The unused-looking space inside an existing masonry chimney is not automatically usable vent area. The review must distinguish the approved liner or vent from the surrounding annular space and trace a compliant path for every connected appliance.
Plain-English Highlights
IMC 801.17 prohibits using the space surrounding a flue lining system or other vent installed inside a masonry chimney to vent another appliance. A separate flue lining is permitted only when it is installed in accordance with its manufacturer's installation instructions and the code. In practical terms, leftover space is not a second flue; a separately designed and approved liner is required.
Permit Proof Chain
Each appliance and fuel identified -> applicable venting code path confirmed -> existing chimney interior and liner arrangement documented -> each connector and vent path traced -> separate liner selected where a separate path is needed -> liner size, material, standard or listing, and appliance compatibility identified -> installation instructions submitted -> cleanout, inspection access, support, clearances, and termination coordinated.
Where To Show It
M-001: existing-chimney, separate-liner, appliance-compatibility, and code-basis notes. M-101: every appliance, connector, liner path, and chimney connection. M-501: liner separation, support, cleanout, access, clearances, penetration, and termination details. M-601: appliance schedule with fuel, input, draft or vent category, connector size, and liner basis. A/LS sheets: chimney location, shafts, rated assemblies, roof conditions, and combustible clearances. Specifications/submittals: liner listing and manufacturer installation requirements.
**Do** - Show a complete, labeled vent path for every appliance. - Identify each liner by size, material, listing or standard, and appliance served. - Provide a separate approved liner when a separate venting path is required. - Coordinate cleanout and inspection access before the liner layout is accepted. - Verify appliance-specific common-venting rules rather than assuming that separate space creates permission to combine appliances.
**Do Not** - Use the annular space around a liner as a vent for another appliance. - Treat unused chimney space as an approved flue. - Draw multiple connectors entering an existing chimney without identifying their actual liner paths. - Assume that adding a separate liner resolves sizing, listing, draft, condensate, clearance, or termination requirements. - Leave existing chimney configuration or liner selection to field judgment.
Masterbuild QA Lens
Use a one-appliance-to-one-approved-path markup. Starting at each appliance outlet, trace the connector, liner or vent, chimney route, and termination without entering an undefined void. If the drawing depends on the space around another liner, or the path changes identity inside the chimney, the design is incomplete.
Drawing / Submittal Check
Compare the chimney survey, appliance schedule, liner product data, and plan detail. Confirm the number and location of liners, clear inside dimensions, liner size and construction, installation method, cleanout and inspection access, connector entry elevations, support, separation, termination, and all manufacturer limitations. Document concealed existing conditions that must be verified before construction.
Common Review Risk
A replacement or added appliance is shown entering the same masonry chimney as an existing liner, but the new connector is effectively assigned to the space outside that liner. No second listed lining system is specified. The result can be uncontrolled flue-gas movement, poor draft, leakage, condensation, corrosion, carbon-monoxide exposure, and an installation that cannot be properly inspected or cleaned.
When To Escalate
Escalate multiple-appliance chimney reuse, missing interior surveys, uncertain liner continuity or condition, incompatible fuels or appliance categories, insufficient space for listed liner installation, common-vent proposals, inaccessible cleanouts, reduced-clearance conditions, or any installation that depends on undocumented existing masonry. For gas-fired appliances, follow the applicable fuel-gas code and the appliance listing; IMC Chapter 8 alone is not the complete venting path.
Field Tip
On the chimney cross-section, shade every approved liner and label the appliance it serves. Anything left unshaded is not a vent. This simple markup exposes the extra-space assumption before it becomes concealed fieldwork.