Reusing the same chimney does not mean keeping the same effective size. Appliance removal, replacement, addition, fuel change, or efficiency change can alter flue-gas temperature, mass flow, draft, and condensation behavior.
Plain-English Highlights
IMC 801.18.1 requires an existing chimney or vent to be resized as necessary to control flue-gas condensation and provide the required draft for the appliance or appliances served. For oil-fired appliances vented to masonry chimneys, resizing must comply with NFPA 31. The code does not say every reuse requires resizing; it requires evidence that the retained or revised size performs correctly.
Permit Proof Chain
Existing chimney or vent inside dimensions, shape, height, material, liner, route, and termination documented -> removed, remaining, and new appliance inputs and venting characteristics identified -> applicable sizing method, table, calculation, manufacturer instruction, or NFPA 31 basis selected -> draft and condensation performance evaluated -> final vent or liner size shown -> connector, common-vent, cleanout, clearance, and termination conditions coordinated -> field verification and acceptance criteria assigned.
Where To Show It
M-001: existing-vent sizing method, condensation criterion, draft criterion, and verification note. M-101: appliances, connectors, common sections, chimney or vent route, and effective height. M-501: final liner or vent dimensions, connector transitions, cleanout, support, clearances, and termination. M-601: appliance input, fuel, efficiency or category, draft type, flue outlet, connector, and final vent size. Calculations: retained-versus-required sizing comparison and governing assumptions. P/A/LS sheets: fuel equipment, chimney, shaft, roof, rated construction, and clearance coordination.
**Do** - Record actual existing dimensions and effective height before accepting reuse. - Re-evaluate the remaining common vent when an appliance is removed. - Evaluate both draft and condensation; passing one is not enough. - Identify the governing sizing method and selected final size in the permit documents. - Apply NFPA 31 to oil-fired appliances vented to masonry chimneys.
**Do Not** - Assume the previous chimney size works for a different connected load. - Treat bigger as automatically safer; oversizing can increase flue-gas cooling and condensation risk. - Treat smaller as automatically acceptable; undersizing can restrict flow and impair draft. - State that higher-efficiency equipment is always incompatible without performing the applicable evaluation. - Leave final liner or vent size to the installer without a documented design basis.
Masterbuild QA Lens
Put two columns beside the appliance schedule: existing vent condition and required vent condition. Compare dimensions, effective height, connected input, venting characteristics, sizing basis, draft result, condensation result, and final action. This makes the reuse decision auditable instead of implied.
Drawing / Submittal Check
Verify the field survey and appliance data against the governing sizing method. Confirm all connected appliances, common and individual connector geometry, equivalent height where applicable, liner condition and material, available draft assumptions, termination, and manufacturer restrictions. The drawings and calculations must agree on the retained or revised size.
Common Review Risk
One appliance is removed from a common masonry chimney and the smaller remaining load is left on the original flue without evaluation. The flue gases cool, condensate forms, masonry and liner deteriorate, and draft becomes unreliable. The opposite risk occurs when added load or restrictive geometry leaves the system undersized.
When To Escalate
Escalate common vents after appliance removal, fuel conversion, condensing or higher-efficiency replacements, low flue-gas temperatures, exterior masonry chimneys, uncertain effective height, damaged liners, missing appliance data, oil-fired masonry-chimney work without an NFPA 31 basis, or conflicting manufacturer and calculation results. Gas-fired systems follow the applicable fuel-gas sizing and venting path rather than relying on IMC 801 alone.
Field Tip
Start the reuse review with two numbers: connected appliance input and actual chimney inside size. Then add effective height and flue-gas characteristics. If the plans cannot show how those inputs support draft and limit condensation, the existing size has not been accepted.