Creosote leaking from a chimney (C) Daniel FriedmanCreosote Deposits & Fire Hazards in Chimneys

  • CREOSOTE FIRE HAZARDS - CONTENTS: What is creosote, how does it form in a chimney. Why is chimney creosote a serious fire hazard. How can we minimize the fire risk from creosote deposits. How often should a chimney be cleaned
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All about chimney creosote: cause, cure, prevention, chimney creosote & chimney cleaning advice:

This article describes the formation of creosote in wood-burning heating appliances such as woodstoves and fireplaces and gives advice on reducing the fire risk. Creosote deposits accumulate in all types of chimneys - masonry, or metal, where wood burning appliances are vented.

Creosote is combustible and if it is set afire the resulting chimney fire is extremely hot. Roaring like a freight train, a chimney fire can melt through even a well-built "safe" chimney, setting the building on fire.

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Check Chimneys for Creosote Deposits - a Serious Fire Hazard

Chimney Creosote Hazards (C) Carson Dunlop Associates

Definition of Creosote

Creosote is a black, oily wood-tar condensate that forms in chimneys over fireplaces or woodstoves is a black tarry condensate that forms inside of chimneys and flues when burning wood in fireplaces and woodstoves.

Carson Dunlop Associate's Sketch at left shows common areas where creosote forms in a fireplace chimney. Creosote is a significant risk in all wood-burning appliances (such as wood stoves) and can also form in other heaters.

Watch out: creosote will burn at a very high temperature - it is combustible. That's why creosote deposits risk a dangerous chimney fire.

Creosote, in the form of coal tar, is chemically similar to wood-tar (but not identical) but is formed in bituminous coal or lignite coal-burning appliances. Creosote is also be formed by burning peat, and even fuel oils such as home heating oil.

Creosote tar, a natural byproduct of burning organic-based fuels (wood, coal, peat), is carried into the chimney as a vapor in the smoke. But this creosote tar vapor condenses as a liquid on to the cooler chimney sides as smoke passes up the chimney and flue.

As the creosote deposits cool they harden to a shiny black coating that can be difficult to remove, especially if the creosote deposits are left over a period of time.

Creosote will continue to accumulate on chimney surfaces until it is removed by one of the methods we describe below.

How & When does a Creosote Fire Ignite in a Chimney?

Chimney fire (C) D Friedman

Watch out: One of the most common and serious fire hazards in chimneys occurs when deposits of creosote build up on the chimney interior are later set on fire. A serious chimney fire can occur regardless of the chimney type: masonry, metal, insulated metal, etc.

Our photo (left) shows a home damaged by a house fire: a chimney fire originated at a chimney venting a woodstove. The owners were away and a house-sitter made a nice hot fire in the woodstove.

The fire ignited at a metal chimney (not shown, around the right corner of the home in the photo), spread through the attic roof, and burned back down the building.

A creosote-based chimney fire occurs when an accumulated deposit of creosote is ignited by a hotter-than-usual fire below.

Once ignited, the deposits of creosote burn at very high temperature, so hot, in fact, that depending on the amount of creosote that is burning, a runaway chimney fire occurs, making a sound like a roaring freight train. A chimney fire damages the chimney leaving it unsafe, and worse, there is a high risk of setting the building itself on fire.

The chances that a chimney fire will occur depend on the amount of accumulated creosote deposits, its location, and the temperature of fires being ignited below.

The chance that a chimney fire will ignite the building on fire also depend on the chimney condition, materials, construction, building materials, and most important as well, the fire clearances that were maintained when the building and chimney were constructed. See the "Fire Clearances" articles at the "More Reading" links at the bottom of this article for details.

What Determines the Rate of Creosote Build-up in a Chimney? Causes of Creosote Trouble:

Fire damaged home (C) Daniel FriedmanOur photo (left) illustrates extensive fire damage to a home by fire originating not in a chimney but in the home's kitchen. The use of fire-retardant plywood in the roof was credited with slowing the fire's spread to the rest of the home.

The rate of creosote build-up in a chimney depends on these factors:

  1. The temperatures in the chimney or flue: A long low-smoldering wood fire produces more creosote than a hotter faster-burning wood fire.

