Creosote and chimney fire — when combustion makes the chimney dangerous

Common soot is annoying but harmless in itself if removed regularly. Glazed soot is something else – it is a hard coating that can make a chimney genuinely dangerous, and it is one of the things you as a chimney sweep should be able to recognize and handle.

Creosote forms when the temperature during combustion is too low. Over time, the creosote can burn permanently on the inside of the chimney and form an almost ceramic layer, which is very hard and therefore difficult to remove.

Chimney fires often occur when igniting or when loading firewood, where there is plenty of air for combustion, and where therefore very high temperatures can develop. During a chimney fire, the glazed soot expands greatly, which reduces the chimney's free cross-section — or in rare cases blocks it completely.

• 01Loud, whistling or humming sound from the chimney
• 02Severe heat development and glowing or black spots on the chimney surface
• 03Sparks or heavy smoke from the chimney top
• 04Cracks or damage occurring or worsening during the fire

• 01Use dry firewood — wet firewood gives a lower combustion temperature and more gloss soot.
• 02Light from the top so ignition gives more even and cleaner combustion.
• 03Ensure regular cleaning so soot and condensation don't accumulate.
• 04Avoid overfilling the fireplace with firewood it is not sized for.

If the customer discovers a chimney fire, air to the fireplace must be closed and 112 must be called — water should not be poured directly into the chimney. After a chimney fire, the chimney must always be checked by a chimney sweep before being used again, as the severe temperature fluctuations may have damaged the inside of the chimney.

“A chimney fire is not just an unpleasant experience — it is a sign that something in the heating or maintenance has not been as it should.”

— Professional rule of thumb in chimney sweep craft.