What Creosote Is and How Chimney Fires Start

Every fire in a stove or fireplace produces smoke, and smoke is fuel that has not finished burning. When that smoke meets a flue wall cool enough, it condenses. Repeated over weeks, the deposit builds into creosote: first a light soot, then a flaky crust, and eventually a hard, shiny glaze that behaves like fuel because it is fuel.

EPA puts the consequence plainly in its material on pellet stoves, noting that creosote is the main cause of chimney fires.

Key figures

• below 20 percent EPA firewood moisture target
• at least six months Minimum seasoning before checking firewood moisture, EPA

What produces it

• Wet wood. Water in the wood absorbs heat and cools the gases. EPA’s guidance is to burn wood below 20 percent moisture, seasoned for at least six months.
• Smouldering fires. Shutting the air down to make a load last overnight is the classic cause, and it is usually a sign the stove is too large for the space.
• A cool flue. Exterior chimneys on a cold wall, oversized masonry flues and uninsulated runs all let gases condense before they reach the top.
• An oversized flue for the appliance, which is why a modern stove vented into an old fireplace chimney without a liner is a bad combination.
• Burning the wrong things. Rubbish, cardboard, treated or painted wood: all produce more deposit, and worse deposits.

What a chimney fire is like

The dramatic version is unmistakable: a roaring or rumbling sound like a freight train, dense smoke and sparks from the chimney top, and a connector pipe that may glow. Neighbours often notice before the household does.

The quiet version is more common and more dangerous in the long run. A slow-burning chimney fire may make little noise and produce no visible drama, while heating the flue enough to crack a clay liner or damage the masonry. The household then keeps using a chimney that no longer contains fire safely, and the next one has a route into the structure.

That is the real argument for an annual inspection: a sweep can see the evidence of a fire you did not know you had, including puffy or honeycombed creosote, warped connectors, discoloured caps and cracked flue tiles.

If you think you have one, right now

1. Get everyone out of the house.
2. Call the fire service. Do it early. A chimney fire is contained until it is not, and the crew would far rather attend a false alarm than a house fire.
3. If it is safe, close the stove’s air controls, which starves the fire of oxygen. Do not open the stove door.
4. Do not throw water into a hot stove or flue. Rapid steam and thermal shock can crack the liner or burst the appliance.
5. Move anything burnable away from the hearth and the connector if that can be done safely from a distance.
6. Watch outside for sparks landing on the roof or in nearby vegetation, and tell the crew where the chimney runs inside the house.

After any chimney fire

• Do not use the appliance again until the chimney has been inspected professionally.
• Ask for a written report covering the full flue: liner, connector, crown, cap and the structure around it.
• Expect repairs. A clay liner that has cracked is no longer a liner, and the usual fix is a stainless steel relining.
• Tell your insurer. Chimney fires are commonly covered, and an undocumented one becomes an argument later.
• Change what caused it. Almost always the answer is wet wood, a smouldering habit, or a flue that never suited the appliance.

Prevention, in the order that works

1. Burn dry wood. Below 20 percent moisture, split and seasoned for at least six months, checked with a meter rather than by eye.
2. Burn hot, not slow. Bright fires with adequate air, and heat you can live with, rather than a damped-down load that smoulders all night.
3. Right-size the appliance, so that burning it properly does not cook you out of the room.
4. Line and insulate the flue to suit the stove, so gases stay hot enough to leave.
5. Sweep and inspect annually, and more often if you burn heavily.
6. Keep the three-foot rule around the appliance: the US Fire Administration’s guidance is that anything that can burn stays at least three feet from the heat source.

Related reading: Clearances, Hearth Pads and the Inspection and How Often a Chimney Actually Needs Sweeping.

Questions people ask

What causes creosote in a chimney?

Unburned smoke condensing on a flue wall that is cool enough. Wet wood, smouldering fires with the air shut down, an oversized or uninsulated flue and burning rubbish all increase it. EPA notes creosote is the main cause of chimney fires.

What does a chimney fire sound like?

The obvious kind roars or rumbles, with dense smoke and sparks from the chimney top. The more common kind is quiet and slow, which is why annual inspections matter: a sweep can find evidence of a fire the household never noticed.

What should I do if my chimney catches fire?

Get everyone out and call the fire service first. If it is safe, close the stove's air controls to starve the fire, but do not open the stove door or throw water into a hot flue. Have the chimney professionally inspected before using it again.

Do creosote-removing logs work?

They can change a hard deposit into one a brush removes more easily, which is useful only if a sweep then removes it. They do not clean a chimney, do not inspect it and do not repair a cracked liner or an oversized flue.

Where these figures come from

1. Pellet Stoves US Environmental Protection Agency, Burn Wise · epa.gov
2. Best Wood-Burning Practices US Environmental Protection Agency, Burn Wise · epa.gov
3. Heating Fire Safety US Fire Administration, FEMA · usfa.fema.gov

Figures last checked September 2026.