Having a fire in your wood-burning fireplace or stove is one thing… a fire in the chimney is another! In this article we’ll discuss what causes chimney fires, how you can tell if you’re having a chimney fire, what to do, and what to do to prevent it from happening again.
What Causes Chimney Fires?
Debris in the chimney such as leaves or bird’s nests, as well as creosote and soot buildup can provide excellent fuel that will incinerate if the temperature inside the chimney reaches a certain point.
What causes creosote, which is the #1 cause of chimney fires in chimneys with a proper cap?
Poor Burning Practices
Creosote is a byproduct of fires that burn too low, and improper fuel.
Too much fuel in the fire box or logs that are too big for the firebox will cause suffocation that leads to low-burning fires. Low heat results in incomplete combustion and large amounts of gases. As these gases move up the chimney flue and cool down, they liquefy and stick to the sides of the flue, creating creosote. Creosote is extremely flammable. If you routinely have a low-burning fire and then on an exceptionally cold day you build a “roaring” fire, you risk igniting the creosote and causing a chimney fire.
Burning unseasoned and/or wet wood will cause creosote to form not only in the chimney, but on the glass of your fireplace or stove. You can be sure if you have creosote on your glass, you have it in the chimney too!
What else can cause a chimney fire?
Improper Installation
The second cause of chimney fires is how they’re installed. Improper clearances, where the chimney is too close to combustible materials, can cause chimney fires as the heat builds up. These types of chimney fires often quickly escalate to house fires since they immediately affect the area adjacent to the chimney.
Always be sure that your installation team is certified by the NFI (National Fireplace Institute) and the CSIA (Chimney Safety Institute of America). Do not assume that general contractors know what they’re doing.
A Neglected Chimney
Chimneys need periodic cleaning and inspection. Regular cleaning will remove creosote and other flammable substances. Just as importantly, inspections will reveal problems that could potentially cause fires.
For example, a masonry chimney that has developed cracks over the years will allow water to infiltrate the chimney. The combination of fire and water can cause cracks. Eventually, creosote can build up in these cracks and one unlucky day, ignite.
For most customers, we recommend annual chimney sweeping and inspections. If you are a heavy fireplace or stove user, you may need service twice a year.
How Do You Know You’re Having a Chimney Fire?
There are two types of chimney fires, and they can affect any kind of chimney whether you’re burning an open fireplace, a fireplace insert, or a stove.
Fast-Burning Chimney Fires
The first type of chimney fire is fast-burning, which happens when the flames (or the heat rising from the flames) is hot enough to ignite any flammable materials in the chimney, including creosote.
This often happens in the spring, when a winter’s worth of creosote buildup creates enough fuel in the flue to ignite (or, in chimneys that haven’t been swept in a long time).
Fast-burning chimney fires are pretty obvious. You may see thick, dense smoke coming from the chimney and/or the fireplace or stove, hear very loud cracking or popping sounds coming from the chimney, see flames or sparks shooting up from the top of the chimney, smell “something burning,” and may even hear a loud roaring sound.
If you notice any of these things, DO NOT assume the fire will burn itself out. Get out immediately and call 911!!
Slow-Burning Chimney Fires
Slow-burning chimney fires happen when flammable substances in the chimney flue become hot enough to catch fire but lack enough oxygen to produce the fast-burning fire that is visible and/or audible. Slow-burning fires can cause severe damage to the flue. They burn hot enough and the heat, if not outright flames, can still cause damage.
You may not notice that you’ve had a slow-burning chimney fire - but, you may also suddenly (and too late) realize that you’ve had one going for some time and your house is on fire.
Evidence that you’ve had a slow-burning chimney fire that has burned itself out usually can’t be seen without a thorough chimney inspection. Signs include:
Evidence of smoke, escaping through the flue, indicating cracks in the chimney flue
Warped or discolored metal in the flue and the cap
Damaged flue tiles (cracks, missing pieces) can be caused by fire or water damage
Cracked exterior chimney masonry (this can be caused by intense heat or water infiltration, or both)
Heat damage to nearby roof structures such as shingles, vents, or antennas
Creosote that is honeycombed, gray, or ashy (normally, creosote is black and shiny, looking “baked on”)
How to Prevent Chimney Fires
Prevention is much cheaper than the cure. This goes for your body, your car, and your chimney!
Burn Dry Firewood
Only burn dry, well-seasoned firewood. Inspect your firewood for moisture content using a visual inspection and by comparing its weight to wood that you know is dry (wet wood is heavier). You can also do a sound test on wood: whack two pieces of wood together and if the sound is dull or heavy, the wood is wet. Dry wood sounds hollow and makes a higher-pitched sound when hit. You can also invest in a moisture meter if you want to be absolutely certain.
Burning wood that is too dry can be problematic too. Wood that is too dry can burn very hot, which could cause a chimney fire if your chimney has any creosote build-up. If you have some old firewood that’s been stored in a shed, you can mix it with less-seasoned firewood to prevent the fire from burning too hot; in other words, to create a “just right” fire that’s neither too hot nor too low.
In general, freshly cut wood that is split into sizes ready for burning and stacked for optimal airflow will be ready to use in 6-12 months. A good rule of thumb is if you cut and split it this year, you can safely burn it next year.
Observe Good Burning Practices
Minimize creosote build-up by burning smaller, hotter fires using well-seasoned wood. As tempting as it may be to stuff the firebox full so you don’t have to add fuel as often, a fire needs good airflow to prevent creosote build-up!
Install a Chimney Cap
All chimneys should have a cap, but many don’t. The lack of a chimney cap will allow moisture to fall into the chimney. This moisture will cool the smoke exiting the chimney, encouraging creosote build-up. And of course, any debris or animal nests that wind up in the chimney is an excellent fuel source.
Get Your Chimney Serviced!
Have your chimney cleaned and serviced at least once a year by a certified chimney sweep who will clean and inspect your chimney to ensure safety.
We make things easy by scheduling your annual chimney service well in advance, so you never have to worry about it or risk forgetting this essential part of fireplace safety. Give us a call today to schedule service - and then, we’ll take care of future appointments.