    That's because burning wood at a lower temperature results in more incomplete combustion of the wood itself. Incomplete combustion of a fuel results in a higher level of off-gassing of incompletely burned chemicals in gaseous form, including, in the case of wood fires, wood-tar.

    Watch out: your wood stove or coal stove manufacturer will provide the recommended operating temperature range for the appliance. Typically the chimney flue temperatures should be over 400 degF. Too cool means rapid creosote formation. Too hot also increases the risk of a chimney fire by threatening to ignite creosote deposits or by exceeding the temperature ratings of the chimney.
  2. Level of usage: The frequency of wood/coal fires and the amount of wood or coal burned
  3. The wood-sap content of the wood being burned. Green wood that has not cured contains more sap and produces more wood tar than thoroughly-dried firewood.
  4. Moisture content of the wood being burned. Wood, even wood that is well seasoned, should be protected from rain and snow. If the wood is wet it will burn more slowly and will create more creosote in the chimney.
  5. The wood species (or to a smaller degree the type of coal or peat) being burned.
  6. The chimney size, height, location, and construction materials serving the heating appliance. Cool chimney surfaces in a large masonry chimney venting a very small wood-fire will cause more creosote to condense on the chimney sides at a faster rate than a smaller chimney flue with better draft and warmer surfaces.
    1. Chimney flue size: a chimney flue that is too large for the heating appliance may fail to develop adequate draft, further increasing creosote deposition. Installing a wood-stove insert or coal stove insert into an older fireplace and masonry chimney may result in a chimney that is too large to properly vent the new heater.
    2. Unlined flues: a professional woodstove installer may insist on re-lining the chimney by one of several methods, such as by installing a smaller-diameter chimney flue liner up the entire height of the chimney. At installations we made we also sealed the bottom of the large original chimney flue at the base (around the new smaller flue insert) and poured a non-combustible insulation around the new flue liner (vermiculite).
    3. Chimney location: a chimney constructed on an outside wall of the building, whether masonry or metal, will be cooler than a chimney routed through the building interior.
    4. Chimney height: a tall chimney improves draft at the fire once it has become fully heated, but may take longer to warm sufficiently, collecting creosote in its cooler upper length.

      Watch out: In some cases, such as a home venting only a gas fired water heater into an older masonry flue, the heating appliance never was able to heat the chimney sufficiently to cause adequate draft, leading to dangerous carbon monoxide gas escape into the building. This carbon monoxide hazard can also occur with coal stoves r even woodstove.
    5. Metal flue chimneys and vents, particularly single-wall flue vent connectors tend to be cooler and to collect creosote faster than warmer, insulated metal flues.
    6. Chimney cap: a chimney cap not only prevents rain from entering (and possibly damaging) the chimney, it improves draft and prevents downdrafts - conditions that can increase the rate of creosote formation.

      Watch out: using an un-listed decorative chimney top shroud is inviting a dangerous chimney fire. At CHIMNEY SHROUD FIRE we describe a fire damage ascribed to an un-listed chimney top decorative shroud on a wood frame chimney chase top at a home in Moorpark CA.
    7. Chimney cleanout openings: a masonry fireplace and chimney often includes an ash pit cleanout door below the fireplace. If this door is left open the result is reduced draft in the chimney (and a fire hazard).
    8. Wood-framed chimney chase (to enclose a factory-built chimney assembly) construction, materials, fire blocking, code compliance: see CHIMNEY CHASE CONSTRUCTION for details.
  7. Adequacy of combustion air supply: just as a flue that is too large to heat up and properly vent the wood or coal stove venting into it, if the heater has inadequate combustion air the result will be incomplete combustion of the fuel and a higher creosote production rate.
  8. Anything else that affects chimney draft. Slowing the chimney draft, such as by closing a damper in a woodstove flue, increases the residence time of the smoke in the flue, increasing the rate of creosote deposition.
  9. The frequency of chimney cleaning and the thickness of existing creosote deposits. As creosote builds up inside of a chimney or flue, its thickness reduces the internal dimensions of the chimney, thus reducing draft, thus slowing the combustion rate of the fire below still more.

    As the combustion rate slows, it becomes still less complete, thus producing creosote at an increasing rate. In other word, a dirty chimney with a thick deposit of creosote causes more creosote to be formed and deposited at a still faster rate.

