Carbon monoxide (CO) is a poisonous gas you cannot see, smell or taste. It is produced when a fuel such as gas, oil, coal or wood does not burn completely, and a faulty or poorly maintained heating appliance is one of the most common household sources. Staying safe rests on two things: keeping appliances serviced so they burn cleanly, and fitting an audible CO alarm that warns you when levels rise.
CO is dangerous because it interferes with the blood's ability to carry oxygen. At low levels it causes vague symptoms that are easy to mistake for other illnesses; at high levels it can cause collapse and death within minutes. The gas builds up unnoticed, which is why early detection matters so much.
Why heating appliances can produce carbon monoxide
The root cause is incomplete combustion — fuel burning without enough oxygen, or burning in a way the appliance was not designed for. When combustion is complete, fuel mostly turns into carbon dioxide and water. When it is incomplete, carbon monoxide is produced instead.
Several faults can push an appliance into incomplete combustion. Common causes include:
- Blocked or restricted flues and chimneys, so combustion gases cannot escape properly.
- Poor ventilation in the room, starving the burner of the air it needs.
- Cracked heat exchangers, worn burners or misaligned components inside the appliance.
- Soot, debris or birds' nests obstructing a chimney or external vent.
- Appliances that have never been serviced, or were not installed correctly.
Any fuel-burning appliance can be a source: gas boilers, gas fires, cookers, water heaters, oil-fired boilers, solid-fuel stoves and open fires. Electric heating does not produce CO because nothing is being burned. The risk rises sharply when an appliance is old, badly maintained, or used in an enclosed space without proper airflow.
This is why an annual service by a suitably qualified engineer is widely recommended. For gas appliances in the UK, that engineer should be registered with the Gas Safe Register — the official list of businesses and individuals legally allowed to work on gas. A service checks that the appliance is burning cleanly and that flues and vents are clear.
The warning signs to watch for
Carbon monoxide (CO) is a poisonous gas you cannot see, smell or taste.
Because CO has no smell, you often have to rely on other clues. These fall into two groups: signs from the appliance itself, and symptoms in the people using the building.
Signs that an appliance may not be burning correctly include:
- A lazy yellow or orange flame instead of a crisp blue one on a gas appliance.
- Sooty stains, dark marks or discolouration around the appliance or its flue.
- A pilot light that keeps going out for no clear reason.
- Increased condensation inside windows in the room where the appliance sits.
- Soot or yellow-brown staining on or near radiators, fires or boilers.
Physical symptoms of CO poisoning are easy to confuse with flu, food poisoning or simple tiredness. The classic clues are headaches, dizziness, nausea, breathlessness, collapse and loss of consciousness. A tell-tale pattern is when symptoms improve away from the home and return on coming back, or when several people — or pets — feel unwell at the same time. Symptoms that ease when you go outside are a strong hint that something indoors is producing the gas.
If anyone in the household reports these symptoms and you cannot explain them, treat CO as a possibility rather than dismissing it. Drowsiness and confusion are particularly worrying, as they can stop a person realising they need to get out.
Where to place an alarm
A CO alarm is an electronic detector that sounds when it senses dangerous levels of the gas. It is not the same as a smoke alarm, which detects particles from fire, so the two are not interchangeable. Look for an alarm that meets the British and European standard BS EN 50291, which is usually marked on the packaging.
Sensible placement makes the difference between an early warning and a missed one. General guidance is to:
- Fit an alarm in any room containing a fuel-burning appliance, such as a boiler, fire or stove.
- Position it roughly between one and three metres horizontally from the appliance.
- Mount it at head height — either high on a wall, or on a shelf about the height of a seated or standing person's breathing zone.
- Keep it away from windows, doors, extractor fans and air vents, where draughts could disperse the gas before it reaches the sensor.
- Consider alarms in rooms where people spend a lot of time or sleep, especially if a flue passes through.
Always follow the manufacturer's instructions, as recommended positions can vary by model. Test the alarm regularly using its test button, and replace batteries when prompted. CO alarms have a limited working life — typically printed on the unit — after which the sensor should be replaced.
What to do if an alarm sounds
Treat a sounding CO alarm as a genuine emergency unless you are certain it is a test. The priority is to get fresh air and to leave the building.
A clear sequence to follow is:
- Stop using the appliance immediately and, if it is safe to reach, turn it off.
- Open doors and windows to let fresh air in.
- Get everyone out of the property into the open air, including pets.
- If anyone feels unwell, seek medical help and tell them you suspect CO poisoning.
- For a suspected gas leak or emergency, call the National Gas Emergency Service on 0800 111 999.
- Do not go back inside until the cause has been found and the appliance checked by a qualified engineer.
If symptoms are severe — someone has collapsed, is struggling to breathe or is unconscious — call 999 straight away. Do not re-enter a property to fetch belongings while the alarm is still active. Only return once a competent engineer has identified the fault, made the appliance safe, and confirmed it is sound to use again.
Carbon monoxide safety comes down to prevention and detection working together. Regular servicing reduces the chance of incomplete combustion in the first place, while a correctly placed, working alarm catches problems that develop between checks. Neither replaces the other.