Fireplaces and wood stoves are designed to safely contain wood-fueled fires while providing heat for a home. A chimney expels the by-products of combustion which are 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 is usually crusty and flaky. It is tar-like, drippy, sticky, shiny and hardened. Often, all forms will occur in one 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. If creosote builds up in sufficient quantities it will sustain a long, hot destructive chimney fire.
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.
Anatomy of a flue fire
Heated wood releases hydrocarbon gases. When they get hot enough (about 1100 degrees F) and mix with air can catch fire. When open or wood heater fires smolder, unburned gases condense and deposit on the stove pipes and the flue as runny acids and liquid tars that harden into what is known as creosote. Both a cool flue and steam from green or wet wood encourage this condensation. Creosote can appear as a sooty powder, gummy mess, hard glaze or a deposit that looks like burnt marshmallows.
A creosote fire can burn with such blast-furnace intensity that it sets off this frightening chain of events. Balls of flaming creosote shoot out of the chimney top onto the roof, crumbling and cracking mortar, brick chimneys crack open, stainless steel liners warp, buckle and separate at the seams and masonry in the chimney expands with such force that sections of the chimney can blow out. Flames can spread to the structure or roof of the house even explode into the room.
Creosote collected in a flue. A roaring fire waiting to happen.
The air supply on fireplaces may be restricted by closed doors or by failure to open the damper wide enough to move heated smoke up the chimney rapidly. The longer the smoke remains 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 or by improperly using the stovepipe damper to restrict air movement.
Burning unseasoned firewood
Much energy is used initially just to drive off the water trapped in the cells of the firewood. Burning green wood keeps the resulting smoke cooler, as it moves through the system encouraging the formation of creosote.
Cool flue temperatures
In the case of wood stoves, fully-packed loads of wood contribute to creosote buildup. Condensation of the unburned by-products of combustion also occurs more rapidly in an exterior chimney than in chimneys that run through the center of a house and exposes only the upper reaches of the flue to the elements.
Damage to Masonry 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. 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.
Metal pipe chimneys are designed to vent wood burning stoves or pre- fabricated metal fireplaces. They 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. They should no longer be used and be replaced.
Avoiding chimney fires
Chimney fires don't have to happen. Here are some ways to avoid them.
Use seasoned woods only (dryness is more important than hard wood versus soft wood considerations).
Build smaller, hotter fires that bum more completely and produce less smoke.
Never burn cardboard boxes, wrapping paper, trash or Christmas trees; these
can spark a chimney fire.
Install stovepipe thermometers to help monitor flue temperatures where wood
stoves are in use, so you can adjust burning practices as needed.
Have the chimney inspected and cleaned on a regular basis.
Remember, clean chimneys don't catch fire.
What do if in case of a chimney fire
Close the door and air intakes to your wood stove ONLY if you can do safely. Get everyone, including pets, out of the house, because fire can flash right through the house with incredible speed. Then call the fire department from a safe distance. Do not try to put out the fire and do not hesitate leave immediately. Once outside, use a garden hose to spray your roof in the area near the chimney so the fire won't spread to the roof. DO NOT SPRAY THE CHIMNEY ITSELF. Remember, homes can be replaced. Humans can not.
Afterwards, if your house is still standing, call a chimney service professional to clean and inspect the chimney as well as assess the damage. Your chimney will need to be repaired and probably relined to make it safe for further use.
20 Safety Tips
1. Dirty woodstove glass? Try dipping a dampened piece of newsprint in the fine white ashes after your fire has died. Wipe it onto the glass in circular motions -- it works well if the glass isn't terribly dirty to begin with.
2. Putting a chimney cover or chimney cap on top of your flue can save you a lot of money in the long run. The covers keep out damaging moisture, which wears away masonry and steel chimneys -- not to mention that they keep birds and other critters out.
