How does it get dirty?
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. 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 flue temperature is high enough – the result could be a 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 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 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 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 buildup.
How Chimney Fires Hurt Chimneys
When a chimney fire occur in masonry chimney – whether the flue is an older, unlined type or 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, thermal shock occurs and tiles crack and mortar is displaced, which provides a pathway for flames to reach the combustible wood frame of the house. This event is extremely dangerous, call 911 immediately.
Pre-fabricated, factory-built, metal chimneys
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. 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.
Chimney cleaning may not be the first item on life’s “To Do” list but if overlooked too long can leave you without house and home! More important then just doing it, is knowing when to clean a chimney, and when it is done correctly. Many times I’ve witnessed a chimney be cleaned that didn’t need it, and on the flip side, a chimney that needed a proper cleaning get one that did not bring it back to a safe condition. If the chimney is built correctly, then cleaning it is a breeze. But cobble one together and it will be a headache from day one.
The chimney is an ancient concept as simple in nature as heat rising, however with the multiple variables involved in the demand put on a chimney these days, improper or insufficient maintenance or installation can become life threatening. In “the old days” a chimney was the only heat source and was much better then the alternative, freezing. It is not uncommon in old homes to see many fireplaces throughout a house. However, in today’s high efficiency, forced air environment, a chimney is a gaping hole of energy loss continuously drawing a volume up to the size of its flue. Lets look at a typical flue size. In Texas a 13×13 is pretty common. The amount of air going up the flue is like having a 12×12 hole in the side of your house.
This rising air is the heated air your furnace produced and is being replaced through every crack in every door and window in the house, and is the temperature of the air outside.
For this reason the traditional “open” fireplace is negative efficient in today’s house. It may feel warm close to it but it’s almost guaranteed to be colder in the rooms farther away from it or with less tight windows and doors. Your furnace will usually work overtime to maintain the same temperature. All while the chimney keeps slurping the heat out of your house. Most people pay to have these installed in a home spending big bucks and thinking they will benefit from heat, but really just get looks. If this is you, don’t despair. There are options to make the energy consuming chimney you may have into an alternative fuel heater of modern efficiency, with little risk if correctly installed, AND PROPERLY MAINTAINED!
The fact is in the “open” fireplace it is much more likely to develop a dangerous batch of creosote due to the principles involved. Creosote forms due to two conditions.
1. Lack of oxygen resulting in incomplete burning of the particulate.
2. Temperature. If you’ve ever seen a thermometer for measuring the performance of a wood burning stove alot of them have an indicator below 250F that warns of creosote forming conditions possible.
In an “open” fireplace the large size of the fire and inefficiency (all the heat, and then some going up the chimney), the temperature has a tendency to be higher through out more of the chimney then a closed unit with more efficiency. The fire also gets as much air as it wants provided glass doors are not used ( and generally not recommended be closed by manufacturers when burning) so the build-up of creosote is generally not much of a problem. The majority of chimneys swept above an open fireplace will have some soot, ash, and cobwebs, but not dangerous creosote. However excessive use of green wood or improperly seasoned wood will cause 3rd stage creosote build-up. It looks like a black shiny mirror. Low burring temps will cause excessive smoke and a sort of liquid
No, In fact the efficient solid fuel appliance of today is generally the culprit behind most chimney fires as it provides the perfect recipe for creosote in an improperly installed location. By being so efficient, these units pump most of the heat they produce into the room and provide a great substitute for the standard primary source of heat in a home. But in doing this the chimney never really gets hot like it would without it. They also run off the principle of limiting the air supply to the edge of extinguishing the fire to get the longest burn possible and much reduced fuel consumption. This combination can lead to an extremely fast development of creosote to dangerous levels in an unexpectedly short period and left neglected can become a cache of stored energy waiting to ignite.