Creosote Buildup Explained (What It Is & How To Prevent And Get Rid)

In Chimneys by James O'KellyLeave a Comment

Creosote is one of a number of unwanted byproducts that can be released from the burning of firewood. In many cases creosote production can be kept to a minimum by burning low moisture content wood, but can be found in much higher quantities when the wood isn’t properly combusted.

Creosote can have both short-term and long-term effects on fireplaces and chimneys if allowed to build up, so what is creosote?

Creosote is a black tar-like byproduct of burning wood that can line the inside of flues or chimneys and cause longer-term issues until it’s removed. Creosote can be produced in higher quantities when there’s a problem with the wood or the airflow to the fire.

It’s important to understand what creosote is so that you can keep your fireplace or stove clean and operating efficiently. We’ve explained below what causes creosote buildup, what issues it can cause and how you can help to prevent too much creosote from building up in your chimney.

What Is Creosote?

When wood is burnt it releases energy in the form of heat, but can also release a number of other unwanted byproducts such as smoke and waste gases.

Creosote is one of the unwanted byproducts from burning wood. It’s a black tar-like substance that can be found in chimneys in different forms depending on the severity of the buildup.

Creosote is defined as:

A dark brown or back flammable tar deposited from especially wood smoke on the walls of a chimney.

Merriam-Webster

The amount of creosote that a wood fire produces can vary depending on how well the fire burns the wood.

What Is Creosote Buildup?

Creosote buildup is the accumulation of creosote within the internal walls of a chimney or flue.

Creosote buildup can cause long-term issues and depending on the severity of the buildup must be removed through sweeping or through the use of more specialized tools.

When wood isn’t combusted properly and creosote is produced, the movement of hot air up a chimney causes creosote to rise. As creosote rises it can cool and solidify on the lining of the chimney.

Minimal creosote buildup can be expected with any normal wood burning fire, but if more creosote is being produced than usual, such as a result of regular poorly burning fires, then the creosote can buildup into thicker layers and start to cause more prominent issues.

As creosote buildup thickens it can lead to problems such as a decrease in the diameter of a chimney or flue, which can affect how well it draws on the fireplace or stove.

Creosote buildup is one of the main reasons why it’s recommended to have your chimney or flue swept at least once per year, ideally before the burning season.

The amount of creosote can be categorized into stages that summarize the level of buildup and therefore the associated levels of risk posed by the creosote.

Stages Of Creosote Buildup

There are three stages of creosote buildup that are used describe the increased severity of buildup within chimneys.

Stage 1 Creosote Buildup

Stage 1 creosote buildup, also known as First Degree Buildup, can be found mainly in the form of soot, which is the easiest form of buildup to clean and can typically be removed as part of your annual chimney cleaning using a brush.

It can be normal to see small amounts of creosote building up in your chimney throughout the year when having clean burning fires with dry wood.

Stage 2 Creosote Buildup

Issues with your fires such as insufficient air supply or burning wood that is too wet can lead to thicker deposits of creosote building up within your chimney.

Stage 2 creosote is harder to remove and can resemble black tar-like flakes, and is at more of a risk of starting chimney fires. More specialized tools may be required to remove this Second Degree Buildup.

Stage 3 Creosote Buildup

Extremely unfavorable burning conditions can result in a concentrated layer of thick tar-looking creosote lining a chimney.

This highly concentrated buildup of creosote is much harder to remove and can be a result of incomplete complete combustion of wood due burning wood that is far too high in moisture content, severely restricted airflow to the fires or a reduced draft due to existing creosote buildup.

This Stage 3 buildup can result in the highest risk of chimney fires occurring.

What Does Creosote Buildup Look Like?

An example of creosote flakes
An example of severe creosote buildup

Signs Of Creosote Buildup

If you have a stove, fireplace insert or damper blocking the view of your chimney or flue it can be hard to see if there are any visible signs of creosote building up.

However, there are a few things to look out for that can be as a result of creosote buildup causing your fires to behave differently.

Signs of creosote buildup include:

  • A reduced draft. If you’re noticing that air isn’t being pulled up the chimney as well as before, it can be a sign that creosote buildup is causing a reduction in the amount of draw on your fireplace or stove.
  • Poorly burning fires. If your fires are struggling or producing more smoke than usual then it can be a sign that the flow of air up the chimney has been reduced due to creosote buildup.
  • Black soot visible around the fireplace, which can indicate increased creosote production from your fires.

How Much Creosote Is Normal?

If a fireplace is swept as part of the recommended annual cleaning then it can be normal to see a couple of cups worth of creosote being removed.

The amount of creosote that is normal, and how fast it can buildup, can typically depend on the type and moisture content of the firewood that you’re burning, and how well the fires are receiving a fresh supply of oxygen.

