What is a chimney flue?
Updated August 2026
The chimney flue is the passage inside a chimney that carries smoke and combustion gases from the fireplace or furnace up above the roof. In any chimney built to modern code that passage is lined, usually with stacked clay tiles or a stainless steel pipe, because bare masonry cannot safely contain 1,000-degree gases or the acidic condensate a gas appliance produces. Each appliance gets its own flue, and one chimney can hold two or three of them side by side.
Where it is
There are two ways to see yours. From outside, the clay tiles poking above the chimney crown are the tops of the flues, and a round metal pipe up there means a steel liner has already been installed. From inside, open the damper, put your head into the firebox with a flashlight, and look up: past the smoke shelf you should see a smooth square or round shaft with even mortar joints between tile sections. Gaps, missing mortar, flaking tile, or a shiny black tar coating are all findings worth a sweep's camera. Furnace and water heater flues hide better; follow the metal vent pipe from the appliance to where it enters the chimney.
What it does
A flue does two jobs at once. It contains heat, and it creates draft, the pressure difference that pulls air through the fire and pushes exhaust out the top. Sizing controls both: a flue too large for its appliance drafts weakly and lets flue gas cool and condense on the walls, while one too small spills smoke into the room. The failure modes are the whole reason chimneys get inspected. Creosote glazing on the tile is fuel for a chimney fire that can reach 2,000 degrees and crack every liner section it passes. Cracked tile, or an unlined masonry flue serving a gas furnace, lets carbon monoxide seep through the brick into bedrooms, which is why a working CO alarm on every level belongs in any house with a chimney.
Maintenance
The sweeps' rule is a measurement, not a calendar: sweep when creosote reaches 1/8 inch on the flue walls. For a household burning a cord or two of wood a season that lands at once a year, twice for heavy burners or anyone feeding it unseasoned wood. Book the inspection and sweep in late summer, before sweeps are booked three weeks out. A sweep runs $150 to $350, and a camera-assisted Level 2 inspection, standard at a home sale or after a chimney fire, is $200 to $500. Gas-only flues build no creosote but still need annual eyes on them for rust, blockages, and bird nests. Skipping it is how a $200 habit becomes a fire claim.
What it costs to replace
A replacement chimney flue runs $2,000–5,000 for a stainless steel reline, against $150–350 for a yearly sweep.
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Frequently asked questions
Does my chimney need a liner?
If it vents anything, effectively yes. Modern codes require a liner sized to the appliance, and most insurers and home inspectors treat an unlined masonry flue as a defect. Clay tile liners came standard from roughly the 1940s onward, so older chimneys are often bare brick inside. A stainless steel liner is also required when a wood stove insert or a high-efficiency appliance goes into an old fireplace, because the original flue is far too large to draft it properly.
How often does a chimney flue need sweeping?
Once a year is the working answer for a wood-burning fireplace or stove, with the real trigger being 1/8 inch of creosote. A household burning two or three cords a winter, or burning wood over 20 percent moisture content, can hit that by midseason and need a second visit. Pellet and gas appliances build little or no creosote, so they need an annual inspection rather than a full sweep, and the inspection still finds nests, rust, and blockages.
Can two appliances share one flue?
Almost never in a modern install. A wood-burning fireplace needs its own flue, and pairing a wood appliance with a gas one is prohibited outright: the two draft differently, and exhaust from one can spill into the house through the other. Older homes sometimes have a furnace and water heater sharing a correctly sized flue, which is allowed under specific conditions. Any appliance swap on that chimney should trigger a fresh sizing check.
What does relining a chimney cost?
A stainless liner for a typical two-story chimney runs $2,000 to $5,000 installed, driven by flue height, diameter, whether the run has offsets, and whether insulation gets wrapped around the pipe. Cast-in-place liners cost more and get used when the masonry itself needs reinforcement. It is a real bill, but it buys 20-plus years, and the alternative on a badly deteriorated chimney is a rebuild at roughly three times the price.