What is Creosote? 


I hated chemistry in high school. But somehow I got into an industry that involves chemistry on a daily basis. The whole chimney industry is rooted in fire and we sweep away the result of fire, which is creosote.

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Creosote is a dangerous byproduct that comes from burning wood and lingers in your chimney until you remove it. When you use your fireplace, your goal should be to move the fuel (oil, natural gases, wood, pellets, etc.) through all four stages of combustion as completely as possible.

Four Stages of Combustion: 

  1. Moisture evaporation
  2. Vaporization of the hydrocarbon compounds
  3. Gas vapor ignition and combustion
  4. Char burning.

Complete combustion of wood produces water vapor, carbon dioxide, heat, and noncombustible ashes. The less complete the combustion is, the more carbon monoxide, combustible hydrocarbons, and other gasses are left behind. 

Unfortunately, achieving complete combustion is rare because to do so your fire must meet three conditions: 

  1. There must be sufficient heat to breakdown wood molecules into combustible hydrocarbons
  2. There must be enough oxygen mixing with the hydrocarbons
  3. There must be enough fuel

If those three conditions are not met, you get more creosote. Creosote originates as condensed wood smoke, tar, condensed vapors, and other compounds. It is a dark brown or black and sticks inside your flue. Once inside the chimney, creosote builds on itself and doesn’t just go away on its own. 

It’s a very dangerous, naturally occuring, by-product that you can’t prevent. But this guide will tell you all that you need to know so that you can understand it. 


What Causes Creosote Deposits?

Your fireplace is designed to safely contain the fire, while providing heat for your home. Your chimney’s job is to move the smoke and other by-products of combustion up and out of your home. As the smoke flows up and into the cooler parts of your chimney it begins to condense. This results in the creation of creosote. 

Several factors encourage the creation of more creosote that you should take into consideration when building and maintaining your fires:

Proper Stove Operation

Operating your heating appliance properly is the single most important contributing factor in minimizing creosote buildup. This means you should be mindful of keeping temperatures, air supply, and fuel load in the proper balance. 

Smoke Density

Smoke density is how much unburned hydrocarbons are in the flue gas. The smokiest fires produce the most creosote. 

Moisture Content

Too much water keeps combustion temperatures low, which results in the release of gases and creosote. The ideal moisture content for wood fuel is between 15-25%.

Flue Gas Temperature 

The temperature of the gases moving through the flue influence how much accumulation occurs in the flue. Cold chimney walls can cause the flue gases to condense and deposit creosote on the chimney liners. 

Residence Time 

This is how long smoke remains in the venting system. Slow smoke velocity means that the smoke remains in the chimney longer and has a greater opportunity to cool. If the smoke cools, it can increase the chance of the smoke to condense to create creosote. 

Large Fuel Loads 

Too much wood causes poor combustion efficiency, high smoke density, slow flow rate, and cooler flue gases. As you know from the previous bullet points, these all contribute to the build up of creosote.

Appliance Factors 

You should have an appliance that matches the size of the space you are heating. For example, if a large wood stove is used to heat a small area, usually it will operate at a low heat output. This causes smoky, smoldering fires that produce more creosote. When installing a heating appliance, you should make sure it is specific to the room size.

Why is Creosote Dangerous? 

Exposure to creosote can have a wide-range of effects on your health. Of course, several factors contribute to health effects, such as the level (amount) to which you are exposed, the length of time, and the number of times you’re exposed. And some people (like children, the elderly, and pregnant women) may be more sensitive to creosote. 

While some symptoms of exposure are mild and irritating, some symptoms can be more severe. Here are some of the most common symptoms associated with exposure to creosote: 

  • Skin and Eye Irritation – exposure to even a small amount of creosote, whether direct skin contact or through vapors, can cause blistering, peeling, or reddening of the skin. It can also cause damage to your eyes and increase your eyes’ sensitivity to the light.
  • Respiratory Issues – long exposure to creosote vapors can irritate your lungs. 
  • Cancer – the Environmental Protection Agency (EPA) determined that creosote is a probable human carcinogen. They found that even low levels of creosote had led to skin cancer. 
  • Birth Defects – while the studies have been done on animals and not on humans, researchers have found that creosote exposure can cause birth defects, such as cleft palate, among babies born to mothers exposed to creosote during pregnancy. 

