What Is Creosote? (And Why It Causes Chimney Fires)

Where creosote comes from

Burning wood is a two-part process. The fire heats the wood until it releases combustible gases, and then those gases burn. When a fire is running hot and getting enough air, most of them burn in the firebox and what goes up the flue is mostly carbon dioxide and water vapour.

When the fire is cooler, or starved of air, a large share of those gases leave unburned. They travel up the flue still carrying tars and moisture, and as they cool against the liner wall they condense onto it — the same way steam condenses on a cold window. What sticks there is creosote.

That is the whole mechanism, and it explains everything else about creosote: anything that makes smoke cooler or smokier makes more of it, and anything that keeps the flue hot makes less.

The three stages of creosote

Creosote hardens over time and with repeated heating, and it is conventionally described in three stages. The stage matters practically, because each one takes a different method to remove and costs a different amount.

A brush will not touch stage-three glaze at any price — it is chemically fused to the liner and has to be broken down before it can be removed at all. If someone offers to take glazed creosote out in one visit with a brush, they are either not going to remove it or not going to mention that they did not.

Why it is dangerous

Two reasons, and the second one is less obvious than the first.

The obvious one: creosote is fuel, and it is sitting inside a shaft that regularly gets hot. A sufficiently hot fire — an overloaded firebox, a load of dry pallet wood, a flare-up — can raise flue temperatures enough to ignite it. A chimney fire can burn at temperatures far above what the flue was built to handle.

The less obvious one: the damage a chimney fire does is often not visible and not immediately dangerous. It cracks clay liner tiles. A cracked liner is a direct path for heat and combustion gases into the framing built against the chimney — which means the house fire that follows a chimney fire can happen days or weeks later, in a home whose occupants believed the incident was over. This is why NFPA 211 calls for a Level 2 camera inspection after any chimney fire, and why "it went out on its own" is not the end of the story.

What makes creosote build up faster

• Unseasoned wood. The single biggest cause. Fresh-cut wood is close to half water by weight, and burning it spends the fire's energy boiling that water off rather than producing heat. The result is a cool, smoky fire — exactly the condition that produces creosote.
• Damping the fire down. Choking the air supply for a long overnight burn produces a smouldering fire and maximum condensation. It is the same problem, deliberately induced.
• An oversized flue. Too much cold surface area means the smoke cools before it reaches the top. Common where a fireplace was built for coal, or where a modern insert vents into a full-size masonry flue.
• An exterior chimney. A chimney on an outside wall, particularly a north-facing one, stays cold. In a Central New York winter it condenses on almost every burn.
• Softwood and construction scrap. Resinous softwoods produce more creosote than seasoned hardwood. Treated lumber, painted wood, and manufactured board should never go in a fireplace at all.
• Restricted air supply. A partly closed damper or a tightly sealed house that starves the fire of combustion air.

How to slow it down

1. Season your wood properly. Split, stacked off the ground, covered on top but open at the sides, for six to twelve months. In a climate as damp as Central New York, closer to twelve. Seasoned wood is darker at the ends, has visible checking, and sounds hollow rather than dull when two pieces are knocked together.
2. Burn hot, shorter fires rather than long smouldering ones. A hot fire burns the gases in the firebox instead of depositing them in the flue.
3. Burn hardwood. Oak, maple, ash, beech, birch — all common in this region and all better than softwood for this purpose.
4. Give it air. Resist the temptation to damp right down overnight. If you need long burns, a modern appliance designed for them is a better answer than choking an open fireplace.
5. Correct an oversized flue. If your appliance vents into a flue far larger than it needs, no amount of good burning practice fully solves the problem. A correctly sized liner does.
6. Get it swept annually. The point at which buildup warrants a sweep is roughly an eighth of an inch on the liner wall, which most regular burners reach in one season.

How to tell if you have a problem

• A hard, shiny, black coating visible on the damper or the top of the firebox.
• A strong tarry or barbecue smell from the fireplace, particularly in humid weather.
• The fire drawing poorly, or smoke pushing back into the room.
• Black flakes or debris falling into the firebox.
• Two or more heating seasons since the chimney was last swept.

A flashlight and a look at the damper tells you a lot. What it cannot tell you is how far the buildup extends up the flue, because the middle of a chimney is only visible on camera.

Creosote in gas and pellet appliances

Gas appliances produce essentially no creosote — there is no wood tar to condense. What they do produce is water vapour and acidic condensate, which attacks clay tile and mortar over time. So a gas flue still needs its annual inspection, just not a sweep.

Pellet stoves produce far less creosote than a wood fire because the fuel is dry and manufactured to a consistent moisture content, and the burn is fan-controlled. They still produce ash and fly ash that accumulates in the vent, and they still need annual service.

Related reading

• Signs of a chimney fire
• How often should a chimney be cleaned?
• Chimney cleaning cost by creosote stage
• Chimney sweep cost