The liner is the surface the fire actually touches. Its job is to contain heat and combustion gases so they go out the top instead of into the wall cavity beside it. Older chimneys use clay tile, which cracks with age and heat. Some very old chimneys were built with no liner at all. Either way, this is the one item on the list where the right advice is sometimes to stop using the appliance today.

Signs you'd actually notice

Illustrated Apex technician holding a broken piece of clay flue tile found in a firebox

Pieces of clay tile turning up in the firebox
An inspection report that flagged a cracked, gapped, or missing liner
A new furnace, water heater, or insert vented into an old flue
A chimney built before liners were standard
You've had a chimney fire, whether or not it seemed serious

What happens on the visit

Illustrated Apex technician pointing at a cracked flue tile on a chimney camera monitor

Camera the flue
A video scan is the only honest way to assess a liner. You'll see the footage — cracks, gaps, and missing sections are unmistakable once they're on screen.
Size the liner to the appliance
An oversized flue drafts badly and lets exhaust cool and condense; an undersized one can't vent. The liner has to match what it's serving, not just fit down the hole.
Install stainless, insulated
Stainless steel liner, insulated along its length, connected properly at the appliance and terminated correctly at the top with a new cap.
Test the draft before we leave
A liner that's installed but doesn't draw isn't finished. We verify it works with the appliance running.

Why it matters

This is the failure that hurts people. A cracked liner lets heat reach framing that was never meant to get hot, and it lets combustion gases — including carbon monoxide — into the house instead of out the top. If an inspection flags your liner, treat it as the thing you fix first.