The health risks are serious. But the most concerning issue of creosote is chimney fires. Creosote is flammable and when the deposits catch on fire it creates a scorching hot fire. The leading cause of chimney fires is due to the build up of creosote deposits inside the flue.

The masonry and flue lining are only designed to handle certain levels of heat. And creosote-caused chimney fires typically far exceed that level. In fact, during chimney fires, temperatures can often exceed 2,000℉. The first chimney fire may not show any visible damage, but it will limit your home’s ability to handle another one. 

Here are nine signs that a professional sweep looks for to see if you’ve had a chimney fire during an inspection: 

  1. “Puffy” or “honey combed” creosote
  2. Warped metal on the damper, metal smoke chamber connector pipe, or on metal chimney of a factory-built chimney
  3. Cracked or collapsed flue tiles
  4. Discolored and/or distorted cap
  5. Heat-damaged TV antenna attached to the chimney
  6. Creosote flakes and pieces on your roof or ground
  7. Roofing material is damaged from the hot creosote
  8. Cracks in your exterior masonry (this could lead to chimney leaks)
  9. Evidence of smoke escaping through mortar joints of masonry or tile liners

How to Recognize Creosote

Creosote deposits appear in many different ways, depending on the moisture content. It’s not uncommon to find more than one type of creosote in a chimney. Here are the four main types: 

  1. Sooty, ash-like deposits – found in flues because of their unlimited access to combustion air. These may be less combustible but should still be removed and taken seriously to prevent blockages, moisture retention, and chimney deterioration. 
  2. Dry, flaky, black tar deposits – these are the result of deposits that have been heated and pyrolyzed. This type is easy to catch fire, even though much of the flame-producing gases have been driven off. 
  3. Dense, hard, shiny black deposits – this is a tar glaze that has stuck to the chimney walls and driven off the moisture. Because the moisture is gone, but the flame-producing hydrocarbons are still present, these deposits retain a high energy content. Ignition of this type of creosote deposit burns at extremely high temperatures. 
  4. Sticky, tacky, or runny deposits – this form of creosote has a consistency similar to chewing gum. Creosote in this form can be liquid in nature when the tar fog condenses with a high concentration of water vapor. 

How Do Chimney Sweeps Deal with Creosote?

During an inspection, your technician will determine what stage or degree your chimney’s creosote deposits are at.  

Stage 1 (first degree) Creosote Build Up

At this stage only small amounts of creosote are present and it consists primarily of soot. It also has a thin, flaky texture. Stage 1 creosote build up is easier to remove from your chimney and only needs a brush to remove it.  

Stage 2 (second degree) Creosote Build Up

At this stage, there is a medium build up, but it’s more tar than soot. This level is thicker and harder, and has a variety of hard, shiny black flakes that contain hardened tar. The best way to remove Stage 2 creosote is with a stainless steel rotary loop attached to a powerful drill. 

Stage 3 (third degree) Creosote Build Up

At this stage, you have a large amount of creosote build up. It is highly concentrated and will feel sticky and oily or hard and shiny, depending on how fresh it is. Stage 3 creosote build up is a tough problem to deal with. We try to use chemical solvents and rotary loops, but it’s not unusual to just replace the liner to get rid of it. 

If you follow the recommended annual inspection, your chimney technician will be able to catch creosote build up before it gets to the higher levels. You don’t want to wait until Stage 3, or else you are putting yours and your family’s health, property, and wallet in harm’s way.

Tips to prepare for a Chimney Sweep: 

Before your chimney sweep appointment, we recommend following a few guidelines: 

  1. Do not use your fireplace for 24 hours before we get there to help let it cool down. Masonry fireplaces can store heat for a long time and the fireplace can still be hot when we get there. 
  2. Move furniture away from the fireplace. We need about 6 feet of space in front of the fireplace to use our equipment. Don’t worry about dirt and dust – we have a no mess guarantee. If you can’t move your furniture, our technicians can do it!
  3. On the day of your sweep, please keep your children and pets away from the room the fireplace is in. This is for their safety and ours.

Inspection & Sweep – Starting at $190

At Patriot Chimney, we include a video inspection for all chimneys we sweep! Schedule an inspection and a sweep today to see why hundreds of customers in Southwestern Virginia have trusted us to take care of their chimney.

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