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The Facts About Chimney Fires

Your chimney and the flue that lines it adds architectural interest to your home, but its’ real function is to carry dangerous flue gases from your fireplace, wood stove or appliance safely out of your chimney. A chimney helps your household air stay breathable, just as your windows and your bathroom, attic and kitchen vents do. Unlike those other exhaust points in your home, however, fireplace and wood stove chimneys need a special kind of care, the chimney should be swept as per your chimney sweeps advice. 

As you snuggle in front of a cozy fire or bask in the warmth of your wood stove, you are taking part in a ritual of comfort and enjoyment handed down through the centuries. The last thing you are likely to be thinking about is the condition of your chimney and chimney flue. However, if you don’t give some thought to it before you light those winter fires, your enjoyment may be very short-lived. Why? Dirty chimneys can cause chimney fires, which damage chimney and chimney flue structures, can destroy homes and injure or kill people.

No One Welcomes a Chimney Fire

A chimney fire in action can be impressive. Indications of a chimney fire have been described as creating a loud cracking and popping noise a lot of dense smoke, and an intense, hot smell Chimney fires can burn explosively noisy and dramatic enough to be detected by neighbors or people passing by. 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 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 fuel to be dramatic or visible. But, the temperatures they reach in the chimney flue 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, regular chimney sweeps, fires are entirely preventable.

Creosote & Chimney Fires: What You Must Know

Fireplaces and wood stoves are designed to safely contain wood fuel fires, while providing heat for a home. The chimneys that serve them have the job of expelling the by products of combustion the substances produced when wood burns. These include smoke, water vapor, gases, unburned wood particles, hydrocarbon, tar fog and assorted minerals. 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 in the chimney flue.

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 the internal chimney flue temperature is high enough the result could be a chimney fire. Certain conditions encourage the buildup of creosote in the chimney flue.

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 may be restricted by closing the glass doors, by failing to open the damper wide enough, and the lack of sufficient make up air to move heated smoke up the chimney rapidly (the longer the smoke’s “residence time” in the chimney flue, the more likely is it that creosote will form.y highlighting part of me and selecting the options from the toolbar.

A wood stove’s air supply can be limited by closing down the stove damper or air inlets too soon or too much. Burning unseasoned wood because so much energy is used initially just to drive off the water trapped in the cells of the logs keeps the resulting smoke cooler, than if seasoned wood is used. In the case of wood stoves, overloading the firebox with wood in an attempt to get a longer burn time also contributes to creosote build up in the chimney flue.

How Chimney Fires Hurt Chimneys

Masonry Chimneys

When a chimney fire occur in masonry chimney whether the flue is an older, unlined type or pot lined to meet current safety codes the high temperatures at which they burn (around 2000°F) can “melt mortar, crack pot liners and chimney pots, cause liners to collapse and damage the outer masonry material”. Most often, thermal shock occurs and pot liners crack and mortar is displaced, which provides a pathway for flames to reach the combustible wood parts of the house. This event is extremely dangerous, call 999 immediately.

Pre-fabricated, factory-built, metal chimneys

Factory built, metal chimneys that are designed to vent wood burning stoves or pre-fabricated metal fireplaces must pass special tests. Most tests require the chimney to withstand flue temperatures up to 2100°F without sustaining damage.

Under chimney fire conditions, damage to these systems still may occur. When pre-fabricated, factory-built metal chimneys are damaged by a chimney fire, they should no longer be used and must be replaced.

Special Effects on Wood Stoves

Wood stoves are made to contain hot fires. The connector pipes that run from the stove to the chimney are another matter. They cannot withstand the high temperatures produced during a chimney fire and can warp, buckle and even separate from the vibrations created by air turbulence during a fire. If damaged by a chimney fire, they must be replaced.

Nine Signs that You’ve Had a Chimney Fire

Since a chimney, damaged by a chimney fire, can endanger a home and its’ occupants and a chimney fire can occur without anyone being aware of them it’s important to have your chimney regularly inspected and the chimney swept. Here are the signs that a professional chimney sweep looks for....

“Puffy” or “honey combed” creosote.

Warped metal of the damper, metal smoke chamber connector pipe or factory-built metal chimney.

Cracked or collapsed flue liners, or brickwork with large chunks missing.

Discolored and/or distorted rain cap.

Heat-damaged TV antenna attached to the chimney.

Creosote flakes and pieces found on the roof or ground.

Roofing material damaged from hot creosote.

Cracks in exterior masonry.

Evidence of smoke escaping through mortar joints of masonry or flue liners.

 If you think a chimney fire has occurred, call Chimney Sweeps Horsham on

0800 061 2234

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