A carbon monoxide alarm sounds when it detects carbon monoxide, a gas with no colour or smell that comes from anything burning fuel: boilers, fires, stoves and heaters. Buy a branded alarm made to BS EN 50291 with a sealed 10-year battery, for about £20 to £30. Our pick is the FireAngel FA3820. Avoid unbranded ones: Which? says "every unbranded carbon monoxide alarm we've tested has failed to detect the killer gas".
Buying through our links costs you nothing extra. Some of them earn us a small commission, and that's what keeps Survival Bunker going: it pays for the time it takes to keep our picks and figures up to date. Thank you for helping. How we choose and how we earn.
The alarm samples the air and sounds when carbon monoxide builds up. People usually fit one in each room with a boiler, gas fire, log burner, open fire or portable gas heater. It detects carbon monoxide only: smoke alarms are a separate device.
| If you rent your home | When a new or replacement appliance is fitted | |
|---|---|---|
| England | Private and social landlords must fit an alarm in any room used as living accommodation that has a fixed fuel-burning appliance, except a gas cooker. They must check it works at the start of each new tenancy, and repair or replace it if you report a fault. | An alarm in the room with any new or replacement fixed gas, oil or solid-fuel appliance, except one used only for cooking. This applies to every home, owned or rented. |
| Scotland | Every home, owned or rented, must have an alarm in each room with a fixed fuel-burning appliance or a flue, except appliances used only for cooking. | Building standards also apply, including appliances in connected spaces such as an integral garage. |
| Wales | Landlords must keep a working alarm in every room with a gas, oil or solid-fuel appliance. | Since 6 January 2025, an alarm is needed with any new or replacement fixed flued appliance burning oil, gas, LPG or solid fuel, including flued cookers. |
| Northern Ireland | Private landlords must fit an alarm in any room or circulation space (a hall, stairs, landing or corridor) with a flue or fixed fuel-burning appliance, other than a gas cooker or oven. This has applied to all private tenancies since 1 December 2024. | Building regulations have required a carbon monoxide alarm when a fuel-burning appliance is installed since 31 October 2012. |
Sources: England, England building guidance, Scotland, Wales (renting), Wales (building), Northern Ireland. A summary, not legal advice: some tenancies are exempt, and the full rules have more detail.
Scotland's fire service says carbon monoxide detectors "should be placed in all rooms where there is a carbon-fuelled appliance". Each alarm's instructions give its position; FireAngel's adds that alarms in bedrooms "should be located relatively close to the breathing zone of the occupants". For new appliances, England's building guidance puts the alarm:
About 10 years for the sealed-battery alarms here. FireAngel rates the FA3820's sensor at "10 Years - Replace unit after 10 years of operation", and Aico gives the Ei208 a "product life 10 years". Kidde says its alarms signal the end of their life "seven or ten years after initial power", depending on the model. Each alarm has a "replace by" date on its label, and it starts beeping when it reaches the end.
Each make uses its own pattern, so check the manual for yours. For the two most common alarms here, the manuals say:
| Signal | FireAngel FA3820 | Aico Ei208 |
|---|---|---|
| Carbon monoxide detected | 4 chirps repeated, with a red flash every second | 3 sound pulses then a pause, repeating |
| Low battery | 1 beep a minute; replace the alarm within 30 days | A beep and a yellow flash about every 48 seconds |
| Fault | 2 rapid beeps a minute: "May not detect CO - replace immediately" | 2 short beeps about every 48 seconds |
| End of life | 3 rapid beeps a minute: "May not detect CO - replace immediately" | 3 short beeps about every 48 seconds |
From the FireAngel FA3820 manual and the Aico Ei208 instructions. For what to do when an alarm sounds, follow the instructions on the alarm and in its manual. A sealed-battery alarm can't take a new battery: when it signals low battery or end of life, the whole alarm is replaced.
Prices checked 4 November 2026, including VAT, from the shop named on each button. Specs are from the makers' pages. Which? makes every alarm that passes all its carbon monoxide tests a Best Buy; which ones did is for its members.
A sealed 10-year battery alarm approved to the current standard, BS EN 50291-1:2018, sold at Screwfix, B&Q and Toolstation. FireAngel's manual rates the sensor and battery for 10 years and gives a five-year warranty. Which? has tested it; its result is for members.
Another sealed 10-year battery alarm approved to the current standard, with Aico's AudioLINK diagnostics, which let you read the alarm's history on a phone. Like the FireAngel, it's a standalone alarm: it doesn't link to other alarms. Which? tested it in 2025; its result is for members.
A sealed 10-year battery alarm with a 10-year warranty, twice the others'. Which? tested it in 2023; its result is for members.