Everyone enjoys a cozy, warm fire on a chilly evening. However, many neglect to have annual chimney inspections and cleaning done. Dirty chimneys can cause chimney fires, which damage structures, destroy homes and injure or kill people. Mariposa County Fire has already responded to a few calls for chimney fires. The information below is being shared and hopefully will eliminate that call for help.

Chimney fires can burn explosively - noisy and dramatic enough to be detected by neighbors or passersby. Flames or dense smoke may shoot from the top of the chimney. Homeowners report being startled by a low rumbling sound that reminds them of a freight train or a low flying airplane.

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. With proper chimney system care, chimney fires are entirely preventable.

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.

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 2,000 degrees) 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.

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 a chimney fire damages pre-fabricated, factory built, metal chimneys, they should no longer be used and must be replaced.

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 burn 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

Clean chimneys don't catch fire. To read more about chimney safety, visit the Chimney Safety Institute of America at www.csia.org.