When burning wood, a range of particles originating from incomplete combustion, such as a tar-like deposit, in a chimney that consists of soot and chemicals. This is what we call Creosote and is highly flammable. This substance encompasses a range of materials, with the composition of each deposit being virtually unique.
There are several factors that affect the composition of creosote in a chimney, such as: the species and moisture content of the wood being burned, the appliance size and operation, the characteristics of the chimney and flue system, and the environmental conditions.
Combustion by-products cool as they are drawn up through the chimney, condensing along the walls to form creosote deposits.
Creosote deposits contain acetic acid which can trap the carbon from smoke produced by your fire, creating an extremely flammable substance that grows as you burn.
With large amounts of creosote in a chimney, it poses dangers such as chimney fires and carbon monoxide poisoning. The occurrence of creosote in your chimney can be significantly reduced through the responsible use of your stove: burning wood with a moisture content of less than 20% to ensure that there is sufficient airflow for your fire to burn hot, and regularly getting your chimney swept will help decrease the rate at which creosote forms in your chimney.
Types of creosote build-up:
Creosote build-up varies due to a variety of factors; the structure and composition of your flue system, the weather, and the wood you are burning impact how quickly deposits can build-up.
There is no set rate of build-up – all appliances, fires and chimney systems are different.
Here are three classifications commonly used to differentiate the level of creosote build-up in a chimney:
First degree creosote build-up
Resembling a coating of dark dust on the walls of the chimney, first degree of creosote is primarily composed of soot and is the easiest to eliminate. First degree creosote build-up causes minimal issues; the main concern is if the build-up increases in size.
This level of creosote can easily be swept and is a natural occurrence when burning wood.
If you are burning your fires correctly, the build-up of creosote should be slow. Having your chimney swept twice a year should be sufficient to keep the build-up low and manageable.
Second degree creosote build-up
Second degree creosote is significantly more difficult to remove, as it forms thick and shiny black flakes. The increased size of these deposits can cause your chimney draught to become restricted, resulting in smoke and other gases such as carbon monoxide re-entering the room. When deposits are larger, there is an increased chance that they cause a significant chimney fire if they ignite.
When power sweeping tools are required to effectively remove this level of creosote; a standard brush won’t have the power to dislodge more stubborn deposits.
This level of build-up most commonly occurs due to irresponsible burning or when your chimney hasn’t been swept for a prolonged period.
Third degree of creosote build-up
Third degree creosote build-up is a serious issue that requires immediate attention, when they are starting to look glossy and like thick tar dripping down your chimney.
This level can liquefy at high temperatures, causing it to drip down your chimney, resembling thick wax as it cools. At large deposits, creosote can reduce the cross-sectional area of your chimney, your chimney’s draught is significantly reduced.
This can result in smoke and carbon dioxide leaking back into the room.
With third degree creosote, it can be difficult to remove, a rotary chain is often the most suitable option to reduce large deposits. When removing creosote of this level can damage clay flue liners and, if build-up is severe enough, a replacement flue liner may be advised, significantly increasing costs.
Irresponsible burning practices is the result of build-up to this level of third degree. Burning wet wood, having an unsuitably large flue for your appliance, and neglecting regular chimney sweeps all contribute to this severity of creosote build-up.
Dangers of creosote in a chimney:
The main cause of a chimney fires is combustible deposits being ignited – creosote is a combustible deposit that will continue to accumulate as you use your stove, increasing the likelihood of a chimney fire.
Creosote creates financial, structural, and physical risks when it is present in your chimney. The likelihood of these risks occurring increases as the build-up increases.
It is also important to burn responsibly and get your chimney swept to prevent these risks.
In continuing with burning or using your fireplace while having creosote build-up in your chimney, these deposits will continue to be heated. When the creosote rises in temperature, it undergoes the same process of pyrolysis as wood. This process emits gases that can be used as fuel for flames or embers in your flue, starting a fire.
These gases carry a high amount of energy, producing large and dangerous flames when ignited. There is a high possibility that it can ignite other creosote deposits in the chimney, causing a chain of events that lead to a catastrophic chimney fire.
Creosote doesn’t need flames to cause a fire
Creosote’s danger arises from its ignition susceptibility. A low flash point means an ember from your fireplace can ignite it with ease, creating a fire. Although, there is another danger with creosote, arising from its auto-ignition point.
- Creosote’s flash point is just 73.9°C
- Flash point is the minimum temperature an ignition source must be for a material to ignite.
- Material with a lower flash point is more susceptible to being set alight; less temperature is needed to create a flame.
- Creosote’s auto-ignition point is 232.8°C
- Auto-ignition point is the minimum temperature required for a material to set alight when reaching a high temperature without an external source of ignition – spontaneous ignition.
- Material with a low auto-ignition point is more susceptible to being set alight; less heat is required to create a flame.
