What is a chimney draught? How it works and why it fails
The draught is what makes the smoke rise up the flue. We explain how it works, what affects it and the most common causes of poor draught.
Draught is the airflow that carries smoke upwards. If it fails, smoke enters the house or the fire does not light properly. The most common causes are a cold flue, dirt or incorrect sizing.
When a chimney draws well, the smoke leaves without trouble and the fire burns easily. When it fails, smoke comes into the room or the chimney smells bad even when it is not lit. It is one of the most frequent problems we come across on inspections.
What is draught?
Draught works on a simple physical principle: warm air weighs less than cold air. When the fire heats the gases, they rise up the flue, create a pressure difference and draw fresh air in from below to feed the combustion. A good draught depends on the temperature of the flue, its height, its diameter and the outside pressure.
Why does the draught fail?
- A cold flue at the start: the column of cold air acts like a plug. Solution: warm the flue with burning paper before putting the wood in
- A dirty or blocked flue: soot, creosote or a bird's nest reduces the usable cross-section. It is the most frequent cause
- Incorrect sizing: the diameter must be proportional to the area of the firebox opening
- A flue that is too short: if it does not rise far enough above the roof line, the wind can create a reversed draught
- Negative pressure indoors: in very airtight houses with mechanical ventilation. It usually disappears when a window is opened
- A damaged exterior terminal: a broken or badly oriented chimney cap that creates negative pressure
Signs of poor draught: smoke in the room when you light the fire, a fire that is hard to get going, a smell of smoke with the fireplace unlit, insert glass that blackens quickly, a low yellow flame.
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Does the draught improve after the flue is swept?
In most cases, yes, noticeably so. If it is still poor after the sweep, it may be a structural problem that needs assessing.
Why does the chimney only smoke when it is windy?
It is usually a problem with the exterior terminal. Depending on the wind direction, the chimney cap can create negative pressure.
Can I solve a draught problem myself?
If it is a cold flue, warming it before lighting usually solves it. If it is dirt, you need a professional sweep. If it is structural, the installation has to be assessed.
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