Where this starts
A chimney works because hot gas is less dense than cold air, so a column of it rises and pulls more behind it. Everything that goes wrong is a failure of that column: it has not formed yet, it is losing its heat, it is obstructed, or something else in the house is winning the competition for the same air.
THE TIMING IS THE FIRST READING and it costs nothing. Smoke only in the first few minutes is a cold flue: until the gases inside it are warmer than the outside air, the chimney has no lift, and in an external or an unused flue that can take several minutes. Smoke throughout the burn is different — the flue is not developing draught at all, or it is obstructed, or the appliance is being asked to do something it cannot.
THE HOUSE IS THE SECOND READING, and it is the cause that has grown as buildings have tightened. A fire consumes a substantial volume of air and sends it up the chimney, and that air has to be replaced from somewhere. In a draughty old house it arrives through the fabric without anybody noticing. In a well-sealed one it does not, and the moment an extractor fan, a cooker hood or a tumble dryer starts, the house goes to negative pressure and the easiest available opening for makeup air is the chimney — which then flows backwards, bringing the smoke with it.
WEATHER IS THE THIRD. Wind blowing onto a nearby roof, a tree or rising ground can produce a downdraught at the terminal, so a chimney that behaves for months misbehaves in one wind direction. That pattern — always the same wind — is specific and worth recording rather than guessing at.
The distinction that decides urgency: smoke in the room is unpleasant and visible. CARBON MONOXIDE is neither, and the same conditions that push smoke back push it back too. A room with a solid fuel or gas appliance needs a working carbon monoxide alarm regardless of what this page concludes, and symptoms of headache or drowsiness around a fire are a reason to stop using it and ventilate, not a reason to keep reading.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A cold flue that has not started to draw
Most commonSmoke only in the first minutes of lighting, clearing once the fire establishes, worst on the first fire of the season and on an external or unlined flue.
How to check: Whether it clears after a few minutes. A flue that draws once warm was simply cold.
The house is at negative pressure
Most commonSmoke arriving when an extractor, cooker hood, tumble dryer or another appliance starts, and stopping when it is switched off. Often in a house that has had windows, draughtproofing or insulation upgraded.
How to check: Open a window on the same side of the house and see whether it stops. If it does, the fire is short of air.
An obstructed or unswept flue
CommonPoor draught throughout the burn, sometimes with debris in the grate, staining around the appliance, or a flue that has not been swept within the last year.
How to check: When it was last swept, and whether anything has fallen into the grate. Birds nest in unused flues in a single season.
Downdraught in particular wind conditions
CommonA chimney that behaves normally except in one wind direction, often where a taller building, rising ground or a tree stands close by.
How to check: Whether it correlates with wind direction. Record the direction each time before concluding anything about the flue.
A flue too large, too short or wrongly sized for the appliance
CommonPersistent poor draught from the day an appliance was installed, commonly a small stove connected into a large old fireplace flue without a liner.
How to check: Whether it has ever drawn well. A flue that never worked is a sizing or a lining question, not a maintenance one.
Burning unseasoned or unsuitable fuel
CommonHeavy smoke, a fire that will not get going, rapid tarry deposits on the glass and in the flue, and wood that hisses or shows moisture at the end grain.
How to check: The fuel. Wet wood spends its heat boiling its own water instead of establishing the draught that carries the smoke away.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
At lighting, throughout the burn, or only when something else is running — the three answers point at a cold flue, the flue itself, and the house respectively, and nothing else separates them.
- 2
It is a free, immediate test for the house being at negative pressure, and it identifies the cause that has become commonest as buildings have been sealed up.
- 3
It isolates which appliance is competing with the fire, which turns a vague problem into a specific one with a specific remedy.
- 4
A chimney that only misbehaves in one wind is telling you about a downdraught at the terminal, and that is a terminal or a height question rather than a flue one.
- 5
Both are cheap to correct and both are the first thing any professional will ask, so arriving with the answers shortens the visit considerably.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where anyone has had headaches, nausea or drowsiness around the appliance — stop using it, ventilate, and treat it as carbon monoxide until a professional says otherwise.
- Where smoke persists throughout the burn rather than only at lighting, since that points at the flue's condition or its sizing and needs an internal inspection.
- Where the appliance burns gas or oil, which is a controlled installation that only a registered engineer may work on.
- Where the flue is unlined, serves a stove rather than an open fire, or has never drawn correctly — a liner is a specification decision based on the appliance's output and the flue's height.
- Where there is staining, heat damage or a smell of smoke in a room the flue passes through rather than the room with the fire, which indicates the flue is leaking into the structure.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.