What Is Soot In Chimney Flue And Why Should I Be Worried?

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Find out what soot in a chimney is, why you should be worried about it, and how to prevent it from happening.

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Have you been noticing black stains in or around your fireplace walls? That’s probably because of soot and creosote buildup inside your chimney walls.

Soot and creosote in your chimney are a serious problem. If ignored, it can lead to various health and safety concerns. In extreme cases, it can even cause chimney fires.

But before we talk about cleaning chimney soot and creosote and the best tips to prevent soot buildup, it’s important to understand what exactly is chimney soot and why it occurs. We’ll also talk about creosote, its various stages, and how to tackle (and prevent) creosote buildup.

Let’s get started.

What Is the Black Stuff in the Chimney? And Where Does It Come From?

Since chimneys are narrow and confined, there’s not enough airflow (oxygen) for proper combustion. When you burn wood or coal in such a place, the partial combustion results in unburnt carbon particles that form soot and creosote (we’ll talk more about it later on).

Chimney soot is a black (or dark brown) powder-like, slightly oily substance. Soot occurs due to incomplete combustion of an organic substance such as wood or coal. It is a foul smelling and dangerous substance that consists of toxins, harmful chemicals, debris, and tiny metal particles.

Is Soot Dangerous?

Soot is a dangerous substance not only for your chimney but also for you, your loved ones, and even your home.

Let’s discuss all the dangerous aspects of soot in detail:

Can Chimney Soot Cause Health Problems?

Soot consists of tiny particles that can easily travel through the air and into your lungs. In large enough quantities, soot can cause various health issues; most notably lung and respiratory problems.

If inhaled, soot can cause irritation in your lungs and lead to lung diseases such as asthma and Chronic Obstructive Pulmonary Disease (COPD). Soot can also enter into your eyes or be absorbed into your body through the skin and cause serious ailments such as bronchitis, coronary heart disease, or even cancer.

Children and elderly are more susceptible to these health issues and may often feel discomfort after house fires.

Can Soot Catch on Fire in Chimney?

According to the United States Fire Administration, around 4,000 house fires were caused by wood stoves. While the NFPA blames uncleaned heating equipment (containing soot and creosote) e.g. chimneys to be the primary cause of house fires.

Soot is a flammable substance. When there’s a lot of soot buildup inside your chimney the heat from the fire below can ignite it resulting in a chimney fire. Since chimneys are not created to contain or withstand fire, a chimney fire can be quite disastrous.

In case of a chimney fire, your chimney liner (flue) can expand due to considerable heat and, eventually, crack. Since the nearby combustible materials such as walls are now exposed, they catch fire. And before you know it, your whole house is in flames.

Can Soot Clog the Chimney?

If your chimney is not cleaned regularly, there’s a chance that the soot buildup will increase so much that it will obstruct airflow inside the chimney. Smoke from the fire will not exit out through a blocked chimney flue properly and some of it will backdraft into the room. Not only can it ruin your room but it is also unhealthy to inhale smoke and other by-products combustion.

Why Is Soot Falling Down Your Chimney Flue?

When you burn wood or coal in your chimney, the heat from the burning causes air and by-products of combustion (including soot) to rise up. Since soot is slightly sticky, it settles down on the mortar between chimney bricks as dark brown or black residue.

However, when your fireplace is not in use, especially during the warmer months, the hot air from outside flows down your chimney (since it’s lighter). This hot air brings with it the soot particles attached to your chimney walls.

You’ll also notice mortar pieces falling down from your chimney into the firebox. That’s because soot eats away mortar (due to its toxicity) causing bits of it to fall down along with it. So, not only is soot dangerous for your health, it also compromises the structural strength of your chimney.

Are Soot and Creosote the Same Thing?

soot in a chimney

Throughout the article, we’ve mentioned creosote along with soot. But, what exactly is creosote? And are these two the same thing?

Well, creosote is also a by-product of poor combustion and is highly flammable (more so than soot). Creosote can be in the form of flakes, tar, or layers of gooey substance.

During wood burning, hot air from the firebox carries unburnt wood, hydrocarbons, exhaust gasses, and debris up into the chimney. When the airflow inside the chimney isn’t fast enough, it gives time for these particles to cool down and stick to the chimney walls as creosote residue.

While there are several similarities, soot and creosote have some differences, as listed in the table below:

SootCreosote
Dark brown or black

powder-like substance

produced as a result of incomplete burning of wood, gas, or oil

Less flammable (due to the presence of ash)
Black

Flaky, tar-like, or gooey

Produced as a result of incomplete burning of wood or coal

Highly flammable

Stages of Creosote Buildup

While most people know that creosote is hazardous, they usually don’t know about its various stages. It’s important to know about the various stages of creosote formation for better removal and prevention.

The different forms of creosote are classified into three main stages as discussed below:

Stage 1 – First Degree Creosote

First degree creosote contains a high quantity of soot and has a flaky texture. It is formed when the airflow is relatively good (e.g. in open fireplaces) and is the easiest to remove. You can easily remove it using a chimney brush.

