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Preventing Chimney Fires

Dealing with a chimney fire

Preventing Chimney Fires

A Fine Mess

Chimney fires are often dramatic events, with flames and cinders leaping high enough to come to the attention of neighbors and passersby. But they're not all like this. They can also burn quite slowly if they aren't being fed by much air or fuel. These sleeper fires are no less dangerous than the more visibly dramatic ones. They still reach high temperatures and can damage the chimney and nearby combustible parts of the house. The heat can be so intense that it can actually pick the mortar out from between bricks or stones.

Most chimney fires happen for one simple reason: improper usage and care of wood-burning appliances like fireplaces and woodstoves. (Faulty installation is another key reason, but it ranks a distant second to the first.) We'll talk about proper usage and maintenance in a minute. First, let's address the problem at hand.

A roaring sound—some people describe it as sounding like a freight train or a low-flying airplane—is typically the first indication of a chimney fire. If you hear this sound, and it's growing louder, you have every reason to believe there's a fire raging in your chimney.

Clouds of black smoke and sparks pouring out the top of your chimney are other indications of a chimney fire. If it's a big fire, flames can leap several feet above the top of the chimney.

If you see or hear any of this, here's what to do:

1. Call the fire department. The fire could be out before firefighters arrive, but it's a good idea to call them anyway.

2. Get everyone out of the house.

3. Close the damper or the air inlet controls to the fireplace or stove. This will limit air supply and reduce the fire's intensity.

4. Grab your fire extinguisher (you do have one, right?). Open the door to the fireplace or stove just enough so you can insert the extinguisher's nozzle. Shoot the contents of the entire canister inside and shut the door. If you don't have a fire extinguisher, baking soda or salt pellets work, too. But you'll have to use a lot of either substance. What you don't want to use is water. It could make things worse by causing more steam and gas to enter the chimney, which could crack or warp it.

5. Go outside. If the water to your hose connections is still turned on, wet down the roof and the area around your house. This will lessen the chances of sparks igniting other combustibles like shrubs and trees. Keep the water away from the chimney—wetting down a hot chimney could cause it to crack or even collapse.

In the Nick of Time

A quick way to snuff out a chimney fire is to use a chimney-fire suppressor. Available under several brand names—Chimfex Fire Suppressor is one—these flare-type devices snuff out flames by filling the chimney with a mixture of gases that rise up the chimney and cut off the oxygen supply to the fire. If you use your fireplace regularly, it might be worth keeping several of them around.

Don't go inside your home until the fire department tells you it's safe to do so. When you do, don't be surprised if things don't look too good. A large chimney fire can dump a ton of smoke and soot inside. There might be water damage to deal with as well. For information on putting your house back in order, turn to Clearing the Smoke after a Fire.

Don't use your fireplace or woodstove until you've had a professional such as a fire-place or woodstove installer or a chimney sweep come out and inspect the chimney. Chimney fires can cause a lot of damage to the chimney structure, including cracks and holes in the chimney wall, the flue, the flue pipe, and the flue liner. If the damage isn't repaired, any subsequent fires could move beyond the flue and engulf adjoining areas.

How do chimneys work?

Tool Chest

A flue is the void or passageway through which the products of combustion are removed from the fire to the outside. A flue pipe is a pipe used to connect a fire or appliance to a chimney. A flue liner is the material used to form the flue in the chimney. They can be made of fire clay, refractory quality concrete, or metal. Some older chimneys may not have liners.

If you're not that familiar with fireplaces or woodstoves, some of the terminology used to discuss them might be unfamiliar to you. So, let's start with a quick anatomy lesson. While the following terms primarily apply to fireplaces, woodstoves are pretty similar in design. If you have one, you can follow along, too.

Chimneys are the physical structures that allow fire by-products to escape outside. They're usually made of brick or stone. The following is inside a chimney:

• Flue. This is the actual tube that smoke travels up through.

• Flue liner. This is the material that the flue is made out of. It's usually constructed of fire clay, refractory quality concrete, or metal. Some older chimneys may not have liners. Older unlined masonry chimneys may be used for gas appliances, but they should not be used for wood-burning appliances. The flue gases are corrosive to mortar and can weaken the chimney. Unlined chimneys should be relined before using them with woodstoves or fireplaces.

• Flue pipe. This is the pipe that connects a fire or appliance to a chimney.

• Smoke chamber. The area where the flue starts. It sits right above the damper.

• Damper assembly. Located between the smoke chamber and the firebox, these are adjustable louvers that open and shut to control airflow.

• Firebox. Where the fire burns. Consists of firebrick, outer hearth, inner hearth, and sometimes an ash dump door.