3. The National Fire Protection Association (in NFPA 211) recommends you have your chimney checked at least once a year, and cleaned if needed. Heavy users need more frequent check-ups.
4. If you have a newer EPA-rated woodstove, you might have a catalytic combustor in there somewhere. Make sure to check the owner's manual about cleaning it -- and stick to the schedule. Combustors should last 5 or 6 years, but a clogged or dirty one will fail rather quickly.
5. Do you have a smelly fireplace? Chimney breath is most often caused by moisture, rain, or high humidity. Have your chimney cleaned early in the spring to make the humid summer days less odiferous.
6. Ever wonder what wood is the best to burn as firewood? Oak is an American favorite. Other hardwoods are also a good choice. You can burn other softer wood also, as long as it is split and dried long enough. It's much more important to burn dry wood than to worry about what kind of wood it is.
7. During a chimney fire call the fire department (911) and get everyone, including pets, of the house -- just like any other house fire.
8. Springtime is the right time to get your chimneys checked! Sweeps are generally less frantic in the spring (vs. the crazy fall season) and if your chimney needs repairs, they can be made before the cold weather hits!
9. Mild winters mean more chimney fires! It's true. People choke back their wood stoves in mild weather -- leading to more creosote accumulation -- but many don't realize this, so they skip getting it cleaned, thinking it doesn't need it as bad as it would after a cold winter.
10. Black stove pipe (and furnace pipe, for that matter) should be securely fastened together at each joint with no less than three sheet metal screws or pop-rivets. Stove and vent pipe should be inspected at least yearly, and replaced when signs of rusting or wear are evident.
11. Gas logs release a lot of water vapor when they are burning. You should be wary of mold and mildew, especially if you have asthma and respiratory problems, when using them for longer than a few hours. A CO detector is a great gas-log accessory. You can find one in many home-improvement and mass chain stores.
12. Make sure you have working smoke detectors installed and check their batteries regularly.
13. Have your chimney checked every year (no matter how you heat your home) to make sure the chimney can do its job to properly vent hot, toxic gases and carbon monoxide from the heating system to the outdoors.
14. Have a high-quality, long-lasting chimney cap installed to keep out debris and prevent birds, animals and insects from nesting in your chimney.
15. Following a violent storm, earthquake, flood or lightning strike, have your chimney inspected for damage -- inside and out. This includes checking for cracks and fallen bricks. For safety's sake, DO NOT USE YOUR CHIMNEY until it is checked by a professional.
16. Install a carbon monoxide detector to warn of harmful gases that may be entering your home because of a blocked or damaged chimney.
17. Have your chimney waterproofed to prevent long-term corrosion and masonry damage.
18. Have your chimney flashing (the seal between the chimney and the roof) inspected and maintained. Flashing prevents rain water and snow melt from entering your home and causing costly damage to your walls and ceilings.
19. Save energy dollars and eliminate unpleasant off-season odors. Have a sealing damper installed in your wood-burning chimney system.
20. Have your chimney sweep ensure that your chimney has an appropriate liner. Chimney liners are required in new construction to separate hot heating system emissions from the structure of your home.
Fireplace inserts became popular in the 1970's during the nation's first oil crisis. Homeowners were told they could just slip their wood burning insert into the fireplace opening and, presto, have a cozier, more economical way to winter warmth. It soon became apparent, however, that inserts presented a unique installation and maintenance challenge. Their safe use as originally hooked up became highly suspect. The Consumer Product Safety Commission and chimney service professionals began to view them with alarm. The incidence of house fires traced to the inadequate installation and maintenance of fireplace inserts escalated.
Many smoke chambers and chimneys ARE TOO BIG to properly vent an insert stove. A typical masonry chimney designed to vent an open fire place, has a 12 by 12-inch tile liner (144 square inches). The average insert stove requires an 8-inch round chimney, or about 51 square inches. This helps speed the much smaller volume of flue gases up and out of the chimney before they have a chance to cool down and cause trouble.
