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CHIMNEY FIRES can burn explosively. Flames or dense smoke may shoot from the top of the chimney. Described as sounding like a freight train running at high speeds.

Chimney fires typically can be caused in three ways:
1. Creosote deposits ignite and burn at very high temperatures,
2. Clearance from combustibles is not adequate, and
3. Pyrolysis, a chemical composition of the home framing that is changed by the heat and is capable of spontaneously igniting.

Sometimes chimney fires burn unnoticed by the occupants of your home because they as they smolder, they are slow burning fires and sometimes spread into the home. 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 both to chimney structures and ignite other structural or flammable sections close to the house. Other factors such as bird's nest, inadequate clearance to combustibles contribute to structural chimney related 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. It can be crusty and flaky ... tar-like, drippy and sticky ... or 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. 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. 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.

Air supply
Closed glass doors may restrict the air supply on fireplaces 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 smoke cooler, as it moves through the system, than if dried, seasoned wood is used.

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.

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.

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),
 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.

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Providing Dryer Vent and Chimney Cleaning Service for Montgomery County,
Bethesda, Damascus, Gaithersburg, Germantown, Olney, Potomac, Poolesville, Rockville,
Silver Spring, and Frederick County, Maryland since 1991.

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