While chimneys are designed to contain heat from normal fires in a woodstove or fireplace, chimney fires can easily burn at 2000 degrees Fahrenheit, damaging mortar and even starting a house fire.
When wood in your fireplace burns, the byproducts of the fire are released up the chimney. Smoke, water vapor, bits of unburned wood, tar, and a number of other hot materials rise up the chimney. As they rise up the cool chimney, some condense and stick to the interior of the chimney, forming creosote.
Creosote is extremely flammable and burns at very high temperatures. Small quantities are not a concern, but if allowed to accumulate, the creosote could fuel a long, hot, destructive fire. Creosote can burn at 2000 degrees Fahrenheit, which is hot enough to melt mortar, crack flue tiles, and damage the outside masonry. Metal flues fair only slightly better. While metal flues are designed to withstand temperatures up to 2100 degrees, damage can still occur. Metal flues that have been subject to a chimney fire must be replaced.
The real danger of chimney fires is the fire spreading to the rest of the house. As the chimney fire cracks the flue, the heat from the fire can escape the chimney and ignite flammable parts of your home. Chimney fires can easily go from a contained burn to a completely destroyed home.
If your chimney does catch fire, leave the house immediately and call 911 from outside the house. Do not light another fire until after your chimney has been inspected by a professional chimney sweep.
Having a chimney fire extinguisher such as Chimfex on hand is a wise investment, but once you have a chimney fire, it is too late to shop for one.