More creosote can be expected if you’re burning firewood that is too wet, but if you’re burning kiln dried hardwood logs that are sufficiently low in moisture content then you may see very minimal creosote in your annual clean.

It’s also normal to see Stage 1 creosote buildup, which is creosote deposited in your chimney in the form of soot. An annual clean using a brush will generally be able to remove this form of creosote buildup without any issues.

If you’re seeing Stage 2 or 3 creosote buildup, which is more flaky deposits and a hard tar coating respectively, then it’s a cause for concern and you’ll need to understand why your fires are producing more creosote than normal.

Dangers Of Creosote

Small amounts of creosote can be expected as part of a normal operation of a wood burning fireplace or stove, but in larger quantities creosote can become more of a danger to your home.

The dangers of creosote buildup in your chimney include:

  • A reduction in draft due to a reduced opening, which in turn can cause further creosote to be deposited and exacerbate the situation.
  • The potential for chimney fires, especially with Stage 3 buildup as creosote is a highly flammable byproduct of fires.

In terms of creosote being hazardous to health, the Agency for Toxic Substances and Disease Registry (ATSDR) explains that ‘you are exposed to creosote only by coming in contact with it’.

Causes Of Creosote Buildup

In order to help prevent creosote from causing long term issues with your fireplace it’s important to understand how creosote is produced in the first place, and why it builds up within a chimney or flue.

The main causes of creosote buildup in a chimney can include:

  • Incomplete combustion of the firewood. This can be a result of a fire that is struggling due to incomplete combustion with either an issue with the wood or the air supply. Wet firewood, a lack of air supply or a poor draft can be the main causes of a poorly burning fire that produces creosote.
  • A lower temperature flue or chimney. Cooler surfaces within a chimney or flue can lead to creosote condensing and building up on the sides.
  • Existing creosote buildup. If creosote is already lining a chimney or flue it can be affecting the draft as a result of a smaller diameter opening. Air may get trapped or lose velocity as it rises leading to further creosote being deposited.

One of the main causes of creosote buildup is a poorly burning fire due to a problem with either the air supply to the fire or a problem with the wood.

An issue with either of these can lead to incomplete combustion of the wood, where a struggling fire can release more creosote than usual.

If the wood hasn’t been properly seasoned, and therefore still too high in moisture content, then it can be harder for a fire to properly combust the wood. While a fire is having to burn off excess moisture in wet wood, incomplete combustion of the wood can lead to increased creosote production.

Green Wood Moisture Content
This piece of wood is over 20% moisture content and far too wet to burn without causing increased creosote production

For more information about wet firewood and why you shouldn’t burn it see one of our other articles here.

If there isn’t sufficient air being supplied to your wood burning fires then it can also lead to more creosote production.

If your fires are smoldering then look to open any air vents in the same room as your fireplace or stove, or crack open any windows also in the same room.

Fireplace Vent
Keep any air vents of windows open if required to help with air supply to your fires

A poor draft may also cause a fire to struggle because the waste gases aren’t being sucked out of the fireplace or stove effectively, and in turn fresh air isn’t being pulled in to feed the fire.

Existing creosote buildup can also make the situation worse.

Buildup of creosote can reduce the opening of a flue or chimney and reduce the draft. This in turn can lower the speed of the air leaving the fireplace up the chimney and cause more creosote to settle.

A culmination of these factors can lead to poorly burning fires that aren’t producing enough heat to keep the fire efficient.

If wood fires aren’t burning hot enough then it can lead lower temperature air leaving the fires and cooler surfaces within a chimney or flue, allowing creosote more opportunity to condense on the sides.

Excessive creosote buildup can be a combination of all these factors, but in particular burning wet wood, inadequate air supply and a poor draft can be the main cause of excessive buildup.

How To Prevent Creosote Buildup In Chimneys

Understanding how creosote is formed and why it builds up can help you to stop it from being produced and settling. From this information you can follow these best ways to help prevent creosote buildup in your home.

To help reduce creosote buildup:

  • Burn only low moisture content properly seasoned or kiln dried wood. Firewood that is dry enough to burn will have a moisture level of lower than 20%, and you can use a moisture meter to see the exact moisture content of your wood.
  • Ensure adequate air supply to your fires at all times, either through opening vents or windows in the same room or not closing stove vents down too far that would cause a fire to smolder.
  • Have your chimney swept at least once per year, ideally before your burning season. If you’re burning wood regularly throughout the year then having your chimney or flue cleaned more than once per year may be necessary.
  • Burn hotter fires. Cleaner burning and hotter fires can help to reduce both the amount of creosote being produced and the ability for it to form and settle within chimneys.
  • Prime your flues before each fire. Warming up the flue before each fire helps to start the draft and can help your fires to get started and get up to operational temperature more quickly; therefore providing a cleaner burn sooner into a fire.
  • Burn the right sized fires. Fires that are too big or too small for your fireplace or stove can lead to smoldering fires that in turn can produce more creosote.

