The fuel: creosote
Creosote is a black, tar-like residue that forms when combustion gases cool and condense inside the flue. Over time it hardens into a glaze that lines the walls of the flue. Once it's hot enough, it ignites — and burns at temperatures well above what the flue is designed for.
The trigger: a hot fire after build-up
Most chimney fires start after a stretch of low, smouldering burns (which deposit creosote) followed by a hot, fast fire that finally ignites the build-up. A roaring sound, vibration, or sparks from the cowl is often the first sign.
Common contributing factors
- Burning unseasoned or damp firewood.
- Damping the heater right down overnight.
- Skipping annual cleans.
- Restricted flues with poor draught.
- Burning treated timber or rubbish — never do this.
Habits that prevent them
The good news: chimney fires are almost entirely preventable with a few simple habits.
- Book a professional clean once a year, before winter.
- Burn properly seasoned hardwood (12+ months dry).
- Run a hot fire each day to keep the flue clean.
- Avoid heavy overnight damping when possible.
- Watch for early warning signs — smell, slow draw, smoke spillage.
If you suspect a chimney fire
Get everyone out of the home, close the heater's air supply if it's safe to reach, and call 000. Don't re-light the heater under any circumstances until it's been fully inspected — even if the fire appears to have gone out.
A note on safety: chimney and flue issues can become serious quickly. When in doubt, leave the heater unlit and call a qualified sweep — a short visit beats guessing.