Does my chimney need a liner?
The liner is the surface the flue gases actually touch, and it is doing two jobs at once: containing heat so it does not reach the structure around it, and containing the gases so they leave at the top rather than passing into the masonry. When it fails, both of those stop being true at the same time, and that is why it matters more than its low profile suggests.
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What is probably in your chimney now
Older masonry chimneys in this area are commonly lined with clay tile, fitted in sections stacked one on top of another. Newer chimneys, and most framed chases, use a metal flue instead. You can often tell which you have by looking up from the firebox with a flashlight, since clay reads as a squared off orange or buff surface with visible horizontal joints, while metal is round and continuous.
A small number of older chimneys have no liner at all, which was accepted practice long ago. That is worth knowing rather than assuming, because an unlined flue behaves differently and the flue gases are in direct contact with the masonry and any mortar joints in it.
The signs a clay liner has cracked
Pieces of clay tile appearing in the firebox is the unambiguous one. Small angular fragments that are not soot and not mortar have come from a liner section above, and they mean a section has cracked or a joint has opened. It is worth keeping a piece to show whoever comes out.
The subtler sign is a chimney that has started drawing poorly, or smelling, without anything else changing. Gaps in a liner let air in partway up the flue, which disrupts the draw, and they let flue gases into the masonry, which is what produces a smell that seems to come from the wall rather than the fireplace.
Why the coastal climate is hard on metal flues
A metal flue in a framed chase depends on staying dry on the outside. Once the cover at the top of the chase corrodes or lifts, rain enters the chase and sits around the outside of the flue, and it is then working on the metal from both sides. Close to the water, salt in that moisture accelerates it considerably.
The failure often shows at the top first, since that is where water collects, so rust staining running down from the top of a chase is worth taking seriously. By the time it is visible from the ground, the section producing it has usually been wet for a long time.
What a liner problem is actually risking
Two things. Heat reaching material that was never meant to see it, and combustion products entering the house rather than leaving at the top. The second is the reason a compromised liner is treated as a reason to stop using a fireplace rather than something to monitor, because what leaks is not just smoke.
This is also why a liner defect changes the answer on an otherwise cosmetic repair list. A chimney can look sound from outside, with tidy masonry and a good cap, and still not be usable. The outside tells you about weather. The liner tells you whether the flue is doing its job.
Relining or rebuilding
Where the surrounding masonry is sound and the defect is confined to the liner, relining puts a continuous new surface inside the existing structure and is generally less disruptive than it sounds. Where the masonry itself has deteriorated around a failed liner, particularly at the top, the two problems get addressed together rather than separately.
It is a fair question to ask which situation you are in and why. The distinction is not subtle to someone who has looked at the flue, and an answer that explains what was seen inside is more useful than a recommendation on its own.
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Frequently Asked Questions
How do I tell what liner I have?
Look up from the firebox with a flashlight. Clay tile reads as a squared, orange or buff surface with horizontal joints. A metal flue is round and continuous. Some older chimneys have no liner at all.
What do clay fragments in the firebox mean?
They have come from a cracked liner section or an open joint above. Small angular pieces that are neither soot nor mortar are the clearest single sign of a liner problem. Keep one to show whoever inspects it.
Can a chimney look fine outside and still need relining?
Yes, and it is common. The exterior tells you how the chimney is handling weather. The liner determines whether the flue can safely carry combustion products, and the two are independent.
Does a metal flue last longer here?
Not automatically. Once water gets into a chase past a failed cover, a metal flue is exposed on both sides, and salt in the air speeds that up. Rust staining from the top of a chase is worth investigating.