What to do immediately
A carbon monoxide (CO) alarm sounds when the gas builds up to a level that can harm you. You cannot see, smell or taste CO, so do not wait for other signs before acting.
- Open doors and windows to let fresh air in.
- Turn off fuel-burning appliances if you can do so quickly, such as the boiler, a gas fire or a cooker. If you have mains gas, turn it off at the emergency control valve by the meter, with the handle across the pipe.
- Get everyone out of the property into fresh air, including pets. Do not go back in.
- Call 0800 111 999, the National Gas Emergency Service, from outside. In Northern Ireland, call 0800 002 001.
- Get medical help if anyone feels unwell. Call 999 if symptoms are severe, or NHS 111 otherwise, and say you think carbon monoxide is involved.
- Do not use the appliances again until a registered engineer has checked them and confirmed they are safe.
Do not silence the alarm and carry on as normal, and do not assume it is faulty because nobody feels ill. Carbon monoxide can build up and cause harm over hours, and early symptoms are easy to mistake for flu or tiredness.
Symptoms of carbon monoxide poisoning
CO stops your blood carrying oxygen properly. The NHS lists symptoms including:
- a tension-type headache
- dizziness
- feeling sick or being sick
- tiredness and confusion
- stomach pain
- shortness of breath and difficulty breathing
Some patterns point to CO rather than a bug. Symptoms ease when you leave the house and come back when you return. More than one person in the home, and sometimes pets, are affected at the same time. Symptoms are worse when the heating or a gas fire is on. Higher levels can cause loss of consciousness and can kill. Babies, pregnant women, older people and people with heart or breathing conditions are at greater risk.
Even if everyone feels fine once they are outside, mention the alarm to a doctor or NHS 111 if anyone had symptoms beforehand. Doctors can check for carbon monoxide exposure, but levels in the body fall quickly after you get into fresh air, so seek advice promptly rather than waiting to see how you feel the next day.
Who to call
| Situation | Who to call |
|---|---|
| Alarm sounding in a home with gas appliances | National Gas Emergency Service, 0800 111 999 (0800 002 001 in Northern Ireland) |
| Anyone unconscious or with severe symptoms | 999 |
| Milder symptoms after exposure | NHS 111 or your GP |
| LPG appliances | Your LPG supplier's emergency number |
| Oil boiler, wood burner or open fire | 999 if anyone is unwell, then an OFTEC registered technician (oil) or HETAS registered installer (solid fuel) |
| Repair of the gas boiler or appliance | A Gas Safe registered engineer |
| Rented home | Your landlord or letting agent, as soon as everyone is safe |
The emergency engineer who attends a gas call will check the property and make it safe, usually by turning off and labelling any appliance that could be producing CO. They do not repair boilers. A Gas Safe registered engineer then needs to find the cause, which could be a blocked or damaged flue, a boiler case that is not sealing properly, poor combustion or a problem with ventilation. Modern room-sealed boilers have safety devices that shut the burner down when the fan or flue is not working, which is why you may see codes such as Worcester Bosch E9 224 (flue or high limit thermostat) or Baxi E160 (fan fault). Those safety devices do not replace an alarm. If you can also smell gas, follow our steps for when you smell gas near your boiler.
Alarm sounding or just chirping?
Not every noise from a CO alarm means carbon monoxide. Patterns vary between makes, so check the leaflet or the label on the back of yours, but most follow a similar approach.
| Sound | What it usually means | What to do |
|---|---|---|
| Loud, repeating beeps, often with a flashing red light | Carbon monoxide detected | Act immediately, as above |
| A single short chirp roughly every minute | Low battery | Replace the battery, or the whole alarm if the battery is sealed |
| A different chirp pattern or an end-of-life message | The sensor has reached the end of its life | Replace the whole alarm |
| Chirp with a fault light | Alarm fault | Replace the alarm |
End-of-life warnings appear because the sensor wears out. Most alarms last 7 to 10 years, and the replace-by date is usually printed on the unit. Alarms are made by brands such as FireAngel, Kidde and Aico, and many models have a sealed battery designed to last the life of the alarm, so a low battery warning on those means replacing the unit. A battery alarm typically costs £15 to £40. If you are not sure whether you are hearing a chirp or a real alarm, treat it as an alarm.
Where to fit a carbon monoxide alarm
Buy an alarm that meets BS EN 50291, ideally with the Kitemark, and follow the manufacturer's instructions. These usually say:
- fit one in every room with a fuel-burning appliance, such as a boiler, gas fire or wood burner, and consider bedrooms and main living areas too
- place it 1 to 3 metres horizontally from the appliance
- on a ceiling, keep it at least 300mm from any wall
- on a wall, place it higher than any door or window but at least 150mm below the ceiling
- if the boiler is in a cupboard, fit the alarm in the room outside the cupboard, not inside it
Avoid putting it directly above a cooker or sink, next to an extractor fan, window or air vent, in a damp bathroom, or behind furniture or curtains. In England, landlords must fit a CO alarm in any room with a fixed fuel-burning appliance (gas cookers are excluded), and an alarm is required when a new or replacement boiler is installed. In Scotland, every home with a fuel-burning appliance or flue must have one.
Testing and looking after your alarm
- Press the test button once a month to check the sounder and electronics are working.
- Dust it gently, and never paint over it or cover it.
- Check the replace-by date and replace the unit on time, not just the battery.
- Never try to test it with smoke, car exhaust or a flame.
- Keep having your boiler and other appliances serviced every year. An alarm warns you of a problem, but a service helps prevent one. Our boiler service cost guide explains what the engineer checks, including the flue.