Creosote, from soft soot to hard glaze

How smoke becomes a deposit

Wood does not burn all the way in a fireplace. Along with heat and carbon dioxide, the fire sends up water vapor, unburned gases, tiny droplets of tar and fine particles of carbon. As long as that mixture stays hot, it rides the draft out of the chimney. When it meets a surface cool enough, part of it condenses and sticks.

The inside of the flue is that surface. The cooler the flue walls and the slower the smoke moves past them, the more condenses on the way up. Creosote is therefore thickest where the flue runs coolest, which can be the upper part of the chimney and any stretch exposed to outside air, not just above the fire.

Creosote stages, as sweeps describe them

Creosote is graded by how it looks and how hard it is to remove. The stages are trade descriptions, not code terms, but they are useful because each one calls for a different response.

• Stage one: dry, black soot, powdery or lightly flaky. A chimney brush removes it easily.
• Stage two: crunchy, shiny black flakes with bits of tar through them, sometimes thick enough to narrow the flue. Stiffer brushes and more passes take it off.
• Stage three: a glaze. Tar has run, pooled and hardened into a glassy coating that brushes slide over. It needs removal methods beyond ordinary sweeping.

What makes creosote buildup worse

Anything that cools the smoke or makes it dirtier adds to the deposit. EPA Burn Wise stresses two of them: wood seasoned six months or longer, and fires built hot, because smoldering fires waste fuel and are unsafe. Wet wood spends its heat boiling off water, which cools the smoke and adds vapor to it.

• Unseasoned or wet firewood.
• Low, smoky fires with the air choked down to stretch a load of wood.
• A flue much larger than the fireplace needs, so the smoke rises slowly and cools.
• A chimney on an outside wall, where the flue walls lose heat faster.
• Burning trash, cardboard or treated wood, which the EPA advises against.

Why creosote matters

Every stage of creosote in the chimney is fuel. A chimney fire starts when a hot fire below ignites the deposit, and it can burn fast and loud or slow and quiet. Either way, the heat inside the flue can crack clay tiles and push heat toward the framing around the chimney, so a flue that has been through a fire gets checked before its next use.

Creosote also narrows the flue, which slows the draft and can push smoke into the room. And it smells. The sharp, smoky odor of an idle fireplace can be creosote, and it gets stronger whenever air moves down the flue and into the room rather than up.

What a chimney fire looks and sounds like

Some chimney fires announce themselves: a roaring sound like a train or a jet, loud cracking and popping, dense smoke, and sometimes flames or sparks leaving the top of the chimney. If that happens, take everyone outside and call 911 from there. Do not stay to fight it.

Others burn slowly and quietly and are only discovered later. Signs left behind include puffy, honeycombed creosote in the flue, a warped damper or cap, cracked flue tiles, discolored metal at the top and flakes of burnt creosote on the roof or the lawn. A flue with any of those signs is checked before the next fire.

How much is too much

No homeowner test replaces a look at the whole flue. NFPA 211 sets a yearly inspection as the baseline, covering soundness, deposits and clearances, with cleaning whenever that inspection finds it necessary. A fireplace used a few times a season may need little cleaning. One used for long fires every week, or fed wet wood, can build a heavy deposit within a single season.

From the firebox you can get a rough idea. With the damper open and eye protection on, look up at the damper frame and the lower smoke chamber with a flashlight. A thin, dry, sooty coating is one thing. A thick, crusty or shiny black layer means the flue should be cleaned and checked before another fire.

Getting it out

Stage one and stage two deposits are the ordinary work of a chimney sweep, who brushes the flue and the smoke chamber, clears the smoke shelf and removes the debris with the room protected. Stage three glaze is a separate job, because brushes will not lift it. Our creosote removal page covers the rotary and chemical methods used for glaze, and why the liner is checked before either one is chosen.

Removal is also the moment to find out why the deposit formed. Changing the wood, the size of the fires or the flue itself is what keeps it from coming back.

Whoever does the work should be able to tell you which stage they found, where in the flue it was heaviest, and whether anything under it needs attention. That record is also a useful baseline for next year's check.

Metal chimneys and liners collect it too

Creosote is not only a masonry problem. The metal chimney of a factory-built fireplace and a stainless steel liner inside a masonry chimney both collect deposits the same way, and they are cleaned with the tools and methods their makers allow. A chimney fire in a factory-built chimney can damage the chimney sections themselves, and damaged sections have to be replaced with parts from that same listed system.

Creosote and gas fireplaces

Creosote comes from burning wood. A properly working gas fireplace or gas log set does not produce it, though a gas appliance that burns poorly can leave soot. When a fireplace that burned wood for years is converted to gas, the old flue still holds whatever creosote was in it, and it is cleaned and checked before the gas appliance is vented through it, with the venting following that appliance's listing.