12.2 Factory-Built & Masonry Chimneys

A chimney is not a vent. Chapter 2 of both codes treats a vent as a listed factory-made conduit for a specific class of appliance. A chimney is a vertical masonry, metal, or factory-built passageway that can serve appliances the listing actually authorizes — including, when it is a real chimney, solid fuel. Type B is a gas vent. Using it as a “metal chimney” for a wood stove is one of the most reliable M2 fails in this chapter.

2024 IFGC 501.1 and 501.3 split the work: factory-built chimneys, liners, vents, and connectors for gas are IFGC Chapter 5; the same parts for other fuels are IMC Chapter 8; construction of masonry chimneys is the International Building Code. IMC 801.3 repeats that masonry construction is IBC work. You still inspect utilization, lining, sizing, cleanouts, and connectors under the IMC or IFGC.

Factory-built chimneys — listed, supported, terminated

2024 IFGC 503.5.1: factory-built chimneys shall be listed in accordance with UL 103. Factory-built chimneys used to vent appliances that operate at a positive vent pressure shall be listed for that application. IMC 805.1: factory-built chimneys shall be listed and labeled and installed and terminated in accordance with the manufacturer’s installation instructions. Those instructions are the clearance, support, firestop, insulation-shield, and cap packet. 304.1 still requires them on site.

Solid fuel in dwelling units — IMC 805.2. Factory-built chimneys installed in dwelling units with solid-fuel-burning appliances shall comply with the Type HT requirements of UL 103 and shall be marked “Type HT” and “Residential Type and Building Heating Appliance.” Exception: chimneys for open-chamber fireplaces comply with UL 103 and are marked “Residential Type and Building Heating Appliance” without the HT stamp. Open-chamber appliances in buildings other than dwelling units use UL 103 marked “Building Heating Appliance” or “Residential Type and Building Heating Appliance.” IMC 805.3 sends factory-built fireplaces to UL 127, which is a fireplace listing, not a Type B listing.

Offsets — IMC 805.4. Where a factory-built chimney assembly incorporates offsets, no part of the chimney shall be at an angle of more than 30 degrees from vertical at any point, and the assembly shall not include more than four elbows. That is tighter than the 45-degree Type B offset rule in IFGC 503.6.10.2. Do not apply the vent-offset numbers to a UL 103 chimney.

Support — IMC 805.5 / IFGC 503.5.8. Where factory-built chimneys are supported by structural members such as joists and rafters, those members shall be designed to support the additional load. All portions of chimneys shall be supported for the design and weight of the materials. Factory-built chimneys are supported and spaced per the manufacturer — not hung from a single drywall screw or from the roof jack alone.

Insulation shield — IFGC 503.5.11. Where a factory-built chimney passes through insulated assemblies, install a steel insulation shield not less than 0.0187 inch thick to hold the listed clearance. In an attic the shield terminates not less than 2 inches above the insulation and is secured against displacement. IMC 802.8 is the matching vent-shield rule (No. 26 gage).

Medium-heat and shrouds. IMC 805.6: factory-built chimneys for medium-heat appliances producing flue gases above 1,000°F (538°C) measured at the chimney entrance shall comply with UL 959. IMC 805.7 and IFGC 503.5.4: decorative shrouds shall not be installed at the termination of factory-built chimneys except where listed and labeled for that specific chimney system. A sheet-metal “hat” from the cornice shop is not a listed shroud.

Cleanout and inspection. Before connecting a replacement appliance, IFGC 503.5.6 / IMC 801.18.2 require the passageway to be examined, free of obstructions, and cleaned if it previously served solid or liquid fuel or a fireplace. The liner or inner wall shall be continuous and free of cracks, gaps, or perforations that would let combustion products, moisture, or creosote escape.

Masonry chimneys — lining, thickness, 3 / 2 / 10, saddles

Construction. 2024 IBC 2113.1: masonry chimneys are solid masonry, hollow units grouted solid, stone, or concrete. 2113.10 Wall thickness: not less than 4 inches (102 mm) nominal. Medium-heat masonry chimneys jump to 8 inches of solid masonry or concrete, or 12 inches of stone, plus a 4½-inch medium-duty refractory lining (IBC 2113.11.2). High-heat chimneys are double 8-inch walls with a 2-inch airspace (2113.11.3).

Lining. IMC 801.16: masonry chimneys shall be lined, and the lining shall be compatible with the appliance listing. IFGC 503.5.3 and IBC 2113.11: clay flue lining complying with ASTM C315, a lining listed to UL 1777, or other approved material that resists corrosion, erosion, softening, or cracking from vent gases at temperatures up to 1,800°F (982°C). IFGC 501.12 / IMC 801.16.1 publish that same three-item menu for residential-type and low-heat appliances. IFGC 501.13 adds, for Category I only, chimney lining systems listed for draft-hood and Type B-listed gas appliances.

The gas-only liner exception in IFGC 503.5.3 is easy to miss. A masonry flue serving listed draft-hood, Category I, or Type B-listed gas appliances may be lined with a system specifically listed for use only with such appliances. The liner is installed to its instructions. A permanent label at the connection shall read: “This chimney liner is for appliances that burn gas only. Do not connect to solid or liquid fuel-burning appliances or incinerators.” That label is an inspection document. A later wood-stove insert into that liner is a fail even if the clay tile is still in the chase.