To help prevent creosote buildup on the glass doors of wood burning stoves we have a complete guide on how to use a wood stove that will help you to understand how to use the air vents to provide the cleanest burn possible.

How To Get Rid Of Creosote Buildup

To get rid of creosote buildup look to have your chimney swept by a professional in the first instance. They will then advise on the best plan of action if creosote buildup has reached a more severe stage.

Depending on the stage of the creosote buildup more onerous cleaning methods may be required for the worst cases.

However, if you always make sure to burn properly seasoned wood that’s low in moisture content, while also having hot and clean burning fires then getting rid of creosote buildup can be easier.

For Stage 1 creosote buildup, which is more soot-looking, the creosote can typically be removed using a chimney brush as part of your annual sweep by a professional.

For Stage 2 creosote buildup, which is harder and stickier than stage 1 creosote, simple brushing methods may not be feasible and more specialized tools may be required to remove the buildup of creosote.

For Stage 3, which is a high concentration of tar-looking creosote buildup, specialized tools may not be able to clear the creosote and removal and replacement of a chimney lining may be required.

Creosote Buildup

What Does Creosote Smell Like?

Creosote has a tar-like smell, and can be likened to the smell of freshly laid asphalt.

The more severe the creosote buildup is the more apparent the smell can become.

Is Creosote Flammable?

Creosote is extremely flammable. Extensive creosote buildup within chimneys can increase the risk of chimney fires.

Creosote is a highly flammable byproduct that is produced from the incomplete combustion of firewood. Regular poorly burning and smoldering wood fires can lead to buildup of creosote within chimneys over time.

Severe stage 3 buildup of creosote can significantly increase the risk of chimney fires due to creosote being such as flammable substance.

Does Creosote Buildup In Gas Fireplaces?

Gas fireplaces cannot produce creosote and creosote cannot therefore buildup in gas fireplaces.

Creosote is a byproduct of burning wood and wood can’t be used in a gas fireplace.

Although gas fires can’t produce creosote and soot, it’s always recommended to have your chimney inspected at least once per year in line with guidelines. This will help to keep you and your home safe in case there are any other blockages as gas fireplace can still produce other forms of harmful byproducts.

Creosote Buildup On Glass Doors

The glass doors of wood stoves and wood burning fireplace inserts may become stained through normal use.

Ensuring to burn dry and low moisture content fully seasoned or kiln dried wood, providing sufficient airflow to fires and keeping chimneys and flues free from creosote buildup and other blockages, can help to keep creosote buildup on glass doors of fireplaces to a minimum.

If you’re using a wood burning stove or fireplace insert, be sure that you’re not closing down the air vents down too far that would cause the fires to smolder and potentially increase creosote production.

For more information we have a guide on how to use the vents on a wood burning stove to control a fire.

You can also use a dedicated stove glass cleaner to help clean any creosote deposits off your fireplace glass. For more information we have a guide to cleaning your wood stove glass here.

How To Check For Creosote Buildup

Unexplained poorly burning fires, a reduced draft or black deposits around your fireplace can all be signs of creosote buildup.

To check for creosote buildup look for black soot or tar deposits around the opening of your fireplace, as well as around the throat of the chimney. You can also shine a light up your chimney to look for further signs of creosote deposits.

Using a torch or light on your mobile look up your chimney from your fireplace to check for black shiny tar-like creosote buildup. If your fireplace has a damper you may need to fully open it to see beyond the top inside of your fireplace.

If you’re unsure always get professional chimney sweep to check and confirm whether there’s any creosote buildup in your chimney and check its severity before using your fireplace for any fires.

How Fast Can Creosote Buildup?

The things that can cause more creosote to be produced can dictate the rate of creosote buildup in a chimney, including the moisture content of the firewood, the air supply to the fire and the strength of the draft.

Smoldering fires due to a weak draft, poor airflow and wet wood can result in faster creosote buildup compared to cleaner and hotter burning fires.

Creosote Buildup On Chimney Cap

Creosote buildup on chimney caps can be another sign that your fires are producing creosote.

The chimney cap can typically be the coldest part of the chimney, leading to waste fumes from fires condensing on the chimney cap and leaving black deposits.

If your chimney cap keeps turning black ensure that you’re only burning properly seasoned or kiln dried firewood. You can use a moisture meter to confirm that the wood you’re burning is below the recommended 20% moisture content.

Further Reading

A Guide To Wet Firewood And Why You Shouldn’t Burn It

A Guide To Seasoned Firewood

A Guide To Kiln Dried Firewood

How To Season Firewood

How To Stack Firewood For Seasoning

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