/ Field knowledge · IMC 801.15–801.16
“Existing chimney to remain” is not a liner.
The question is not whether a masonry chimney exists. It is whether the chimney and its liner are compatible with this appliance, this fuel, this flue-gas temperature, this condensate exposure, and these installation instructions. “Existing chimney to remain” is not that answer.
Educational 2024 International Mechanical Code (IMC) 801.15 and 801.16 from Daily Code Talk. Confirm the adopted edition. Distinct from existing chimney reuse (801.18) and minimum area (801.6). Gas-fired appliances also follow the fuel-gas code. This is not a liner selection.
- 801.15: masonry chimneys used to vent fuel-fired appliances are located, constructed, and sized as specified in the appliance manufacturer’s installation instructions.
- 801.16: masonry chimneys are lined. The lining must be compatible with the connected appliance per the listing and instructions. Listed lining materials are installed per their listing and manufacturer.
- 801.16.1 (residential-type and low-heat): general lining options are clay flue lining complying with ASTM C315 or equivalent; listed and labeled systems complying with UL 1777; or other approved materials resistant to flue gases and condensate up to 1,800°F (982°C).
On the set: appliance, fuel, venting category, chimney dimensions and construction, liner material / standard / listing / size, connector, cleanout, clearances, termination, and any resizing or relining. Compare appliance data, manufacturer venting instructions, chimney survey, and liner submittal. A generic note cannot replace that comparison.
High-efficiency appliance, old unlined masonry
No liner type or condition documented, flue oversized for the new duty, condensate ignored. Poor draft, corrosion, leakage, carbon monoxide, failed inspection. Escalate fuel conversion, category changes, visible liner deterioration, and any liner outside the 801.16.1 options.
Source: Daily Code Talk — IMC 801.15–801.16 · IBC: 801.3 · leftover space: 801.17 · existing: 801.18 · area: 801.6 · medium-heat: 805.6.
What to send
Appliance cut sheet (vent category and instructions), photos of the chimney and any visible liner, and whether this is a replacement or a new connection.
If a condensing or Category III/IV appliance is hitting old masonry
Send the cut sheet and the chimney photos. Masterbuild will say whether 801.16 even has a compatible liner — or 801.18 still has no four-gate evidence.
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. ASTM C315, UL 1777, and 1,800°F are 2024 IMC 801.16.1 education from Daily Code Talk — not a liner specification. This page is not legal advice.