Creosote & Chimney Fires

Introduction

A lot of people that we work with are shocked to learn that their chimney had a fire that they didn’t know about. In fact, most chimney fires go undetected. Slow-burning chimney fires that don’t have enough air or fuel to be dramatic (or even visible) can go unnoticed until a chimney inspection.

Your chimney’s masonry and flue lining are only designed to handle certain levels of heat. Creosote-caused chimney fires far exceed that level with temperatures over 2000 degrees F. The first chimney fire might be fine and won’t show any damage, but it will limit your home’s ability to handle another one.

Chimney fires are responsible for an average of 22,300 house fires, 20 deaths, and $125 million in property damage each year in the United States. Built up creosote and deficiencies in chimney linings and structures encourage high temperatures, embers, and sparks to reach combustible areas outside of your chimney or fireplace.

As the stats show, chimney fires can have some serious consequences. In the 11 months between your chimney inspections a lot can happen. If you aren’t on the lookout for these warning signs, then you could put your home and family at risk. Luckily, chimney fires can be prevented with your awareness and annual inspections and maintenance by a CSIA certified chimney sweep.

Signs of a Chimney Fire

Before we get started, it’s important to know what a chimney fire in the moment looks and sounds like. Chimney fires can be fast- or slow-burning. Fast-burning chimney fires are extremely obvious as the fire consumes the flammable residue on the inside of your chimney flue.

If you notice any of these – get out of your house immediately and call 911!

A loud, crackling or popping noise from the chimney flue (imagine the sound of a large bonfire)
A lot of dense, thick smoke coming into the fireplace or out of the top of the chimney
Flames or things that are on fire (pieces of flaming creosote) coming from the top of the chimney
A strong, intensely hot smell
A roaring sound, often described as being like a freight train or airplane

What Causes Chimney Fires?

Chimney fires can be caused by debris, such as a bird’s nest or leaves from a tree, that have made its way inside your flue. An easy way to prevent this from happening is by having a chimney cap to keep that stuff out.

The most serious cause of chimney fires is caused by a buildup of creosote. Even during normal use of your fireplace, soot and creosote can build up in your chimney. 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 condensate. And that creates creosote.

Even though creosote is a naturally occuring result of combustion, it can be pretty dangerous, even beyond chimney fires. Exposure to it can have a wide range of health effects including skin and eye irritation, respiratory issues, cancer, and even birth defects.

Creosote comes in many different forms and it’s very common to find more than one type of creosote in a chimney. We typically see the four main types:

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.
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.
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.
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.

While all fireplaces produce creosote, there are a few factors that encourage excessive creosote that you should consider when building and maintaining 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.

Effects of Chimney Fires

Chimney fires can be slow-burning or fast-burning. Slow-burning chimney fires happen when the flammable substances inside the chimney flue, such as creosote, become hot enough to catch fire, but don’t have enough of the elements that can cause it to be seen or heard.

The scariest part about this is that you probably won’t even notice you had a chimney fire until you get an inspection. Here are signs that we look for to spot possible chimney fires:

“Puffy” or “honey combed” creosote – normally creosote is flat, black, and often shiny
Warped metal of the damper, metal smoke chamber connector pipe or factory-built metal chimney, and/or rain cap – this tells us that the metal structures have been damaged by the intense heat
Cracked or collapsed flue tiles, or tiles with large chunks missing – this could contribute to water leakage and water damage in your home
Heat-damaged roof structures near the chimney – TV antenna, shingles, or even vents
Creosote flakes and pieces found on the roof or ground – intense heat, high temperatures can drive these up and out of the chimney
Cracks in exterior masonry – caused by excessive heat
Evidence of smoke escaping through mortar joints of masonry or tile liners – indicates cracks inside the chimney flue, and possibly caused by excessive heat

Masonry Chimneys

Chimney fires in a masonry chimney can have temperatures around 2,000 F, regardless of whether you have an older, unlined chimney or a tile lined to meet current safety codes. The high heat can easily melt mortar, crack tiles, cause liners to collapse, and damage the outer masonry material. Most often, thermal shock occurs. Tiles crack and mortar is displaced. All of this creates a new pathway for flames to reach the wood frame of your house.

Prefabricated, factory-built, metal chimneys

To be installed in most parts of the United States, factory built, metal chimneys designed to vent wood burning stoves or prefabricated metal fireplaces must pass special tests that require the chimney to withstand flue temperatures up to 2100 F without sustaining damage. But with chimney fires, damage may still occur. If your metal-built chimney is damaged by a chimney fire, they should no longer be used and must be replaced.

Wood Stoves

Wood stoves are made to contain hot fires. The connector pip that runs from the stove to the chimney is a whole different story. They cannot withstand the high temperatures produced during a chimney fire and can warp, buckle, and even separate from the vibrations caused by air turbulence during a fire. If the connector pipe is damaged by a chimney fire, it must be replaced.

Carbon Monoxide

Despite the fires, the most dangerous consequence of a chimney fire is that it creates a pathway for carbon monoxide (CO) to enter your home. Using a fireplace with a warped metal flue lining or cracked tile flue lining can allow for the dangerous gases to leak back into your home, with potentially fatal consequences.

Prevent Chimney Fires

Failure to regularly inspect, repair, and clean your chimney can cause malfunctions with disastrous effects. As I mentioned before, there’s a lot that can happen with your chimney in the 11 months between your chimney inspections. But if you are aware of what’s happening with your chimney, you can stay ahead of any potential hazards it may cause.

Follow these 6 tips to help stay safe throughout the year:

Minimize creosote build-up to prevent chimney fires – click here to learn more about creosote
Install a chimney cap to keep leaves, birds, and other debris out of your flue
Use safe fire starters. Never use gasoline or kerosene to start a fire.
Employ clean burning techniques
Clean your chimney when it has a 1/8 in layer of creosote build up or at least once per year
Schedule annual chimney inspections with a CSIA Certified Chimney Sweep

Schedule Your Inspection

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.

Tips to prepare for a Chimney Sweep:

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

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.
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!
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.