6.3 Masonry and Factory-Built Chimneys

6.3 Masonry and Factory-Built Chimneys

Quick Answer: Masonry chimneys need approved, appliance-compatible lining. Required cleanouts sit at least 6 inches below the lowest connector inlet, with the fireplace-opening exception where applicable. Generally allow 2 inches to combustibles for chimneys inside or within exterior walls, and 1 inch only where entirely outside exterior walls, subject to the code’s exceptions. Factory-built systems follow their listing, the applicable Type HT rule and exception, and attic insulation-shield requirements.

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, listed chimney lining systems or units, and other approved materials under R1003.11.1, selected for the appliance. Examples 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 inside the building or within its exterior walls generally require 2 inches of airspace clearance to combustibles; apply the specific R1003.18 exceptions where applicable.
  • Exterior Chimneys: Masonry chimneys entirely outside the exterior walls generally require 1 inch of airspace clearance to combustibles, subject to the R1003.18 exceptions.
  • 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: A masonry chimney generally requires a cleanout opening to remove soot, creosote and debris. The stated exception permits a masonry-fireplace chimney flue to be cleaned through the fireplace opening. 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.

• Gas-flue sizing (G2427 and G2428): Identify the appliance category, connector, chimney liner, available height, and approved sizing method. A familiar area ratio is an alternative in specific circumstances, not a universal test for every gas appliance or chimney.
• Common Venting: When appliances share a chimney, use the applicable Chapter 24 tables or approved engineering method for the particular appliance combination. An existing flue may need a compatible listed liner or other correction if the new configuration cannot draft safely.

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 generally must meet UL 103 Type HT and its required marking. Under the R1005.3 exception, a chimney serving an open-combustion-chamber fireplace complies with UL 103 and has the specified residential/building-heating-appliance marking but is not required to be Type HT. 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.