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.
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.
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