According to a report from the U.S. Fire Administration, between 2017 and 2019, there were an estimated 34,200 heating fires in residential buildings. 77% of those fires — over 26,000 — started in chimneys, and the vast majority of them were caused by creosote.
But what is this dangerous substance? How does it form? How do you get rid of it — and how can you prevent it from forming in your wood heating appliance’s chimney?
At HY-C, we manufacture wood burning furnaces, chimney caps, and several other hearth products and fireplace accessories. We’re also a member of the National Chimney Sweep Guild (NCSG).
We’ve learned plenty about chimneys and flues over 75 years, including the ins and outs of creosote. And we want to tell you everything you need to know about it.
In this guide, we’ll cover all things creosote. We’ll go over what creosote is, how it forms, the dangers it poses, and even how to get rid of creosote and prevent it from coming back.
By the time you’re finished here, you’ll understand creosote in depth, which will help you to burn firewood more safely and get the best performance out of your wood heating appliance.
What Is Creosote?
Creosote is, at its core, a byproduct of incomplete combustion. It starts off as a gas, then cools into a sticky liquid that adheres to the side of a chimney flue. Finally, the tarry liquid hardens on the side of the walls of the chimney, forming a hard, sooty substance.
The chemical makeup of creosote is complex and often depends on the type of wood burned, the temperature at which it was burned, the amount of airflow in the firebox, and more highly variable factors.
In most cases, though, creosote consists of what are called polycyclic aromatic hydrocarbons (PAHs). It also primarily consists of tar, soot, and moisture. Depending on the type of wood fuel from which it formed, creosote may also contain acids, phenols, or aldehydes.
How Does Creosote Form?
Creosote can form in the chimneys of wood stoves, wood furnaces, fireplaces, and other wood heating appliances. It forms in a chimney flue for three reasons:
- A poor draft
- A cold flue
- Wet firewood
A lack of oxygen in the firebox means that the wood fuel cannot burn completely. As a result, the fire gives off smoke which contains vapor, oils, and other harmful chemicals that weren’t able to burn away fully in the initial fire.
As this smoke rises, the temperature around it in the chimney stack falls. If the flue temperature is too low (below about 250 °F), it causes the smoke to cool and collect as a liquid on the inner walls of the chimney. Over time, this liquid hardens as creosote.
All of these problems are exacerbated by burning wood with too much moisture. High-moisture firewood results in a more incomplete burn, leading to more smoke which rises and cools to create more creosote.
Creosote is a compounding problem. A bad chimney draft leads to the formation of creosote, which in turn causes a worse draft, which leads to more creosote, etc. The problem only gets worse over time if left untreated.
What Are the Dangers of Creosote?
As creosote forms, it begins to block and constrict the chimney. This can quickly lead to smoke rollback into the home and the spread of carbon monoxide and other harmful gasses. But perhaps the biggest and most immediate threat from creosote is a chimney fire.
Creosote is a highly flammable substance. As you continue to use your appliance, embers and sparks will rise into the flue. One of these hot sparks will inevitably ignite the creosote that has built up in the flue, leading to a fire throughout the entire chimney.
If your appliance is making loud sucking sounds, or if you hear a large roar or shaking pipes, a chimney fire is likely spreading through your flue. It’s important to keep a fire extinguisher and a fire blanket near your appliance to stop the fire from spreading out of control.
How Do You Get Rid of Creosote?
While some wood burners like to get rid of creosote simply by lighting it and letting it burn away, this method is not safe and is not recommended. Rather, the best way to get rid of creosote is by cleaning it out of the flue.
Chimney sweeps perform this service all over the country. They use a series of wire brushes designed for chimney flues of all sizes to scrape and scour the creosote from the sides of the flue. They can also inspect the chimney for any damage the creosote may have caused.
There are also several DIY chimney cleaning kits available on the market. We manufacture one called SootEater under our Gardus brand name.
Kits like this consist of flexible rods with a chimney whip line at the end. To use it, just attach it to a drill and run it up the length of the flue, adding additional rods as you go. The rotating motion of the whip line knocks soot, ash, and creosote down into the firebox where it can be scooped up and discarded.
Whether you use a chimney sweep or a DIY kit, it’s recommended that you clean your chimney flue at least once a year to prevent creosote-related problems. Another good rule of thumb is to clean your flue after burning three cord’s worth of firewood.
How Do You Prevent Creosote?
Remember that creosote is caused by three problems: a poor draft, a cold flue, and wet firewood. Preventing creosote buildup simply involves mitigating or eliminating these issues.
To ensure a good draft, be sure to leave the air inlets open on your wood heating appliance to provide plenty of air to the fire. You also need to ensure your chimney was built properly. This includes having as few bends as possible between the appliance and the chimney, and making sure your chimney has the proper diameter and height to produce a good draft.
To get your flue up to temperature, you may need to change your burning practices. Start by burning small bits of dried wood for the first 20 minutes to warm your flue up above creosote-formation thresholds. Also, be sure your chimney utilizes an insulated pipe.
Avoiding wet firewood starts with seasoning your firewood properly. Never burn freshly cut cordwood. Allow firewood to sit outdoors for at least six months so excess moisture can evaporate away. You can use a moisture meter to ensure your firewood’s moisture content is below 20% before burning it.
Finally, products like our Creosote Control FireBrick release chemicals that help slow the formation and buildup of creosote. They contain no waxes or oils, and they’re compatible with fireplaces, fireplace inserts, wood stoves, and more wood heating appliances.
The best way to prevent creosote, though, is to stay on top of cleaning your chimney. To learn more about how SootEater can help you do just that, check out the article below to find out if it’s the right DIY chimney cleaning tool for you.
