chimney to withstand flue gas temperatures of up to 2,100°F for brief periods (such as during a chimney fire).
• Clearances: Clearances are determined by the manufacturer's listing (typically 2 inches to combustibles, but always check the product label).
• Attic Insulation Shield (R1005.8): A common inspection failure is attic insulation coming into contact with a factory-built chimney. To prevent this, a metal attic insulation shield must be installed where the chimney passes through an attic.
  • Material: The shield must be constructed of steel not less than 0.0187 inch thick (No. 26 gage).
  • Clearance: It must maintain the manufacturer's specified clearance to combustibles.
  • Height: The shield must extend at least 2 inches above the top of the insulation.
  • Security: It must be secured in place to prevent displacement by wind, attic storage, or shifting insulation.
• Structural Support: Factory-built chimneys must be supported by listed components (ceiling support boxes, wall brackets, or roof supports) and must not place their weight on the appliance. Brand mixing is prohibited; all chimney parts must be from the same listed manufacturer.