Soot vs. Creosote

Two different materials

Soot is mostly unburned carbon: very fine black particles carried up the flue with the exhaust. It settles on surfaces as a dry, powdery or fluffy layer that smudges when touched and brushes off easily.

Creosote forms in a different way. Blaze King's owner's manual explains that when wood is burned slowly it produces tar and other organic vapors, which combine with moisture to form creosote, and that these condense out of the smoke when flue temperatures drop below about 250°F. CSIA describes the result as a black or brown residue that can be crusty and flaky, tar-like and sticky, or shiny and hardened.

In a wood-burning chimney, the two are usually mixed. Sweeps often speak of "soot" for the light dry layer and "creosote" for anything heavier, which is why the words get used interchangeably at home.

The distinction still matters, because the two respond differently to the same habits. More air and drier wood cut creosote sharply. Soot is produced to some degree by almost any fire and accumulates steadily even in a well-run fireplace, which is one reason a chimney that is used regularly needs periodic sweeping no matter how carefully it is burned.

Which fuels make which deposit

CSIA lists restricted air, unseasoned wood and cooler-than-normal chimney temperatures as conditions that encourage creosote. EPA's Burn Wise guidance says to avoid burning wet, rotted, diseased or moldy wood, along with trash, painted or treated wood and other materials that can release harmful chemicals and damage an appliance.

Oil soot is its own problem

Oil burners leave a soot that behaves differently from wood soot. CSIA explains that oil soot is made up of carbon and sulfur, that it can adhere to the sides of the chimney and eventually fall to the base of a masonry chimney or back into the top of the appliance, and that the fallen buildup can restrict the flow of flue gases. The sulfur forms acid in the presence of moisture, which CSIA notes can corrode metal chimneys.

That is why an oil flue is serviced as a system: the connector is taken apart and cleaned, the base of the flue is emptied, and the liner is checked for corrosion. A sudden increase in oil soot, or soot puffing out around the burner or connector, is a reason to call the heating technician for the burner as well. The oil flue guide covers the liner and connector side in more depth.

What gas flues collect instead

CSIA describes gas as generally a clean-burning fuel, but notes that a gas chimney can become non-functional from bird nests or other debris blocking the flue. The more common deposit in a gas flue is water. DuraVent's guide to relining for gas appliances explains that efficient appliances produce cooler exhaust whose water vapor condenses inside the chimney, and that the resulting acids break down clay tiles, brick and mortar. Sand or small bits of masonry at the bottom of the flue can be a sign of that damage.

A gas appliance that leaves black marks around its burner, draft hood or vent is not working as intended. Turn it off if you can do so safely and have it checked by a qualified gas appliance technician before using it again.

How they behave when heated

Both soot and creosote burn. CSIA states that all forms of creosote are highly combustible, and that enough buildup combined with a hot enough flue can lead to a chimney fire. Dry soot burns too, though a thin layer of it carries less fuel than thick creosote.

The difference matters most with glazed creosote. It forms a hard, concentrated layer that is difficult to remove and, once ignited, burns intensely. CSIA's FAQ, summarizing the guidance in NFPA 211, says masonry fireplaces should be swept at 1/8 inch of sooty buildup, and sooner if there is any glaze present. See creosote stages, risk and removal and preventing chimney fires.

Reading the deposits as feedback

What comes out of a chimney is a report card on how the fireplace or stove is used.

• Mostly dry, light soot after a full season usually means the wood was dry and the fires had enough air.
• Crunchy or flaky creosote in thick layers points to fires that were often cooler than they should be.
• Sticky, tar-like deposits or glaze point to long smoldering burns, wet wood, a stove run too low, or a flue that is too large or too cold for the appliance.

On a stove, the glass gives a daily clue. Blaze King's manual notes that the best way to keep the glass clean is to run the stove on high for a period after each reload; glass that blackens quickly usually means the fire is running cool. Our firewood guide covers the most common fix.

How each one is removed

Dry soot and flaky creosote come off with brushing. Harder, crusty creosote may need more aggressive brushes or rotary tools; CSIA's FAQ notes that rotary cleaning can help with crusty deposits. Tar-like or glazed creosote usually has to be changed with a chemical treatment applied by a professional before it can be brushed out, and may take more than one visit.

Oil soot is vacuumed and brushed out of the connector and the base of the flue. In a gas flue, the work is usually about clearing debris and checking for condensation damage.

Products sold to burn in the fire and loosen creosote are a separate subject; see creosote sweeping logs. For the service itself, see chimney cleaning.

Questions homeowners ask

Sources

1. CSIA: Chimney fires and creosote
2. CSIA: Frequently Asked Questions
3. CSIA: Annual service of your oil-fired appliance chimney
4. Blaze King Chinook CK24 catalytic wood stove owner's manual (PDF)
5. DuraVent: Relining Chimneys for Gas Appliances (technical guide, PDF)
6. U.S. EPA Burn Wise: Best Wood-Burning Practices

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