Why does my fire or wood burner smoke back into the room?

Because a chimney works by draught — hot gas rising creates suction that pulls smoke up — and anything that weakens that suction or starves it of replacement air sends smoke the other way.

How the draught is created. Hot flue gas is less dense than the outside air, so it rises, and the column of rising gas pulls more behind it. The strength of that pull depends on flue height, flue temperature, and the pressure difference between inside and outside.

The causes, roughly in order of likelihood:

Not enough replacement air. For every cubic metre going up the chimney, one must come in. Modern draught-proofing, sealed floors and good double glazing have made houses far more airtight, and an open fire needs a substantial air supply. This is the most common cause in a renovated house, and the diagnostic is simple: open a window slightly and see if the smoking stops.

Extract fans competing. A kitchen extractor, bathroom fan or tumble dryer depressurises the house and can reverse a marginal flue outright. Cooker hoods are notorious for it.

A cold flue. Before it warms up, the flue contains a column of cold, dense air that resists being displaced. This is why smoke-back happens at lighting and then stops. Warming the flue first — a rolled newspaper or firelighter held up the throat — usually resolves it.

Blockage or restriction — a bird's nest, soot buildup, or a collapsed liner. This is also a carbon monoxide hazard and needs sweeping.

Flue too short, too wide or badly routed. Excessive bends, an oversized flue for a small appliance, or a chimney that does not clear the roof ridge adequately.

Wind effects. Nearby buildings or trees causing downdraught. An anti-downdraught cowl helps.

Wet or unseasoned wood, producing cool, smoky combustion.

Fit a carbon monoxide alarm, have the chimney swept annually, and get any persistent problem assessed by a HETAS-registered engineer — persistent smoke-back means combustion products are entering the room.