Creosote: Stages, Risk and Removal

What creosote is

Wood smoke is a mix of gases, water vapor, unburned particles and tar-like vapors. CSIA describes how these byproducts move up a relatively cool chimney and condense on its walls. The residue that sticks is creosote.

Soot is the dry, black, powdery part. Creosote is the broader term for the deposits, which range from dry soot to sticky tar to a hard glaze. Sweeps often use "soot" for the light material and "creosote" or "glaze" for the heavier forms, but they are all products of incomplete combustion.

The three forms, often called stages

CSIA describes creosote as crusty and flaky, tar-like and sticky, or shiny and hardened. Many sweeps number these as stages or degrees.

CSIA's homeowner FAQ notes that the glazed form is the one that cannot be removed by brushing alone and needs a chemical treatment before mechanical removal.

Why creosote is a fire risk

All forms of creosote are combustible. When enough builds up and the flue gets hot enough, the deposits can ignite inside the chimney. That is a chimney fire.

Testing by the National Bureau of Standards (now NIST) measured flue gas temperatures during creosote fires ranging from roughly 1,600°F to over 2,500°F, with chimney wall surfaces in some tests exceeding 2,000°F. Temperatures that high can crack clay liners, damage mortar and, in some factory-built chimneys, buckle metal.

CSIA notes that many chimney fires are slow-burning and go unnoticed by the people in the house, while still causing damage that only shows up during an inspection. Glazed creosote is a particular concern because it is dense, hard to remove and can burn intensely.

If you suspect a chimney fire is happening now, such as a loud roaring sound, heavy smoke or flames from the top of the chimney, get everyone out of the house and call 911. For what to look for afterward, see Signs you've had a chimney fire.

What makes creosote build up faster

CSIA lists three main conditions: restricted air supply, unseasoned wood and cooler-than-normal chimney temperatures. In everyday terms:

• Wet or green wood. Energy goes into boiling off water, so the smoke is cooler and wetter. See Burning the right firewood.
• Starved fires. A damper not fully open, glass doors closed on an open fireplace, or a stove turned down for long overnight burns all limit air and lower combustion temperature.
• Overloaded stoves. CSIA notes that packing a stove full to make the fire last longer can increase creosote.
• Cold or oversized flues. An exterior chimney on a cold wall, or a large flue serving a small stove, lets smoke cool before it exits.
• Short, smoky fires. A flue that never fully warms up collects more condensation.

Where creosote collects in different systems

• Open masonry fireplaces. Deposits often concentrate in the smoke chamber, on the smoke shelf and on rough corbeled brick, where smoke slows and turns. See What is a smoke chamber?
• Wood stoves. The connector pipe and the upper part of the chimney, where flue gases are coolest, are common trouble spots. Modern stoves that burn efficiently can produce less creosote, but low, slow overnight burns can still build it quickly.
• Inserts. An insert vented into a large fireplace flue without a properly sized liner can produce heavy deposits in the oversized space above it. That is one reason inserts are normally installed with a liner.
• Exterior chimneys. A chimney on an outside wall runs cooler than one through the middle of the house, so the upper flue often collects more.
• Factory-built chimneys. Deposits on metal surfaces are acidic and can contribute to corrosion, which is why manufacturers set low cleaning thresholds.

What you can see, and what you cannot

With the fireplace cold and the damper open, a flashlight will show the smoke chamber walls and perhaps the first few feet of the flue. Dry, flaky black deposits are common. A shiny, bubbled or dripping black coating is a sign of glaze and worth having looked at.

What you cannot see from below is how much is in the rest of the flue, especially in offsets, at the top where the flue is coldest, or in a stove liner. Inspections use brushes, mirrors and cameras to see the whole run. Please do not climb on the roof or reach into the flue to check.

Mechanical vs. chemical removal

Mechanical removal means physically scraping deposits off the flue walls. Tools range from wire and poly brushes on rods to rotary systems that spin cables or brushes inside the flue. Brushes are matched to the flue size and liner material so they clean without damaging it. This is the core of every chimney cleaning.

Chemical treatments come in two kinds:

• Professional treatments applied by a technician to glazed creosote. They change the hard glaze into a more brittle or flaky material that can then be removed mechanically. This can take more than one application and visit.
• Consumer products, such as creosote-treating logs or powders burned in the fire. CSIA says these can supplement but never replace mechanical sweeping and annual inspection. The loosened deposits still have to be removed, and a product cannot tell you whether the liner is damaged.

When glaze is very heavy or the liner is damaged beneath it, removal alone may not be the end of the job. Relining or flue repair may be discussed after the flue is cleaned and evaluated.

Keeping creosote down

1. Burn dry, split, seasoned wood. EPA recommends moisture content under 20 percent.
2. Open the damper fully and give the fire enough air, especially when starting it.
3. Burn hotter, shorter fires rather than long smoldering ones.
4. In a stove, follow the manual's guidance on loading and air settings.
5. Have the chimney inspected every year and swept when buildup reaches the guideline. See How often should a chimney be swept?

When it is time, see chimney cleaning or, for stoves and inserts, fireplace and wood stove cleaning.

Questions homeowners ask

Sources

1. CSIA: Chimney fires and creosote
2. CSIA: Preventing chimney fires
3. CSIA: Homeowner FAQs
4. National Bureau of Standards (NIST), NBSIR 83-2771: Intensity and duration of chimney fires in several chimneys (PDF)
5. U.S. EPA Burn Wise: Best wood-burning practices

This guide is general information. It can't diagnose a specific chimney; an inspection can.