Carbon monoxide alarm beeping: warning, battery or fault?
A carbon monoxide alarm that sounds loudly and keeps repeating, usually with a flashing red light, is warning you about carbon monoxide: open windows, turn off gas appliances if it is safe, get everyone outside and call the National Gas Emergency Service on 0800 111 999. A single short chirp every minute or so is different. It normally means a low battery, the end of the alarm's working life or a fault in the alarm itself. Check the pattern against the label on the back of your alarm, because makes differ, and if you cannot tell which it is, treat it as a real alarm.
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Discuss your plumbing workFull alarm or a chirp: how to tell them apart
Carbon monoxide alarms sold in the UK are made to BS EN 50291, and they all have to give a loud alarm when CO builds up. What they do for batteries, faults and end of life varies by manufacturer. The table shows the common patterns, but the printed label on the alarm, or its leaflet, is the authority for your model.
If you have lost the leaflet, the make and model are printed on the back of the alarm, and most manufacturers publish the beep patterns on their websites.
| What you hear and see | What it usually means | What to do |
|---|---|---|
| Loud beeps repeating in a pattern, often with a red light | Carbon monoxide detected | Ventilate, get out, call 0800 111 999 |
| One short chirp roughly every 30 to 60 seconds | Low battery on a replaceable-battery alarm | Fit a new battery of the type on the label, then test |
| A chirp or double chirp on a sealed alarm, sometimes with a yellow light | End of life: the sensor has reached the end of its working years | Replace the whole alarm |
| A different chirp pattern with a fault light | The alarm has detected a fault in itself | Follow the leaflet; replace it if the fault stays |
| Alarm goes off, then stops on its own | CO may have been present for a short time | Still ventilate and get the appliances checked |
Never take the battery out to stop a full alarm. A CO alarm that goes quiet when you open the windows has not been proved wrong: fresh air lowers the level, and the source is still there.
What to do if it is a real carbon monoxide alarm
Carbon monoxide has no smell, colour or taste, so the alarm is the only warning you get. Act on it even if nobody feels unwell.
- Open doors and windows.
- Turn off the boiler, gas fire, cooker and any other fuel-burning appliance if you can do so safely.
- Get everyone out into fresh air, including pets, and do not go back in.
- Call the National Gas Emergency Service free on 0800 111 999. The line is open 24 hours a day.
- If anyone has a headache, dizziness, sickness, breathlessness or confusion, get medical help and mention carbon monoxide. Call 999 if they are seriously unwell.
- Do not use the appliance again until a Gas Safe registered engineer has checked it.
Why an alarm chirps when the battery is low
Alarms with a replaceable battery warn you before the battery is too weak to power the sensor and the sounder. The warning is a short chirp that repeats, often every 30 to 60 seconds, and it can go on for days. It is designed to be annoying so that it gets fixed.
Fit a fresh battery of the type printed on the alarm, not whatever is in the drawer, then press and hold the test button until the alarm sounds. If it still chirps with a new battery, the alarm may be at the end of its life or faulty.
Chirps often start at night. That is not a coincidence: battery voltage drops slightly as the house cools, which is enough to tip a tired battery below the alarm's threshold.
End of life: when the whole alarm needs replacing
The sensor inside a carbon monoxide alarm has a limited life. Many alarms now come with a sealed battery that lasts as long as the sensor, and are sold as seven-year or ten-year units. When that time is up, the alarm gives an end-of-life warning, which is usually a chirp with its own pattern or light.
You cannot fix an end-of-life warning with a new battery. Take the alarm down, check the date on the back and replace it. While you are there, look at the dates on any other CO alarms in the house: if they were all fitted at the same time, they will all expire around the same time.
Some alarms let you silence the end-of-life warning for a day or so. That is there to give you time to buy a replacement, not to keep using the old one.
False alarms: what can set one off
Genuine false alarms happen, but they are less common than people assume. Some chemical vapours, such as strong cleaning products, aerosols and solvents used right next to the alarm, can affect the sensor. An alarm that is badly placed, for example directly above a cooker or next to an open window where fumes blow back in, can also give readings that confuse people.
The safe approach is to assume the alarm is right until a qualified person has checked your appliances. If the alarm keeps going off in the same room, that is a strong reason to have the boiler, fire or cooker in that room looked at, not to move the alarm somewhere quieter.
Testing, placing and replacing your alarms
Press the test button once a month. The test checks the battery and sounder, not whether the sensor still responds to gas, which is why the end-of-life date matters.
Fit an alarm in every room with a fuel-burning appliance, positioned as the instructions say. Many households also put one in bedrooms and next to the boiler cupboard. In rented homes in England, landlords must fit a CO alarm in any room with a fixed combustion appliance other than a gas cooker, and repair or replace one when the tenant reports it faulty.
The heating season is when alarms earn their keep. Boilers and fires that have sat idle all summer are switched back on, windows stay shut, and flues can be partly blocked. An annual boiler service by a Gas Safe registered engineer, ideally before the cold weather, is the prevention. The alarm is the backup.
Getting the appliance checked
Once the emergency service has made things safe, the appliance still needs proper attention before it is used again. It may have been capped off or labelled with a warning notice. Loyal Plumbers' gas work is carried out by Gas Safe registered engineers, and we cover London and everywhere inside the M25 around the clock, with 10 vans on the road.
When you get in touch, tell us which appliance was involved, what the emergency engineer found or wrote on any notice, and whether the alarm has gone off before. That helps us send the right engineer to service, repair or replace the boiler, or carry out a landlord gas safety check.
Common questions
- Why is my carbon monoxide alarm beeping every 30 seconds?
- A single chirp at regular intervals is almost always a low battery or an end-of-life warning. Check the label: a new battery fixes the first, but a sealed alarm at the end of its life must be replaced.
- How do I stop my CO alarm chirping?
- Replace the battery if it has a replaceable one, then test it. If it keeps chirping, check the expiry date on the back and replace the alarm. Never silence a full alarm by removing the battery.
- Can a carbon monoxide alarm go off for no reason?
- Occasionally strong chemical vapours near the sensor can trigger it, but treat every full alarm as real. Ventilate, get out and call 0800 111 999, then have the appliances checked.
- How long do carbon monoxide alarms last?
- Most are designed to last between five and ten years, depending on the model. The expiry or replace-by date is printed on the alarm.
- Is a smoke alarm the same as a carbon monoxide alarm?
- No. A smoke alarm does not detect carbon monoxide. You need a separate CO alarm, or a combined unit that is clearly marked as detecting both.
Sources and further reading
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