What happens if you do not sweep your chimney?
The real risks nobody tells you about

Every year hundreds of families in Catalonia light their fireplace without having had it swept since the previous season. Nobody wants to think their chimney might be dangerous — but the reality is that leaving it unswept has real consequences, some of them serious.

In this article we explain, without needless alarm, what happens when soot and creosote build up inside your flue over months or years.

1. The greatest danger: a soot fire

Creosote is a dark, sticky, highly flammable residue that deposits on the flue walls every time you light the fire. Over time this layer hardens and thickens.

When the chimney reaches high temperatures and enough creosote has built up, a fire can break out inside the flue itself. Temperatures can exceed 1,000°C, damaging the internal lining and the walls of the house, and can even cause a structural fire.

2. Carbon monoxide: the invisible danger

A blocked or partially obstructed flue stops the combustion gases from escaping properly to the outside. The result is that carbon monoxide (CO) — an odourless, colourless gas — can seep into the rooms of the house.

Carbon monoxide poisoning is one of the leading causes of accidental death in the home during the winter months. The symptoms (headache, dizziness, nausea) are easily mistaken for a cold or flu.

3. Performance drops: you spend more and heat less

A dirty chimney does not draw well. Built-up soot narrows the flue and hinders the escape of the gases, so the fire does not burn properly. In practical terms, you need more firewood to get the same heat, or the fire goes out more easily.

An annual sweep is not an expense — it is an investment that pays for itself in firewood you do not burn.

4. Structural damage to the flue

A build-up of creosote, combined with damp and temperature changes, can crack the internal lining of the flue. Those cracks let combustion gases into the cavities between walls, where there is no ventilation and where the fire risk is at its highest.

Spotting and repairing these cracks in time is infinitely cheaper than having to rebuild the whole flue — or, worse, doing it after a claim.

5. Problems with your home insurance

Many home insurance policies require the chimney to be properly maintained and swept in order to cover damage arising from its use. If a fire breaks out and you cannot prove that you have kept up regular maintenance, the insurer can refuse to pay out.

A sweeping certificate issued by a professional chimney sweep is the document you need in order to be covered.

How often should the chimney be swept?

The short answer is: once a year, preferably between September and October, before the cold sets in. If you use the fireplace very heavily (lighting it every day for months), twice a year may be necessary.

Frequently asked questions