How Hot Does a Chimney Get?

Why the bottom is the hot end

Every chimney has a temperature gradient. The firebox, the throat and the first few feet of flue take the full heat of the fire, and from there the gas loses heat continuously to the surrounding tile and brick. Masonry is heavy and absorbs a lot of energy before it warms through, so the flue gas reaching the outlet of a working fireplace is far cooler than the flames that produced it.

That gradient explains several things a technician sees on inspection. Firebrick and refractory mortar are reserved for the firebox because ordinary brick and mortar are not made for direct flame. Damage from overheating shows up low, around the damper and smoke chamber. Damage from running too cool, such as creosote buildup and condensation stains, tends to concentrate in the upper flue and around the crown.

Approximate temperatures at a glance

The figures below are approximate and widely used in the trade. Real readings vary with the fuel, the appliance, the size of the fire, the weather and how big the flue is.

How each appliance heats its flue

An open wood fireplace sends a great deal of heat and room air straight up, so its lower flue gets very hot during a big fire, and the masonry around it warms over hours. A wood stove or insert behaves differently. Its connector pipe can run extremely hot close to the stove, but a slow, damped-down burn sends cool, smoky exhaust into the flue, especially when a small stove is set into a large masonry chimney.

Oil and gas boilers heat their flue in cycles. While the burner runs, exhaust rises warm and steady; when it shuts off, the flue cools until the next call for heat. Older gas water heaters and boilers produce warm, very moist exhaust. High-efficiency condensing appliances pull so much heat out of the gas that their exhaust leaves warm rather than hot, which is why their makers call for plastic vent pipe and why they should not be vented into an old masonry flue as if nothing had changed. In a basement where a boiler and water heater both burn gas into one old flue, the chimney may spend much of the winter only lukewarm inside, a condition that matters more than most owners expect.

Running too cool causes damage too

Heat is not the only enemy. When wood smoke cools below roughly 250°F, the tar and vapor in it condense on the flue walls as creosote, first as loose soot, then as crunchy, tar-like flakes, and eventually as a hard, shiny glaze that is difficult to remove and burns fiercely. Slow, smoldering fires, wet wood and an oversized or exterior flue on a cold wall all speed that up.

Gas and oil exhaust has its own cool-flue problem. Once natural gas exhaust drops below about 130°F, its water vapor condenses into liquid carrying combustion acids. In an unlined or oversized masonry flue, that acidic water soaks the mortar, flakes off tile and shows up as white staining on the outside of the stack or a rusty puddle at the base cleanout. Freeze-thaw winters then work on the wet brick from the outside.

Heat through the walls: masonry, clearances and slow charring

Brick conducts heat slowly, but it does conduct it. During a long fire, the chimney breast above an open fireplace can become warm to the touch, and that heat keeps moving outward to anything resting against the masonry: floor joists, wall studs, a wood mantel, trim. Building codes and appliance makers set required air gaps between chimneys and combustible materials for exactly this reason.

Wood exposed to moderate heat over many seasons changes. It dries, darkens and turns into charcoal-like material, a process called pyrolysis, and charred wood can ignite at a lower temperature than fresh lumber. Older houses where framing was built tight against the masonry, or where a mantel was added later without regard to the opening, are the usual places to find scorched wood when a wall is opened. For factory-built fireplaces and metal chimneys, the clearances in the installation instructions are the rule for that unit.

What a chimney fire does at those temperatures

Once creosote ignites, the fire feeds on the deposits along the flue walls and burns far hotter than the fire in the firebox. Clay tile does not tolerate sudden, uneven heating well; it cracks and can split away from its joints. Metal liners can warp and their seams can open. Mortar between tiles can be blown out, leaving gaps through which heat and exhaust reach the brick and the house framing.

Some chimney fires are loud, with a roaring sound, popping and sparks or flame at the outlet. Others smolder quietly and are found only later by the puffy, honeycomb-like creosote they leave behind. NFPA 211 calls for the chimney to be inspected after a chimney fire, or after any other event that may have damaged it, before it goes back into service.

Reading the heat in your own system

On a wood stove, a magnetic or probe thermometer on the connector pipe shows whether you are burning in the range the stove's manual recommends, which helps avoid both overfiring and a cool, creosote-making burn. With an open fireplace, signs of trouble are visual: paint or wallpaper discoloring on the chimney breast, a warped damper plate, crazed or crumbling firebrick, and glazed, shiny deposits above the damper. In the basement, a boiler connector pipe that is rusted through, or a draft hood with soot or scorch marks around its edge, says the exhaust is not leaving the way it should.

During an inspection, a technician looks for those same clues along the full height of the flue, checks the smoke chamber and the joints between tiles, and examines whatever framing and trim can be seen next to the masonry, including from the attic. Heat damage found early is usually a repair; found after years of use, it can mean relining the flue or opening a wall.