What is a flue?
A flue is the passage inside your chimney that carries smoke and combustion gases up and out of the house. In a masonry chimney it's typically a clay-tile-lined shaft; in a factory-built fireplace it's a metal pipe. One chimney can contain several flues — one for each appliance, since separate appliances can't share a passage safely.
Flue versus chimney
People use the words interchangeably, but they're not the same. The chimney is the whole structure — the brick or the framed chase you see outside. The flue is the actual channel inside it that the gases travel through.
A single chimney can hold two or three separate flues stacked together, which is why you'll see a masonry chimney with several clay tiles poking out the top. Each appliance needs its own; you can't safely vent two on the same passage.
Sizing is everything
A flue has to match the appliance. Too small and smoke backs up; too large and it never warms enough to draft — cold air just sits in it like a plug. That's the trouble with putting a sealed gas insert into a big open masonry flue, and it's why inserts get a correctly sized liner instead.
Draft depends on a warm, right-sized column of air, and getting that dimension wrong is behind a lot of the smoky-fireplace calls we take.
What we watch for in older WNC homes
Around Asheville and Weaverville we see plenty of chimneys from another era — cracked clay tiles, undersized or unlined flues, mortar joints gone soft. A compromised flue can leak heat and gases into the framing, which is a fire and carbon-monoxide risk.
Before we install or convert anything, we inspect the flue and line it if it needs it. If your chimney is old and you're not sure it's sound, call (828) 545-0525 for an inspection.
How we can help
Related questions
- What is a chimney liner and what does it do?
- What is a fireplace damper and what does it do?
- How does chimney draft work — why does smoke go up the chimney?
- What does a chimney cap actually do?
- What are the smoke shelf and smoke chamber in a fireplace?
- What is a direct vent gas fireplace and how does it work?
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