/ Field knowledge · IMC 801.17
Leftover chimney space is not a second flue.
The unused-looking space inside an existing masonry chimney is not automatically usable vent area. The space around a flue lining or other vent installed in that chimney cannot vent another appliance. Leftover space is not a second flue.
Educational 2024 International Mechanical Code (IMC) 801.17 from Daily Code Talk. Confirm the adopted edition. Distinct from liner compatibility (801.15–801.16) and dedicated solid-fuel flues (801.11). Gas-fired appliances also follow the fuel-gas code. This is not a liner design. Packet: 801.17 PDF · ES.
- Do not use the annular 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. That is a separately designed and approved liner — not the leftover void.
- Show a complete, labeled path for every appliance: connector → liner or vent → chimney route → termination, without entering an undefined void. If the path changes identity inside the chimney, the design is incomplete.
On the set: chimney survey, number and location of liners, each liner’s size / material / listing / appliance served, connector entry elevations, cleanout, and manufacturer limitations. Shade every approved liner on the cross-section and label the appliance it serves. Anything left unshaded is not a vent.
The new connector assigned to the space outside the existing liner
No second listed lining system. Uncontrolled flue-gas movement, poor draft, leakage, condensation, carbon monoxide, and an installation that cannot be inspected or cleaned. Adding a second liner still does not, by itself, resolve sizing, listing, draft, condensate, clearance, or termination.
Source: Daily Code Talk — IMC 801.17 · tool: Annular-space screen · lining: 801.15–801.16 · existing: 801.18.
What to send
Chimney cross-section or photos of the interior, every appliance proposed on that chimney, and whether a second listed liner is actually specified.
Free sample — the four-gate existing-chimney acceptance
“Existing chimney to remain” is the most expensive four words on a renovation mechanical set. The complete acceptance section from the IMC 2024 Chapter 8 Chimneys and Vents Field Guide — all four gates, the survey discipline, the escalation trigger, and the record it produces — is available as a free PDF. No email required. Download the sample (PDF). The full edition is available in the Field Guides catalog.
If a second appliance is hitting leftover chimney space
Send the photos. MasterBuild will say whether 801.17 still has no second liner — or 801.16 still has no compatible lining for the first one.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
MasterBuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. This page is 2024 IMC 801.17 education from Daily Code Talk — not a liner selection. This page is not legal advice.