Interactive lesson

Chimney, CO and fire, step by step

Interactive lesson: how a chimney works, soot and sweeping, chimney fires, air supply, carbon monoxide, alarms and protecting the house from fire.

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This lesson is general guidance, not professional training. Local regulations and expert advice always take precedence. Read our disclaimer.

The whole lesson as text

1. How a chimney works

A chimney works by itself, as long as there is draught.

Warm smoke rises — The greater the temperature difference and the taller the chimney, the stronger the draught.
Draught pulls in air — Draught pulls fresh air into the stove and carries smoke outside. Without an air supply the house gets negative pressure and pulls smoke back in.
What weakens draught — A cold, uninsulated or too-low chimney, a blocked flue and a flue too large for a small stove. Smoke cools, condenses and makes tar.
Damper never fully shut — While there are embers in the stove, don't close the damper completely. A closed damper and a damped-down fire are a classic cause of CO poisoning overnight.

Why: Warm smoke is lighter than cold air and rises through the chimney. When the draught weakens, smoke and carbon monoxide stay in the stove or come back into the house.

Two appliances on one flue only with the sweep's approval: one pulls smoke from the other.
Building a new chimney is a job for a designer and a builder, under the building regulations.

2. Soot, tar and sweeping

When wood doesn't burn completely, smoke turns into deposits on the cold walls of the chimney.

Three kinds of deposit — Fluffy soot is a low risk, flaky soot medium and glazed tar high: it burns violently and is hard to remove.
What makes tar — Damp wood, a damped-down fire and long smouldering overnight. Dry wood under 20% moisture and a warm, insulated chimney prevent it.
Twice a year — A sweep cleans a wood-burning chimney at least before the heating season and in midwinter. Chemical products don't replace the brush.
Certificate and soot door — Ask for a sweeping certificate and write the findings in your log. The soot door at the bottom must shut tight, and soot goes in a metal bucket.

Why: Tar deposits can catch fire and burn at more than 1,000 °C (1,800 °F). A fire like that can crack the chimney and set the roof, the loft and the house next door alight.

Don't climb onto an icy roof yourself to sweep the chimney. Falls from roofs are more common than fires.
Owner-occupiers usually have no legal duty, but home insurers often ask for a sweeping certificate. Use a registered sweep (UK: HETAS, NACS or the Guild of Master Sweeps; US: CSIA).