IMC 805 is about more than selecting a chimney diameter. Factory-built chimneys are listed systems. The appliance, chimney classification, offsets, supports, termination accessories, clearances, and insulation protection all have to work together.
IMC 805.1–805.3 Listing and appliance compatibility
Factory-built chimneys must be listed, labeled, and installed per the manufacturer’s instructions. Match the chimney to the appliance and fuel. Verify UL 103 Type HT where required for dwelling-unit solid-fuel appliances. Coordinate factory-built fireplaces with UL 127. Do not mix incompatible components.
IMC 805.4 / 805.5 Offsets and support
Where offsets are used: maximum 30 degrees from vertical and maximum four elbows (educational 2024 IMC). Where structural members support the chimney, they must be designed for the added load.
IMC 805.6 Medium-heat appliances
Where flue-gas temperature exceeds 1,000°F at the chimney entrance, the factory-built chimney must comply with UL 959. The temperature at the chimney entrance matters — not just appliance input or pipe size.
IMC 805.7 Decorative shrouds
Decorative shrouds are permitted only where listed and labeled for the specific factory-built chimney system and installed as required. An unlisted shroud can affect draft, heat dissipation, and flue-gas discharge.
IMC 805.8 Insulation shields
Where chimneys pass through insulated assemblies: maintain manufacturer-required clearance, provide the required insulation shield, use minimum No. 26 gage steel where the code-built shield applies, extend the shield at least 2 inches above insulation in attics, and secure it against displacement.
Common failure points
Wrong chimney classification. Unlisted substitutions. Excessive offsets. Unsupported loads. Improvised shrouds. Insulation in the required clearance space. These can lead to overheating, poor draft, chimney damage, fire risk, and failed inspections.
Permit-set checklist
Identify chimney manufacturer and system. Confirm listing and appliance compatibility. Verify required UL standard. Show routing and elbow count. Coordinate structural support. Dimension combustible clearances. Show roof and attic penetrations. Coordinate termination accessories. Detail insulation shields where required.
Field tip
Review the chimney as one continuous listed assembly from appliance connection to termination. If the drawings only show a line labeled “chimney,” the design is probably not coordinated enough for plan review or construction. Hub: factory-built chimneys. Next: IMC 806 metal chimneys.
Project boundary
2024 IMC model-code education from the owner-provided Daily Code Talk recap. Confirm adopted edition, amendments, listing, manufacturer instructions, structural design, and AHJ path. Not a chimney selection.