An open chimney or flue system acts like a distillery. You have a hot fire and organic material source, gases containing water and condensates, and a cooling surface (the exterior brickwork of the chimney) to condense the volatile substances in the vapour. Unfortunately for users of woodburning stoves, these volatiles which solidify on the surface of the internal chimney flue present an unseen danger known as wood tar creosote.
Wood tar creosote is made up of the unburned natural resins, waxes and oils in the wood (wood wouldn't be able to burn without them). It also contains burnt particles of carbon from the wood (which is why it is black in colour), and a whole array of chemical polycarbons and sulphides (which is why it is corrosive). This inflammable and toxic mixture passes through a chimney flue depositing layers of flammable creosote material on the cold inner walls of the chimney. The rougher the texture of the surface of the flue, the more likely it is to stick, and any cracks or damaged masonry areas provide an ideal depository for these chemicals. The more fires, the greater the build up in layers of creosote, the narrower the space for the gases to escape, sometimes blocking the flue entirely.
A hot flue temperature with a good updraught of hot air will prevent creosote from adhering to the walls. Due to areas within a flue being generally inaccessible, it follows that the easiest way of tackling creosote formation in chimneys is a preventative approach. Best working practice is to have a clean well maintained chimney flue, that is at a high enough temperature to discourage any creosote formation.
A general guide is as follows:
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