Chimney draught problems: causes and solutions

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There are few things more frustrating than lighting the fire and having the smoke come into the house. Or having the fire fail to catch and go out on its own. The draught is what moves the smoke upwards, and when it fails, the chimney does not work.

We have seen hundreds of installations with draught problems. These are the most common causes and how they are put right.

What draught is and how it works

The draught is the air current that rises up the flue and carries the combustion gases outside. It works because warm air weighs less than cold air: the heat of the fire warms the air in the flue, which rises and takes the smoke with it.

Three things are needed for it to work well: a clean flue free of obstructions, a big enough temperature difference between the inside of the flue and the outside, and dimensions and geometry of the flue that suit the firebox.

The 6 most common causes of poor draught

1. A dirty flue or one with deposits

Built-up soot and creosote reduce the cross-section of the flue and hinder the flow of smoke. It is the most common cause and the easiest to solve: a professional sweep.

2. A cold flue when you light the fire

If the fireplace has been unused for a while, the flue is cold and the draught does not start straight away. The solution is to warm the flue before lighting the fire: hold a lit paper torch near the mouth of the firebox for a few seconds.

3. A blockage in the flue

Birds' nests, building debris, fallen pieces of render or even stray objects can block the flue partially or completely. It shows up in an inspection and is removed during the sweep.

4. The exterior terminal blocked or in poor condition

The chimney cap or terminal may be clogged with dirt, bent by the wind or fitted with a protective cover that does not open properly. It needs a visual check from outside.

5. Pressure problems in the house

In very airtight houses, or houses with mechanical extraction (cooker hoods, fans), a negative pressure can build up that reverses the draught. Solutions: open a window slightly when lighting the fire, or fit a dedicated outside air inlet.

6. A badly sized flue

If the firebox is too big for the flue, or the flue has too many bends and changes of direction, the draught is structurally poor. This is harder to fix and may call for changes to the installation.

How to work out which case is yours

If the draught only fails when you light the fire but improves after 10-15 minutes, it is usually a cold flue. If it always fails and there is smoke from the outset, a blockage or a build-up is the most likely explanation. If the problem is recent and it used to work well, it is probably accumulated dirt or something that has fallen into the flue.