Thursday, August 9, 2018

How to Avoid a Chimney Fire that Can Lead to a House Fire

House fire caused by flammable
creosote and improper construction
Creosote is a flammable substance that must be removed from the flue and smoke chamber periodically in order to avoid having a chimney fire. 

Since all wood creates creosote, including dry hardwoods, creosote is impossible to avoid when burning wood in a fireplace, wood-burning stove, circulating fireplace, or stove insert.

Creosote can only be removed by brushing the flue and chamber out with wire or poly brushes that are made especially for this purpose. Professional chimney sweeps know what size and type of brush to use for the type of flue system installed. There are many different types of venting systems, so knowing how to properly maintain each type is critical.

In a masonry chimney with a wood-burning fireplace the most common type of flue system is made out of vitreous clay tile, and the smoke chamber is usually made out of brick, stone, or block. This type of venting system accumulates more creosote than a stainless steel flue liner does because it is more porous.

Stainless steel flue liners don't accumulate as much creosote if they are properly insulated and installed correctly, but can be damaged by a chimney fire so maintenance is necessary.

Smoke chambers that are not parge coated with insulating mortar per IRC Code requirements allow more creosote accumulation due to their rough surfaces and corbels which decrease the flow of smoke and tar vapours.

The National Fire Protection Association and the National Chimney Sweep Guild recommend that creosote be removed after 1/8" accumulation on flue walls.

Gene Padgitt Sweeping a masonry chimney
The creosote being removed should be stage 1 which is very light and ash-like, or stage 2 which is more dense. If there is any accumulation of stage 3 glazed baked-on tar-like creosote, something is not right with the system, the fuel being used, or the operation of the appliance. Stage 3 creosote is the most flammable type, and is most often associated with chimney fires.

A chimney fire occurs when a spark ignites the flammable creosote, usually in the smoke chamber just above the damper, and flame may spread through the remaining fuel source (creosote) on the flue walls.

Stage 3 Creosote on flue walls
Chimney fires can be of short or long duration, but almost always cause damage to the smoke chamber and flue liner due to the quick temperature differential that occurs to the masonry during a chimney fire. Damages may occur to the face wall above the fireplace opening inside the house, to the back wall of the chimney, or to the top portion of the chimney and cement cap due to expansion. Fresh breaks may be found in the masonry and cement cap in a masonry chimney.

Burnt creosote that has been on fire
In a manufactured fireplace and metal chimney damages may occur to the metal smoke chamber and metal chimney pipe in the form of buckling, warping, and opening of seams. With a Class A chimney system serving a wood-burning stove, the same type of damage can occur.

In either case, the damage must be repaired, or parts replaced before further use because the system is no longer functional if it has damages.

Unfortunately, most chimney fire damage is found only during inspection of the system during routine chimney cleaning/inspection maintenance by a professional chimney sweep. Most chimney fires go unnoticed by homeowners when the fire occurs because they either don't recognize the whoosh of air and increased draft during a chimney fire, or are out of the room when it occurs and the fires goes out before they return.

Burnt, honeycomb creosote that has
been burned in a chimney fire
After a fire has damaged a chimney system, the flue liner and chamber may have cracks and gaps that would allow a second fire to escape to nearby combustible wood framing. This is how a house fire due to chimney fire commonly occurs. Another way a house fire can occur is if flame and sparks shooting out the top of the chimney catch the roof on fire.

chimney inspection with a Chim-Scan Camera
Water damage from rain and condensing acidic flue gasses often causes mortar joints between tile liner sections to deteriorate and leave gaps, which may also allow heat and flame to escape the flue system and cause a house fire.

The only way to know if the flue system has damages is to sweep it first to remove creosote, then run a special chimney camera through the chamber and flue. Photos may be taken with this type of camera. If damages are found, do not use the appliance until it is properly repaired by a professional chimney contractor.
Break in clay tile flue liner
In the greater Kansas City area, call HearthMasters, Inc. for chimney maintenance, repair, and building.

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