While the first degree of creosote is inevitable, it’s still dangerous and should be regularly removed. If not, its buildup can restrict the chimney airflow, causing second and third degree creosote to develop.

Stage 2 –  Second Degree Creosote

When the airflow is restricted (e.g. in fireplaces with glass doors), second degree creosote is formed. At this stage, creosote consists of much denser, harder, and porous, tar-like deposits that are stuck firmly to the chimney flue.

Second degree creosote is harder to remove. A professional chimney sweep might use harder brushes or, for more stubborn spots, a rotary loop tool attached to a drill.

Stage 3 –  Third Degree Creosote

While all stages are dangerous and flammable, the third degree creosote is the worst of them all. At this stage, it looks like a thick, dark buildup of tar. It usually looks like it’s dripping down your chimney walls. That’s because it melts like wax upon heating. However, it forms a hard glaze buildup upon cooling.

Third degree creosote occurs when the temperature inside the chimney flue is lower than usual.

Since it’s the densest of all creosote stages, it’s the primary source of fuel in case of a chimney fire. You should not burn fire if you have third degree creosote inside your chimney.

Third degree creosote is the hardest to remove, so much so that the process can even damage your clay or ceramic chimney flue. That’s why you’d find professionals recommending a chimney flue replacement if there’s a considerable amount of third degree creosote inside your chimney.

4 Reasons for Creosote Formation

There are certain conditions that cause creosote to form. If you prevent these conditions, you’ll find less creosote in your chimney and fireplace, thus, creating a safer fire. These conditions are listed as follows:

Restricted Airflow

When the airflow inside a chimney flue is restricted, it doesn’t allow smoke and other residual particles to leave. In such a situation, these particles condense and form a buildup of creosote. Debris, small animals or birds, large quantities of soot, etc usually restrict the airflow.

More commonly, you can restrict the airflow inside the chimney flue by tightly shutting the glass door of your fireplace or by not opening the dampers wide enough.

Cooler Flues

Cooler temperatures in chimney flues are among the main causes of creosote formation. Due to cooler temperatures, the smoke rising from the fire condenses into creosote that builds up inside the chimney walls. That is especially true for chimney flues made from metal sheets.

The placement of your chimney can also impact the temperature inside. For example, if a chimney is placed on an interior wall, it will be warmer compared to the one placed on an exterior wall, thus, preventing creosote formation.

Oversized Flues

A chimney flue needs to follow proper sizing guidelines for an effective upward draft of smoke and post-combustion materials. If the flue is too large, the upward draft will be much slower, allowing combustion by-products enough time to cool down and form creosote on the chimney walls.

Unseasoned or Softwood Firewood

The type of firewood you use also affects creosote formation.

Softwoods such as Pine or Cedar contain significant amounts of resins and oils that form creosote upon burning. Soft hardwoods have lesser quantities of resins and oils, producing lesser creosote. While hardwoods are dry woods that burn at higher temperatures and leave behind the least amount of creosote.

You should also ensure that you use seasoned firewood since unseasoned wood contains moisture. Burning such a wood leads to improper combustion and produces more smoke which cools down and forms creosote.

Best Tips to Prevent Soot and Creosote Buildup

  • Schedule annual chimney inspections and chimney cleaning. Regular cleanings and inspections ensure that the chimney is not clogged, allowing proper airflow.
  • Have a quality chimney cap installed. It will restrict debris, small animals, birds, etc. from entering and clogging your chimney. An unclogged chimney has proper airflow and diminishes the chances of creosote formation.
  • We already discussed why it’s not a good idea to use unseasoned firewood. So, it’s important to only burn seasoned firewood that has dried for 6 to 12 months. Having a moisture meter helps ensure that the wood is actually seasoned and will produce less smoke. Firewood with less than 20% moisture is good for burning.
  • Make sure your chimney dampers are wide enough to allow proper airflow. Also, do not shut your stove or fireplace glass doors too tightly.
  • In-between inspections, to ensure safe burning, it is best to use creosote destroyer powder. You can throw one or two pounds (or as per the instructions on the container) into burning wood or coal. The powder turns soot or creosote into ash that you can safely, and easily, remove from the firebox.

The Takeaway

Soot and creosote are flammable by-products of improper combustion caused by low-temperature fires, restricted airflows, or unseasoned firewoods, among other things. Not only is soot a foul and dirty substance, but it’s also quite hazardous. Creosote can even lead to chimney fires owing to its high flammability.

It’s important to ensure that your chimney isn’t clogged with soot and creosote. This can be done through annual inspections and regular cleaning of your chimney. And by following the best soot and creosote prevention practices.

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Thomas Green

Thomas Green

Thomas has worked in the Chimney & Fireplace field for over 12 years. He is an expert in his trade and loves to help People with their needs. Thomas Write helpful articles so that homeowners can make the most informed decisions about their fireplace and chimney.