Soot, creosote and tar: what builds up in your chimney and why it's dangerous

When we talk about sweeping a chimney, people think of soot. But what can make a flue genuinely dangerous is creosote. And if it has been there a while, it can harden into tar, which is virtually impossible to remove without professional tools.

Soot: the most visible, but not the most dangerous

Soot is the solid residue of incomplete combustion — carbon particles that settle on the flue walls. It is relatively easy to remove with a brush and vacuum. The problem comes when it mixes with water vapour and condenses on the walls, forming creosote.

Creosote: the real risk

It forms when the combustion gases cool before they leave the flue. There are three stages:

• Stage one: a flaky, powdery deposit. The easiest to remove.
• Stage two: a harder, sticky deposit, similar to liquid tar. It calls for powerful tools.
• Stage three (hardened tar): the hardest and most dangerous. It may call for chemical treatment or relining the flue.

Factors that speed up the build-up

• Damp firewood: the number one factor. It produces far more water vapour, which condenses in the flue
• Low flue temperature: the colder it is, the more condensation
• Slow or smouldering fires: they produce more unburnt gases
• Large-diameter flues: the gases cool before they get out
• Infrequent use: a cold flue at the start encourages condensation

How to prevent it

• Use only well-seasoned firewood (below 20% moisture)
• Keep a good air supply while the fire is lit
• Burn at a high enough temperature, not slow fires
• Have the flue checked and swept at least once a year