The villain here, the thing that creates a safety hazard, is the excessive production of creosote. Creosote is contained in the volatile flue gases coming up the chimney with the smoke. When these gases are allowed to linger in the flue because of an oversize chimney, they cool, causing the creosote to condense onto the walls of firebox, smoke chamber and chimney.
These creosote deposits constitute a fuel that can cause intense chimney fires when ignited. One chimney fire, or a series of chimney fires, can cause unseen structural damage to the chimney and can eventually cause adjacent flammable materials (joists. studs, wall paneling, roofing, wallpaper and mantel) to catch on fire. Exposed to heat over a period of time, all of these combustibles undergo a process called "pyrolysis," which causes their ignition temperatures to be lowered so they ignite much more easily than was possible before. A combustible doesn't need exposure to direct flame. It can ignite whenever oxygen is available and its temperature is raised high enough.
Tips on fireplace safety
Don't build a fire too big for the fireplace.
Don't use fire starters such as charcoal lighter, kerosene or gasoline to start the fire.
Fireplaces radiate heat just like space heaters so furniture and other combustibles should be kept a safe distance from the fire.
Be sure the chimney is clean and in good condition.
Be sure the damper is open before starting a fire.
Don't burn trash in the fireplace.
A screen or glass doors should cover the front of the fireplace to prevent sparks from flying out into the room.
Seasoned wood is safer than green wood. Hardwoods have less creosote build-up than softwoods.
Ashes should be removed into a metal container and allowed to thoroughly cool before being placed in the trash container.
Don't leave small children alone in a room with a fire.
Do not leave a fire burning when you go to bed or leave the house.
Excessive creosote build-up is caused by one or more of the following:
A flue too big for the appliance it serves.
Most open fireplace flues are too big to be used to vent a wood burning stove or a fireplace insert without a liner. This causes a sluggish flue draft effect and gases expand to fill the space then quickly cool down, allowing creosote to deposit on the chimney walls, where it sticks like glue.
Solution: Install an insulated flue liner that is properly sized for the stove or fireplace insert.
Poor wood burning habits, such as severely limiting the air supply in a stove to achieve an all-night burn. This causes a sluggish draft and a smoldering fire that doesn't get hot enough to burn the volatile gases released by the wood.
Solution: Burn smaller, hotter fires using seasoned firewood with a good draft never air-starve the fire. This way, the heat will quickly warm up the flue and increase the draft, while volatile gases burn up in the stove the way they should.
An oversize or outdated stove:
When a stove is too big for the space it heats, it's likely to be burned in the closed down, creosote-producing mode. Also, many stoves sold in the 70s and early 80s are now obsolete. They're not as clean burning as EPA-certified models designed to meet new emissions standards.
Solution: Replace your old stove with a new high-tech unit correctly sized for the space you want to heat and matched with the proper venting system.
A neglected flue.
Many of us don't give our chimneys a second thought until something goes wrong.
Solution: Find an experienced, certified professional chimney sweep right away. Put your whole heating system on a regular inspection and maintenance schedule.
Stove installation and chimney service work are "NOT" home handyman chores. They call for professional know-how, special training, technical expertise and familiarity with the local building codes.
Solution: Always entrust installation and maintenance to a professional. A wood burning appliance with improper venting and an unlined chimney cannot give the house proper protection.
What can you do?
Ask a professional about good flue liners; this can include stainless steel, cast-in-place (or "poured-in-place"), and modular ceramic pipe systems. They are designed both for unlined chimneys and as replacement for damaged claytile liners.
If your house and chimney were lucky enough to survive that first flue fire, don't wait for a second or third event to do them in. Now that you've learned how a chimney fire can take the whole house down, do use caution and take preventive action.
DO NOT LIGHT ANOTHER FIRE !
If you suffer a flue fire in your stove or fireplace do not light another fire until you've had the chimney professionally inspected and repaired!!