As you snuggle in front of a cozy fire or bask in the warmth of your wood stove, you are taking part in a ritual of comfort and enjoyment handed down through the centuries. The last thing you are likely to be thinking about is the condition of your chimney. However, if you don't give some thought to it before you light those winter fires, your enjoyment may be very short-lived. Why? Dirty chimneys can cause chimney fires, which can damage structures, destroy homes, and injure or kill people.
Chimney fires can burn explosively—noisy and dramatic enough to be detected by neighbors or passersby. Homeowners report being startled by a low rumbling sound that reminds them of a freight train or a low flying air plane. Flames or dense smoke may also shoot from the top of the chimney. However, those are only the chimney fires you know about. Slow-burning chimney fires don't get enough air or have enough fuel to be as dramatic or visible. But, the temperatures they reach are very high and can cause as much damage to the chimney structure—and nearby combustible parts of the house—as their more spectacular cousins. However, with proper chimney system care, chimney fires are entirely preventable.
What Causes Fires
Fireplaces and wood stoves are designed to safely contain wood-fueled fires, while providing heat for a home. The chimneys that serve them have the job of expelling the by-products of combustion—the substances given off when wood burns.
As these substances exit the fireplace or wood stove, and flow up into the relatively cooler chimney, condensation occurs. The resulting residue that sticks to the inner walls of the chimney is called creosote. Creosote is black or brown in appearance and can be crusty and flaky, tar-like, drippy and sticky, or shiny and hardened. Often, all forms will occur in a single chimney system.
Whatever form it takes, creosote is highly combustible. If it builds up in sufficient quantities—and catches fire inside the chimney flue—the result will be a chimney fire. Although any amount of creosote can burn, sweeps are concerned when creosote builds up in sufficient quantities to sustain a long, hot, destructive chimney fire.
Conditions to Watch Out For
Certain conditions encourage the buildup of creosote: restricted air supply, unseasoned wood and cooler than normal chimney temperatures are all factors that can accelerate the buildup of creosote on chimney flue walls.
The air supply on fireplaces may be restricted by closed glass doors or by failure to open the damper wide enough to move heated smoke up the chimney rapidly (the longer the smoke's "residence time" in the flue, the more likely is it that creosote will form). A wood stove's air supply can be limited by closing down the stove damper or air inlets too soon and too much, and by improperly using the stovepipe damper to restrict air movement.
Burning Unseasoned Firewood
Because so much energy is used initially just to drive off the water trapped in the cells of the logs, burning green wood keeps the resulting cooler smoke than if dried, seasoned wood is used. Cooler air obviously doesn’t rise as fast as hot air and therefore spends more time in the flue.
Cool Flue Temperatures
In the case of wood stoves, fully-packed loads of wood (that give large cool fires and eight or 10-hour burn times) contribute to creosote buildup. Condensation of the unburned by-products of combustion also occurs more rapidly in an exterior chimney, for example, than in a chimney that runs through the center of a house and exposes only the upper reaches of the flue to the elements.
How Chimney Fires Damage Chimneys
When chimney fires occur in masonry chimneys—whether the flues are an older, unlined type or are tile-lined to meet current safety codes—the high temperatures at which they burn (around 2000' F) can "melt" mortar, crack tiles, cause liners to collapse and damage the outer masonry material. Most often, tiles crack and mortar is displaced, which provides a pathway for flames to reach the combustible wood frame of the house. One chimney fire may not harm a home, but a second can burn it down. Enough heat can also conduct through a perfectly sound chimney to ignite nearby combustibles.
Pre-fabricated, Factory-built Metal Chimneys
To be installed in most jurisdictions in the United States, factory-built, metal chimneys that are designed to vent wood-burning stoves or pre-fabricated metal fireplaces must pass special tests determined by Underwriter's Laboratories (U.L.). Under chimney fire conditions, damage to these systems still may occur, usually in the form of buckled or warped seams and joints on the inner liner. When pre-fabricated, factory-built metal chimneys are damaged by a chimney fire, they should no longer be used and must be replaced.
Ways to Prevent Chimney Fires
Chimney fires don't have to happen if you’re careful. Start by using seasoned woods only; dryness is more important than hard wood versus soft wood in this case. Also, build smaller, hotter fires that burn more completely and produce less smoke. Never burn cardboard boxes, wrapping paper, trash, or Christmas trees in your fireplace either. And finally, have the chimney inspected and cleaned on a regular basis.
Clean chimneys don't catch fire. Make sure a CSIA Certified Chimney Sweep inspects your solid fuel venting system annually, and cleans and repairs it whenever needed. Your sweep may have other maintenance recommendations depending on how you use your fireplace or stove as well. CSIA recommends that you call on certified chimney sweeps, since they are regularly tested on their understanding of the complexities of chimney and venting systems.
Signs You've Had a Chimney Fire
Since chimney fires can occur without anyone being aware of them and since damage from such fires can endanger a home and its occupants, how do you tell if you've experienced a chimney fire?
Here are the signs a professional chimney sweep looks for:
- "Puffy" creosote, with rainbow colored streaks, that has expanded beyond creosote's normal form
- Warped metal of the damper, metal smoke chamber, connector pipe or factory-built metal chimney
- Cracked or collapsed flue tiles, or tiles with large chunks missing
- Discolored and distorted rain cap
- Creosote flakes and pieces found on the roof or ground
- Roofing material damaged from hot creosote
- Cracks in exterior masonry
- Evidence of smoke escaping through mortar joints of masonry or tile liners
If you think a chimney fire has occurred, call a CSIA Certified Chimney Sweep for a professional evaluation. If your suspicions are confirmed, a certified sweep will be able to make recommendations about how to bring the system back into compliance with safety standards. Depending on the situation, you might need a few flue tiles replaced, a relining system installed or an entire chimney rebuilt. Each situation is unique and will dictate its own solution.
What to Do if You Have a Chimney Fire
If you realize a chimney fire is occurring, get everyone out of the house, including yourself, and call the fire department.
If you can do so without risk to yourself, you can take additional steps to help save your home. Remember, however, that homes are replaceable; lives are not.
Put a chimney fire extinguisher into the fireplace or wood stove and close the glass doors if there are any. Also, close the air inlets on the wood stove. Once you're outside, use a garden hose to spray down the roof (not the chimney) so the fire won't spread to the rest of the structure.
Once it's over, call a CSIA Certified Chimney Sweep to inspect for damage. Chimney fire damage and repair normally is covered by homeowner insurance policies.