How to spot a creosote problem in the chimney & what to do about it

Jotul No. 118 wood stove cleaned of creosote and avoiding a chimney fire (C) Daniel Friedman Paul Galow Jotul No. 118 wood stove cleaned of creosote and avoiding a chimney fire (C) Daniel Friedman Paul Galow

Watch out: a chimney fire that has ignited creosote burns very hot, sounds like a roaring freight train, and can burn down the building. If you encounter these conditions you need to get everyone out of the building and you need to call the fire department immediately. If it is possible to safely close down air supply to the chimney that might help but your first priority is saving lives.

In January 2015 our buddy Paul G. reported having successfully detected and removed a creosote problem in a woodstove just in time before recent very cold weather and just before a predicted record blizzard in New York. Lighting the wood stove Paul observed that the stove was smoking from various leak points at the chimney connections and at various wood stove openings.

Outside he saw that some smoke was coming from the wood stove's chimney top but inside the stove was smoking and draft (a match at the air intake) was poor.

Paul shut down the wood stove air supply immediately. When the fire had cooled enough to safely do so he finished extinguishing the fire by carefully spraying a bit of water onto the coals. He waited a bit because he knew that doing this while a wood stove is hot could make matters worse by causing breakage or even exploding parts.

When the fire was out in the wood stove Paul was able to check the chimney top closely: undaunted by being an old guy in his 80's he clambered up onto the snow-covered roof to check the wood-stove chimney. There he saw that the spark screen was mostly occluded by creosote. Removing the cap gave access to clean the flue - ash and creosote fell down the wood stove flue into the top of the wood stove where it was removed and saved for me.

Paul didn't have a proper chimney cleaning brush kit (see the chimney brush chart at below left) so he banged on the metal flue until the creosote and crud fell into the wood stove.

When the wood stove chimney was clean it was inspected for safety and deemed usable.

Chimney cleaning brush specifications (C) Daniel Friedman Paul Galow High heat furnace cement Paul used to seal wood stove section gaps (C) Daniel Friedman Paul Galow

The wood stove itself needed further attention too. The observation of smoke coming out of wood stove section joints told Paul that the cement sealant at those joints needed to be replaced.

Watch out: leaks at wood stove section joints as well as leaks at a wood stove door due to a lost or worn gasket can make draft control impossible - this is a very dangerous situation.

The result can be a runaway wood stove fire overheating the stove or chimney to unsafe temperatures and perhaps igniting a creosote chimney in the flue or even burning down the building. Or burning up the building. Anyway the same gaps that let smoke out of the wood stove (because the flue was blocked with creosote) let excess air into the wood stove in other conditions, making it unsafe.

In our photo (below left) creosote is leaking out of this chimney on the upper floor of a pre-1900 home.

Creosote leaking from a chimney (C) Daniel FriedmanInside the chimney, creosote looks like a black shiny deposit, sometimes with a bubbly or knobby rounded surface. It can be spotted at the chimney top from outside and it may be seen by looking up into the flue from inside the building.

Watch out: Beware that because creosote is deposited more heavily where the chimney is cooler, a chimney that looks pretty clean just above a fireplace or woodstove may still be unsafe.

It is just about impossible for a homeowner to thoroughly inspect the entire inside of a chimney or flue, but you can inspect the lower chimney from inside the building, and the chimney top can be inspected from outside if you have safe on-roof access.

If you see a thick deposit of creosote, a deposit that looks thicker than a simple "dusting" of 1/8" or less on the chimney surface, you should have the chimney inspected and cleaned.

Even if you do not see a thick creosote deposit, if you are making regular use of a woodstove or fireplace the chimney should be inspected for safety and cleaned no less than once a year. And if you've never had your chimney inspected or cleaned we recommend that you do so before continuing its use.

After the first inspection and cleaning, and based on the accumulated creosote found and your frequency of use of the chimney, you and your chimney professional will be able to have a more clear opinion about how often your chimney needs cleaning.

Creosote odors in buildings: causes & cures

You may smell a chemical odor that is traced to a chimney or fireplace when that system is not in use, typically in the spring and summer and during wet weather. Water entering the chimney from the top (leaks, or a bad or missing cap) or moisture condensation forming inside the flue can dissolve some of the creosote deposits leading to an odor. A downdraft through the chimney brings those odors into the home. Have the chimney inspected and cleaned and close the damper when the chimney is not in use.

