Chimney basics
Flue vs. liner: what is the difference?
People use the two words as if they mean the same thing. They do not, and the difference matters when you are told a chimney needs one.
Short answer. The flue is the passage the gases travel through. A liner is the material that lines that passage. Some chimneys have a clay tile liner built in, some have a metal liner added later, and some older ones have none.
Flue: the passage
The flue is the open channel from the firebox or appliance to the top of the chimney. Every working chimney has at least one. A chimney can have several flues side by side, each serving a different fireplace or appliance.
Liner: what lines the flue
A liner is the inside surface of the flue. Its job is to contain combustion gases and protect the surrounding masonry and house framing from heat and moisture. Common types are:
- Clay tile. Short ceramic sections stacked in the chimney, common in houses built in the twentieth century.
- Cast-in-place. A cement-like lining formed inside the flue.
- Stainless steel. A metal sleeve installed into an existing flue. It is sized to the appliance it serves.
Why the distinction matters
A flue can be sound with a damaged liner, or have no liner at all. When someone says a chimney needs relining, they mean the surface inside the flue needs to be repaired or replaced, not that the chimney needs rebuilding.
Appliance changes matter too. A liner should be suited to the appliance and fuel, which is why the same chimney may need different treatment after a new heater is installed. Read more in Do you need a chimney liner?.
Quick decision guide
| Question | Answer |
|---|---|
| Is the flue the same as the chimney? | No. The flue is the passage inside; the chimney is the whole structure. |
| Does every flue have a liner? | Most modern ones should. Older chimneys may have a damaged one or none. |
| Why does it matter? | The liner contains heat and gases and protects the masonry. |
| What if it is cracked? | Have it inspected and discuss repair or relining. |
Common misconceptions
"Bricks are the liner." Brick is the chimney wall. The liner is a separate lining inside it.
"Liners last forever." They age and can crack.
About this guide. Published by Dr. Sweeper as general homeowner education, last updated October 1, 2026. Sources are linked where they are cited. What applies to your home can depend on your local building code, your appliance and its manufacturer's instructions, and the actual condition of your chimney, so this guide is not a substitute for an inspection.
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