Flue Liners: What the Standard Requires and How Compliance Is Actually Established
Why an unlined flue is not permitted in service, the two compliant routes to fixing one, and why the alloy choice is the specification that decides service life.
A liner is not an upgrade. It is the barrier between combustion products and the structure of the building, and a flue whose liner is breached is not a chimney with a defect — it is a chimney that should not be in service until it is corrected. Here is what compliance actually requires.
01What the liner is doing
Three things. It contains combustion products so they do not reach framing or living space. It presents a smooth, correctly sized surface so the system drafts as designed. And it resists the acidic condensate that certain fuels produce.
A breach defeats all three simultaneously, which is why the response to a confirmed breach is to take the flue out of service rather than to schedule a repair for convenience.
02How a breach is established
By internal camera scan. This is the distinction between a visual inspection and one that includes an internal examination of the flue surface — and it is why the deeper inspection is required after a chimney fire, an impact, or an operating malfunction, when a breach is most likely to have been created.
A verdict on a liner delivered without a scan is an opinion. The scan produces an image with a location, and that image is what a repair scope is built from.
03Two compliant routes, and how to choose
A listed metal liner, sized to the appliance and insulated as its listing requires. Or a cast-in-place system, where a form is set and a cementitious mix rebuilds the flue in place, which also reinforces the chimney structurally.
Metal is reversible and generally cheaper. Cast-in-place is permanent and is the answer where the masonry itself needs the reinforcement. Where a metal liner would genuinely serve, specifying cast-in-place is over-buying and we will say so.
04Alloy is the specification that matters most
316Ti for fuels producing acidic condensate; 304 where the fuel does not warrant it. This is not a brand preference — it is a corrosion-resistance decision, and choosing 304 to save money on the wrong appliance is the most expensive error in this whole category.
Insulation belongs in the same conversation. It is what allows a liner to carry its listing at reduced clearance and keeps flue gases above condensation temperature. Treat it as part of the specification rather than an option.
Get a camera scan before accepting any verdict on a liner, insist the alloy is specified against your fuel, and treat insulation as standard. A liner is the one component where a saving today is reliably a full replacement later.
Every recommendation here starts from one place: a graded, photographed chimney inspection done to one standard, with a paper trail you keep.
The Chimney Standard Team
This article was written and reviewed by the CSIA-certified crew at Chimney Standard. Every recommendation reflects NFPA 211 and CSIA trade standards and the same documented, photographed approach we hold every job to.
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