See ODORS GASES SMELLS, DIAGNOSIS & CURE for help in tracking down and curing smells or odors in buildings.

How to Prevent Creosote Chimney Fires

  • If you are new to the building, have the chimney (and fireplace or woodstove) inspected for safety before use. Ask for a Level-1 inspection as defined by NFPA 211.[4] You can find a qualified chimney professional through the Chimney Safety Institute CSIA [3] or the National Chimney Sweeps Guild (NCSG) [7].
  • Make sure that your heating appliance and chimney installed according to the manufacturer's instructions and complies with local building codes.
  • Review the list of creosote formation causes above at What Determines the Rate of Creosote Build-up in a Chimney? , and correct the ones that affect your installation.
  • Do not use a chimney, wood stove, or coal stove in a home that is unfamiliar to you without first having the installation inspected for fire safety. For example if you buy an existing home with a fireplace or woodstove installed, you should not use it before having had an initial chimney and fireplace or woodstove safety inspection.
  • If you burn wood more than once or twice a season, your chimney should be inspected and cleaned at least annually.
  • Burning dry wood, periodically burning a hot fire, and other measures may reduce the rate of creosote buildup but there is no substitute for a careful and competent chimney inspection and cleaning.
  • At CHIMNEY FIRE ACTION / PREVENTION we provide more chimney fire safety advice expanded from information provided by the US CPSC. There we also discuss the use of Chimflex™ fire extinguisher sticks and fire extinguishers and we outline what to do in the event of a chimney fire.

How Often Should the Chimney be Cleaned to Remove Creosote

Preparing to inspect a chimney flue with a camera (C) Daniel FriedmanIt depends . .. on the conditions we described above, such as the amount of wood burned, the condition of the wood and chimney and fire temperatures.

According to NFPA 211,

Chimneys, fireplaces, and vents shall be inspected at least once a year for soundness, freedom from deposits, and correct clearances. Cleaning, maintenance, and repairs shall be done if necessary". [4]

But depending on the creosote formation factors we described above, your chimney may need cleaning more often in order to be safe.

According to CSIA,

... open masonry fireplaces should be cleaned at 1/8" of sooty building, and sooner if there is any glaze present in the system.

Factory-built fireplaces should be cleaned when any appreciable buildup occurs. This is considered to be enough fuel buildup to cause a chimney fire capable of damaging the chimney or spreading to the home. [3]

We note that "appreciable buildup" and "capable of damaging the chimney" are too vague to guide a homeowner. The judgment of a chimney professional can assist you in learning to recognize when your chimney needs cleaning.

How Creosote Deposits in Chimneys are Cleaned

Also see Chimney Cleaning Advice, Procedures.

Creosote deposits may be removed by using a stiff chimney brush if the deposits are sooty in character. This is the easiest and best condition to handle.

Creosote deposits that are glazed and hard on the chimney walls can be difficult to remove by may be removable using a powered mechanical brush.

Creosote deposits that are sticky, oozy, "gummy" are very difficult to remove without a chemical treatment - chimney cleaning chemicals are not recommended by the CSIA.[3]

The Chimney Safety Institute of America (CSIA), a non-profit, educational institution focused on the prevention of chimney and venting hazards, is concerned about the consumer use of chemical chimney cleaning products to the exclusion of conventional chimney inspections and cleaning. These products often are promoted for their ability to remove a portion of the creosote from a masonry or metal chimney interior through catalytic action when burned in a fireplace or wood stove.

The CSIA believes that the use of these products alone is not an adequate substitute for mechanical chimney cleaning and inspection because it does not provide the same level of protection to the chimney system. Current promotional claims for some of the products may be creating a false sense of security among consumers.[3]

We have observed that a "gummy" creosote deposit may harden when the chimney dries or is (carefully) heated.

Watch out: we have also seen "chimney cleaning sticks" a high-temperature flare that is tossed into a fireplace or woodstove to "burn off" the creosote. In our opinion you're asking for a chimney fire, and we would not rely on such products. Call a professional, certified chimney sweep instead, or ask for advice from your local fire department.

Watch out: creosote itself can affect your health: it is a skin, eye, and respiratory irritant. Depending on the level and location of exposure creosote can also act as a carcinogen.[1][2]


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