· Michael Blair
Creosote is condensed wood smoke. It accumulates on the flue wall and, past a certain point, it burns. Almost everything that causes it comes back to fuel moisture and flue temperature.
The three stages
Stage one is a light flaky soot that brushes off easily. Stage two is harder and granular. Stage three is a hard tarry glaze that is difficult to remove and burns readily.
Each stage results from progressively cooler smoke and more incomplete combustion.
What causes it
Burning unseasoned wood is the main one — water in the wood absorbs heat, cools the smoke, and condenses creosote on the flue. Damping a fire down for a long slow burn does the same.
An oversized flue, or an exterior chimney that runs cold, cools the smoke too and accelerates it.
- Burn seasoned wood only, below about 20 percent moisture
- Never damp a fire down for an overnight burn
- Hot short fires produce less creosote than smouldering ones
- Exterior chimneys run cooler and build faster
- A moisture meter costs very little and settles the question
How often to sweep
Annually for a fireplace in regular use, and more often for a wood stove used as a primary heat source. Industry guidance is inspection annually and sweeping when deposits reach about an eighth of an inch.
A chimney used two or three times a winter still wants an annual inspection, because animals and debris are as much a risk as creosote.
Signs of a past fire
Puffy or honeycombed creosote, cracked or displaced flue tiles, discoloured or warped rain cap, and creosote flakes on the roof or in the garden.
Chimney fires often go unnoticed and leave a flue that is no longer safe. That is what an annual inspection is looking for.