Existing does not mean accepted. When an appliance is permanently disconnected from an existing chimney or vent, or when a new appliance is connected during a new installation, the existing system must be evaluated as part of the design rather than inherited through a generic note.
Plain-English Highlights
IMC 801.18 requires compliance with Sections 801.18.1 through 801.18.4 when an appliance is permanently disconnected or a new appliance is connected to an existing chimney or vent. The acceptance decision has four gates: size for draft and condensation control; flue passageway cleanliness, continuity, and condition; cleanout compliance; and required airspace clearances. Passing only one gate does not make the system reusable.
Four-Gate Existing-Vent Acceptance Matrix
1. Size: resize as necessary to control condensation and provide required draft. 2. Passageway: inspect for obstruction, deposits, continuity, and damage; clean, repair, or reline as required. 3. Cleanout: confirm the required cleanout is present, accessible, and coordinated. 4. Clearance: verify airspace to combustibles, fireblocking, and any listed reduced-clearance liner condition under the applicable code path.
Permit Proof Chain
Existing system survey -> appliances removed, remaining, and added identified -> fuel and venting path confirmed -> chimney or vent dimensions, height, material, liner, route, and termination documented -> four acceptance gates evaluated -> required cleaning, repair, relining, resizing, cleanout, closure, or clearance work shown -> manufacturer instructions and adopted code basis referenced -> final field-verification hold point assigned.
Where To Show It
M-001: existing-vent acceptance criteria, field-verification hold point, and applicable code path. M-101: removed, remaining, and new appliances with complete connector routes. M-501: chimney or vent section, liner, repairs, cleanout, unused-opening closure, clearances, fireblocking, and termination. M-601: old and new appliance inputs, fuels, draft or vent category, connector size, and final venting basis. A/LS sheets: shafts, rated construction, roof conditions, clearances, and fireblocking. P sheets: boilers, water heaters, fuel, oil, and shared-system coordination.
**Do** - Identify the exact trigger: disconnection, replacement, addition, or change in connected load. - Evaluate and document all four Section 801.18 acceptance gates. - Show required cleaning, repair, relining, resizing, cleanout, and clearance work. - Close unused inlet openings by an approved method under the applicable code path. - Coordinate the final design with appliance manufacturer instructions and verified existing conditions.
**Do Not** - Use "existing chimney to remain" as the complete design. - Assume prior service proves current suitability. - Check only chimney size while ignoring condition, cleanout, and clearance. - Connect a new appliance before the concealed passageway has an inspection and acceptance plan. - Leave abandoned connectors or openings, repairs, or liner decisions undefined.
Masterbuild QA Lens
Convert every existing chimney or vent reuse note into a four-row acceptance matrix. Each row needs existing evidence, required action, drawing location, and responsible verifier. This turns an assumption into a permit-reviewable and field-enforceable control.
Drawing / Submittal Check
Require a documented survey showing accessible dimensions, height, construction, liner, cleanout, connector openings, termination, clearances, visible damage, and deposit or obstruction conditions. Compare that survey with the removed, remaining, and new appliance data. The permit set should state what must be verified before connection and what action follows a failed condition.
Common Review Risk
A new appliance is scheduled, but the plan only says to reconnect to the existing chimney. The remaining load changed, the passageway was not inspected, the cleanout is not shown, and combustible clearances are unknown. That gap can produce condensation, corrosion, poor draft, flue-gas leakage, carbon-monoxide exposure, change orders, and failed inspections.
When To Escalate
Escalate inaccessible or concealed chimneys, missing dimensions, damaged or discontinuous liners, fuel conversion, efficiency or vent-category changes, common vents after load removal, reduced clearances, blocked or heavily deposited passageways, oil-fired masonry-chimney repair or relining, and any condition where the field survey contradicts the design basis. Gas-fired appliances follow the applicable fuel-gas code path and manufacturer instructions.
Field Tip
Do not sign off "existing to remain" from the plan view. Open a chimney section and check four boxes: size, passageway, cleanout, and clearance. If any box lacks evidence, reuse is still an unresolved design condition.