Termination — confirm 2024, both books. IFGC 503.5.4 and IBC 2113.9 are the same 3 / 2 / 10 rule for residential-type or low-heat chimneys:

• Not less than 3 feet (914 mm) above the highest point where the chimney passes through the roof, and
• Not less than 2 feet (610 mm) higher than any portion of a building within a horizontal distance of 10 feet (3048 mm).

Chimneys also extend not less than 5 feet (1524 mm) above the highest connected appliance draft hood or flue collar (IFGC 503.5.4). Medium-heat chimneys extend not less than 10 feet higher than any portion of any building within 25 feet (IFGC 503.5.4; IBC 2113.11.2.4).

Saddles. 2024 IBC 1503.5 (Roof Assemblies), not Chapter 21, is the cricket/saddle rule: a cricket or saddle shall be installed on the ridge side of any chimney or penetration greater than 30 inches (762 mm) wide as measured perpendicular to the slope. Coverings shall be sheet metal or the same material as the roof covering. Unit skylights under 2405.5 are the stated exception. A 36-inch-wide mechanical-room chase through a sloped roof without a cricket is a 1503.5 fail even if the flue termination itself meets 3 / 2 / 10.

Offsets and corbels. IBC 2113.7: where a fireclay liner is surrounded by one wythe of masonry, the maximum offset keeps the centerline of the flue above the offset from extending beyond the center of the chimney wall below, unless the offset is supported by masonry below in an approved manner. Each corbeled course still obeys 2113.5 (not more than half the wall thickness from a wall, and a single course not more than one-half the unit height or one-third the bed depth). IBC 2113.6: the wall or lining shall not change size or shape within 6 inches above or below a floor, ceiling, or roof penetration.

Cleanouts, entrance, abandoned inlets. IMC 801.13 / IFGC 501.15.3 / IBC 2113.18: masonry flue cleanout opening not less than 6 inches high; upper edge not less than 6 inches below the lowest inlet; tight-fitting noncombustible cover. Fireplace flues that can be cleaned through the fireplace opening are excepted. Connectors enter a masonry flue not less than 12 inches above the lowest interior of the flue (IFGC 501.9; IMC 801.12). Abandoned inlet openings shall be closed by an approved method (IFGC 501.5; IMC 801.8). A leftover thimble stuffed with newspaper is not an approved method.

Connectors into masonry. IMC 803.10.3: the connector extends to the inner face of the liner, not beyond, and is cemented to masonry in an approved manner. A thimble, if used, is permanently cemented with high-temperature cement. IFGC 503.5.10 / IMC 801.17: the space surrounding a liner, gas vent, special gas vent, or plastic pipe inside a masonry flue shall not vent another appliance and shall not be used as combustion air (except the stated direct-vent fireplace appliance).

Clearances and fireblocking. IBC 2113.19: interior or in-wall masonry chimneys keep 2 inches of airspace to combustibles; chimneys entirely outside, including those that pass a soffit, keep 1 inch. The airspace is not filled except for fireblocking. IBC 2113.20 / IFGC 501.15.4.1 / IMC 801.18.4.1: spaces between chimneys and the floors or ceilings they pass shall be fireblocked with noncombustible material securely fastened in place. UL 1777 liners listed for reduced clearance are the path when the historic chase never had the airspace (IFGC 501.15.4 Exception).

Relining, oversized flues, and the Type B “chimney” fail

2024 IFGC 501.15 / IMC 801.18 apply whenever an appliance is permanently disconnected from, or newly connected to, an existing chimney or vent. Resize as necessary to control interior condensation and to provide the required draft. For Category I gas appliances, resizing follows Section 502 / 504. For oil to masonry, IMC 801.18.1 sends you to NFPA 31.

IFGC 503.5.5 Item 2 is the exam number for a single draft-hood appliance: effective connector and chimney-flue areas shall be not less than the draft-hood outlet area and not greater than seven times that outlet area. Two draft-hood appliances: not less than the larger outlet plus 50 percent of the smaller, and not greater than seven times the smallest outlet. An old 12-by-12 clay tile on a 4-inch water-heater draft hood is a condensation machine. IMC 801.7 is the solid-fuel twin: the flue serving a solid-fuel appliance shall be not greater than three times the appliance flue-collar area.

You cannot vent gas into an unlined masonry chimney, or into a lined but too-large cold chimney, without sizing or relining. IMC 801.16 does not offer an unlined exception. IFGC 503.5.3 requires a liner. 501.15.2 requires a continuous inner wall. “It used to vent the coal boiler” is not a listing.

Field scenario. A restaurant wants a decorative wood stove on the leftover Type B that once served a Category I furnace. The Type B is still in good shape, double-wall, and terminates legally for a gas vent. The installation fails. Table 503.4 sends appliances that can burn solid fuel, and combination gas-and-solid-fuel appliances, to a chimney (503.5). IMC 805.2 requires a Type HT UL 103 chimney for solid fuel in a dwelling-unit setting; a commercial dining room is not a reason to use a gas vent. IFGC 503.6.13’s required label exists for this exact temptation: “This gas vent is for appliances that burn gas. Do not connect to solid or liquid fuel-burning appliances or incinerators.” Type B is not a chimney.

free M2 practice bankPractice questions with detailed explanations