What creosote actually is
When wood burns it releases water vapour, gases, and unburned particles. As those gases travel up the flue they cool and condense on the cooler walls — leaving behind a dark, sticky residue. That residue is creosote. It's flammable, it builds up over time, and it's the primary fuel behind almost every chimney fire.
The three stages
Stage 1 — soft, flaky soot. Easy to brush out during a routine clean. A normal result of regular use.
Stage 2 — crunchy, tar-like flakes. Harder, shinier, and starting to bond to the flue wall. Needs more aggressive cleaning.
Stage 3 — glazed, hardened layer. A black, shiny coating that resists brushing. It's highly flammable and often needs specialised treatment to remove safely.
What encourages it to form
- Damp or unseasoned firewood (the biggest factor).
- Smouldering, low-temperature burns — particularly overnight.
- Heavily restricted air supply.
- Cold flues that never quite reach operating temperature.
- Long flue runs in unheated roof spaces.
Why it matters
As creosote builds up, the internal diameter of the flue narrows. The fire draws more slowly, smoke can back into the room, and the heater becomes less efficient. If a hot fire finally raises the flue temperature above creosote's ignition point, the result is a chimney fire.
What helps
Burn dry, seasoned hardwood. Run a hot fire daily — even after long smouldering burns. And book an annual clean to remove build-up before it can harden into something serious.
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.