Carbon monoxide and chimneys: the real risks and how to prevent them
Carbon monoxide is odourless and colourless. A poorly maintained chimney can produce dangerous concentrations. What you need to know and what to do about it.
Carbon monoxide (CO) cannot be seen or smelled. A fireplace with poor draught or a blocked flue can produce dangerous levels within minutes. CO detector + clean flue = real protection.
Carbon monoxide is the silent risk of any chimney. It has no smell, no colour and builds up in the room without you noticing. At low concentrations it causes headaches and dizziness; at high concentrations it can kill within minutes.
We do not want to alarm anyone, but we do want to explain when the risk is real and how to remove it.
When a chimney produces CO
A chimney in good condition and with a good draught does not produce dangerous CO inside the house: the combustion gases leave through the flue. The risk appears when that path is interrupted.
The most common risk situations:
- Blocked flue: a nest, a build-up of soot or an object obstructs the passage and the gases come back inside.
- Reversed draught: pressure differences or wind make the gases travel down instead of up.
- Cracks in the flue: fissures that let gases escape before they reach the outside.
- Closing the flue damper with the fire still burning: a common mistake in fireplaces fitted with a damper.
- Insufficient ventilation: in very airtight houses, the fire uses up the available oxygen and combustion becomes incomplete, producing more CO.
Symptoms of CO poisoning
The first symptoms are similar to flu: headache, nausea, dizziness, tiredness. The difference is that they disappear once you go out into fresh air. If several people in the house have these symptoms at the same time, that is a warning sign.
If you suspect poisoning: get outside immediately, call 112 and do not go back in until the fire brigade confirms it is safe.
How to prevent the risk
CO detector
This is the single most important measure. CO detectors cost between 20 and 60 euros and are fitted like a smoke alarm. They should be placed in the bedroom or in the room with the chimney, at mid-wall height (CO has a similar density to air, so it neither rises nor sinks).
Annual flue sweeping and inspection
A clean, unobstructed flue is the best guarantee of a correct draught. An annual professional sweep also picks up cracks, blockages or structural problems that cannot be seen from outside.
Adequate ventilation
Do not use the fireplace with every window sealed shut. Combustion needs oxygen. A slightly open window or a dedicated air inlet is enough.
Never close the damper while the fire is burning
It sounds obvious, but it is a common mistake. The damper should only be closed once the fire is completely out and the embers are cold.
Where should the CO detector go?
In the same room as the fireplace or stove, at mid-wall height (not on the floor or on the ceiling). If you use the fireplace in the living room but sleep upstairs, fit one in the bedroom too.
How often should the CO detector be replaced?
Standard models have a service life of 5-7 years. They have an indicator that warns when they need replacing. Do not reuse expired detectors.
Does the fire have to be lit for there to be a CO risk?
In practice, yes. CO comes from incomplete combustion. With the fire out and the embers cold, the risk is minimal. The greatest danger is while it is in use with a poor draught.
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