Woodstoves have stovepipes instead of flue pipes. They also have more vents for controlling airflow and temperature.

Now that you know how a chimney is put together, let's take a closer look at how they work, and why they sometimes don't work as well as they should.

Keeping chimneys clean

Dirty Business

Tool Chest

Creosote is a flammable tar-like substance that's created when hot smoke flows up into the chimney.

As mentioned, dirty chimneys—or, more properly put, dirty flues—are prime breeding grounds for chimney fires. But it's not just the ashes and wood particles that travel through chimneys that make them dirty. Creosote, a natural by-product of burning wood, makes them dirty, too, and is at the heart of most chimney fires.

Wood smoke consists of tar acids, organic vapors, and water. When it goes up into a chimney, it wafts over cooler surfaces. As it does, the substances it contains condense and forms a residue—creosote—that sticks to the inner walls of the chimney.

How much creosote forms and how quickly it forms depends on the following:

• How thick the smoke and fumes are

• How hot the fire burns

• The temperature of the stove pipe or flue

A Fine Mess

Creosote buildup in a chimney or stovepipe can be compared to cholesterol building up in arteries. Both substances reduce the diameter of the structure they flow through. As they build up, things don't function as well as they used to. Creosote, which has amazing insulating properties (even better than asbestos), impairs chimney function by putting down a layer of insulation on chimney walls. This hampers effective heat passage in and out of the chimney. When this happens, the temperature inside the chimney goes up fast. If it reaches 1,000°F, the creosote can ignite.

A Fine Mess

Some people believe that having a chimney fire from time to time is a good way to clear creosote from a chimney. Bad idea! Allowing a chimney to catch fire on a regular basis increases the chances of damaging your home. Even a small fire can make a chimney unsafe to use.

Cooler temperatures and slow-burning fires cause greater buildup.

Creosote is pretty unsightly stuff. It can be black or brown, crusty or tar-like, and drippy, or even shiny and hard. Look up inside your chimney, and you'll probably see more than one form of it. But appearance is secondary. What's most important to know is that all forms of creosote are highly flammable. Even small amounts of it can cause a fire. Let it build up over time, say over a winter or two, and there will be enough to fuel a long, hot fire that can destroy your chimney -- and possibly your home.

Getting rid of creosote buildup in its entirety isn't possible. Moisture and cooler temps produce the gases that form creosote. All wood contains moisture and no fire can burn bright and hot forever. But you can keep creosote in check by using wood-burning appliances correctly, and by taking good care of them.

As mentioned, every fire produces creosote, and it builds up in every chimney. However, certain factors can make it worse. They are…

• Poor air supply. Fires need oxygen to burn well. On fireplaces, not opening the damper wide enough or closing glass doors restricts the amount of air that flows into the fire. When this happens, the heated smoke from the fire can't travel as quickly up the chimney as it should. The longer the smoke stays in the flue, the more creosote it forms. With woodstoves, stopping down the damper or air inlets too soon or too much or not using the stovepipe damper correctly can cause the same problems.

• Green wood. Green wood is higher in moisture than seasoned wood is. Burning it takes a lot of energy, which results in a cooler-than-normal fire that doesn't burn very efficiently.

• Cool chimney temperatures. Creosote-creating condensation forms faster in exterior chimneys that are exposed to the elements than in chimneys that run through the interior of a house. Packing woodstoves tightly for an all-day burn creates large, cool fires instead of small, hot ones. When fires burn cool, chimneys can't heat up like they should.

• Burning things other than wood. Plastic substances in things like boxes, wrapping paper, and trash can emit a corrosive acid that can worsen an existing creosote problem.

In the Nick of Time

Wondering how much creosote is in the stove pipe on your wood-burning stove? Here'san easy way to find out: Give the pipe a sharp rap or two with your knuckles. A clean stove pipe will answer back with a ping. If you hear a dull thud, creosote buildup is hampering the pipe's ability to sing.

Woodstoves, fireplaces, and the chimneys they're attached to cause an estimated 100,000 residential fires every year and some 200 deaths. Makes you want to avoid one at your home, doesn't it? What's more, chimney fires are not only deadly, they're expensive. A good-sized chimney fire can warp a metal chimney and crack or break the tile liner on masonry chimneys.

These are major repairs, and they should be made before using the chimney again. Let even a small crack or hole in a flue go unfixed, and the next chimney fire could be much more serious. Sparks from a regular fire could float into the attic or framework near a chimney and cause a house fire.

As already mentioned, there are two basic ways to keep chimney fires from happening: using fire-burning appliances correctly and keeping chimneys clean.