Safety Guide
Carbon Monoxide Alarm: Why Every Fireplace Needs One
If you have an open fire, wood burner or multi-fuel stove, fitting a carbon monoxide alarm is one of the simplest and most important safety steps you can take. This guide explains why CO alarms matter, what to look for when buying one, where to install it, and what to do if it sounds.
What is carbon monoxide?
Carbon monoxide (CO) is an extremely poisonous gas produced when fuels such as gas, oil, solid mineral fuel or biomass are not burned under the correct conditions. It has no colour, smell or taste, which makes it impossible to detect without an alarm. Even low levels over time can cause serious health problems, and high levels can be fatal.
HETAS, the UK's recognised solid fuel heating organisation, describes carbon monoxide as "the silent killer" because of how difficult it is to notice before it starts to affect you. The risk is real for any home with a fuel-burning appliance.
Why a CO alarm is essential
CO is invisible and silent
Carbon monoxide has no colour, smell or taste, so you cannot detect it without an alarm. Symptoms can be mistaken for flu, tiredness or food poisoning, which is why an early warning is so important.
It can leak from any fuel-burning appliance
Wood burners, open fires, multi-fuel stoves, gas boilers, oil boilers and biomass appliances can all produce CO if the fuel is not burning correctly or the flue is not working properly.
Chimneys can become blocked or damaged
Bird nests, soot build-up, cracked flue liners, or a dislodged cowl can all stop fumes escaping. A CO alarm picks up the problem before it reaches dangerous levels.
Insurers and regulations expect it
Building Regulations Approved Document J recommends a CO alarm in rooms with a solid fuel appliance. Some insurers also expect an alarm to be fitted and maintained as part of your home safety obligations.
What standard should the alarm meet?
Not all alarms are equal. A CO alarm intended for use in a home with solid fuel or gas appliances should:
- Choose an audible alarm that conforms to BS EN 50291.
- Position it in the same room as the appliance, following the manufacturer and Approved Document J guidance.
- Keep it away from draughts, vents, damp areas, and direct steam.
- Test it regularly and replace batteries or the unit as instructed.
- Never treat the alarm as a substitute for sweeping, servicing and proper ventilation.
HETAS recommends an audible CO alarm conforming to BS EN 50291 and positioned in accordance with Building Regulations Approved Document J. If you are unsure which alarm to choose, a HETAS-registered sweep or installer can advise on the right type and position for your room.
Where to install a carbon monoxide alarm
The alarm should be installed in the same room as the appliance, in a place where it can be heard clearly, and away from draughts, vents, damp or very steamy areas. Always follow the manufacturer's instructions and the positioning guidance in Approved Document J. If you have more than one appliance, you may need more than one alarm.
Symptoms of carbon monoxide poisoning
The early signs of CO poisoning are often similar to common complaints such as flu, tiredness or an upset stomach. The common symptoms can include:
- Headaches
- Breathlessness
- Nausea and/or vomiting
- Dizziness or collapse
- Chest and/or stomach pains
- Erratic behaviour or visual problems
If more than one person in the home feels unwell at the same time, or if symptoms improve when you leave the building, carbon monoxide could be the cause.
What to do if your CO alarm goes off
If your alarm sounds, or you suspect fumes are escaping from your appliance:
- Close the appliance vents to let the fire die down.
- Open doors and windows to ventilate the room.
- Leave the building immediately and do not return until the appliance has extinguished and the air is clear.
- Seek medical advice if anyone feels unwell — call NHS 111, or 999 in an emergency. Tell them you suspect CO poisoning.
- Do not use the appliance again until it has been checked by a HETAS registrant or Gas Safe engineer and declared safe.
HETAS also runs a consumer helpline on 01684 278170 (Monday–Friday) for advice on CO safety.
An alarm is not a substitute for sweeping
A CO alarm is a vital safety net, but it does not replace the maintenance that stops CO being produced in the first place. Regular chimney sweeping removes soot, creosote and blockages that can affect the flue's performance. Your appliance also needs to be serviced and installed correctly, with good quality fuel and proper ventilation.
For wood burners in regular use, HETAS recommends sweeping the chimney every three months during the burning season. Smokeless fuels should be swept at least once a year, and house coal twice a year. The best defence against carbon monoxide is the combination of a well-maintained appliance, a clear flue, and a working CO alarm.
Source and further reading
This guide is based on HETAS's official carbon monoxide safety advice. For the full article, safety videos and downloadable leaflets, visit the HETAS Carbon Monoxide advice page.
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