An existing chimney can look serviceable from the room or roof and still fail internally. Section 801.18.2 moves the reuse decision inside the passageway: the full path must be evaluated from the connector inlet through the liner or vent wall to the termination.
Plain-English Highlights
IMC 801.18.2 requires the flue-gas passageway to be free of obstructions and combustible deposits. A passageway previously used for a solid- or liquid-fuel appliance or fireplace must be cleaned. The flue liner, chimney inner wall, or vent inner wall must be continuous and free of cracks, gaps, perforations, damage, or deterioration that could allow gases, moisture, or creosote to escape. Where an oil-fired appliance connects to an existing masonry chimney, the chimney flue must be repaired or relined in accordance with NFPA 31.
Permit Proof Chain
Existing appliance and fuel history identified -> chimney or vent route, material, liner, openings, and access documented -> inspection and cleaning scope assigned -> passageway evaluated for blockage, deposits, continuity, and deterioration -> defects mapped and photographed or otherwise recorded -> repair, relining, replacement, or rejection scope established -> sizing, cleanout, clearance, and appliance compatibility rechecked -> EOR disposition and required closeout evidence shown in the permit record.
Where To Show It
M-001: adopted-code basis, existing-passageway acceptance criteria, cleaning trigger, inspection hold point, and EOR disposition requirement. M-101: existing and new appliances, connector inlets, common sections, chimney or vent route, and limits of work. M-501: liner, repair or relining, connector, cleanout, access, abandoned opening, penetration, and termination details. M-601: appliance fuel, input, draft or vent category, flue outlet, venting basis, and existing-versus-required condition.
Specifications / submittals: inspection method, cleaning, repair materials, liner listing, manufacturer instructions, test documentation, and closeout record. P/A/LS sheets: boiler or water-heater coordination, chimney or shaft construction, roof, rated assemblies, and clearance interfaces.
**Do** - Record prior fuel use and every connected, removed, and remaining appliance. - Define who inspects, what portion of the passageway is evaluated, and what evidence is returned. - Require cleaning when the adopted code trigger applies; distinguish cleaning from condition acceptance. - Map cracks, gaps, perforations, corrosion, moisture damage, and deposits to a repair or relining disposition. - Reconcile the passageway result with sizing, cleanout, clearances, listing, and manufacturer requirements before reuse.
**Do Not** - Accept a chimney from an exterior-only observation or a generic existing-to-remain note. - Treat sweeping or cleaning as proof that the liner is continuous and sound. - Use a contractor verify note without acceptance criteria, required evidence, and an EOR disposition path. - Assume a new liner resolves sizing, clearance, fireblocking, termination, or appliance-compatibility requirements. - Connect the appliance while the passageway condition or repair scope remains unresolved.
Masterbuild QA Lens
Use a four-column acceptance ledger: segment or condition, evidence reviewed, required disposition, and accepting party. Mark each item PASS, REPAIR, RELINE, REPLACE, or HOLD. This separates field observation from engineering acceptance and prevents a cleaning invoice from being mistaken for a complete reuse decision.
Field Inspection / Closeout Check
Confirm the evaluated path extends from each connector inlet to the termination and includes accessible common sections, offsets, thimbles, liner joints, cleanout area, abandoned openings, and the termination. Retain the inspection and cleaning record, dated condition evidence, repair or relining submittal, listing and manufacturer instructions, discrepancy resolutions, and final EOR acceptance required by the project documents. The code establishes the required condition; the EOR and AHJ determine the acceptable project evidence and inspection method.
Common Review Risk
The chimney is swept before an appliance replacement, but the documents never address a cracked clay liner or deteriorated mortar joint. Deposits are removed, yet the passageway can still leak gases, moisture, or creosote into the building or concealed construction. Cleaning and continuity are separate acceptance questions.
When To Escalate
Escalate unknown prior fuel use, inaccessible offsets, incomplete full-length evidence, cracked or missing liner sections, corrosion, perforations, moisture staining, odor or spillage history, heavy soot or creosote, common-vent changes, conflicting inspection results, and any oil-fired connection to an existing masonry chimney without an NFPA 31 repair or relining basis. Also escalate when the proposed appliance listing or manufacturer instructions conflict with reuse of the existing path.
Code-Path Guardrail
Daily Code Talk explains the 2024 IMC model code. The adopted code, local amendments, permit date, AHJ procedure, appliance listing, and manufacturer instructions control the project. Gas-fired chimney reuse follows the adopted fuel-gas code path, including its existing-chimney inspection and passageway provisions, rather than relying on IMC 801 alone.
Field Tip
Ask one direct question: is the passageway clean, continuous, and intact from every connector inlet to the termination? Then ask what evidence supports the answer. If the permit record cannot answer both questions, the existing chimney or vent has not been accepted for reuse.