Best battery powered smoke and carbon monoxide detector

Everyone knows how important it is to have a smoke alarm in the home. If a fire breaks out while you’re asleep, the smoke it emits will set off the alarm, hopefully giving you time to get out before being asphyxiated by the fire’s deadly concoction of chemical gases.

But there’s another gas that is possibly even more deadly because you can’t see it, smell it or taste it. Carbon monoxide (CO) is produced during incomplete combustion of fuels, usually because of a faulty appliance such as a gas boiler, gas-powered fireplace, gas cooker, wood burner or blocked coal fire flue. Sadly, this writer has been on the receiving end of this silent killer – I lost a sister-in-law and her three dogs – so I’m doubly mindful of just how horrendously dangerous it can be. Like an invisible ghost in a horror movie, it enters a room, slowly sapping the life out of its occupants without anyone realising.

Thankfully, there's an early-warning solution in the form of a carbon monoxide alarm that costs only a few pounds more than a smoke alarm. Be aware, though, that not all CO detectors are sensitive enough and some no-name brands are downright useless – and, therefore, dangerous. Ensure you opt for a reputable brand such as FireAngel, First Alert, Kidde, Nest or Lifesaver.

We’ve called in a selection of CO alarms from top-name brands and put them to the test using the simple but effective “incense in a plastic bag” method. We’ve also scoured the web for both professional and user reviews to back up our results. Read on for the lowdown and stay safe, people, stay safe.

Best carbon monoxide alarm: At a glance

• Best all-in-one, app-connected CO and smoke detector: Nest Protect
• Best combination smoke and carbon monoxide alarm: X-Sense SC01
• Best Wi-Fi-connected safety system: FireAngel Pro Connected FP1820-W2-R 
• Best budget-priced combined CO and smoke alarm: First Alert SCO5
• Best cheap CO alarm: Lifesaver 5CO

How to choose the best carbon monoxide alarm for you

What is carbon monoxide poisoning?

Carbon monoxide is known as the silent killer because you can’t smell it, taste it or see it. Once it enters your bloodstream, it invades the red blood cells (haemoglobin), preventing them from carrying oxygen around the body. Symptoms of carbon monoxide poisoning include a flu-like headache, dizziness and nausea followed by tiredness, confusion, breathlessness, weakness, loss of muscular coordination and – dare I mention it – death. CO is so odourless that even dogs can’t detect it.

How do CO alarms work?

The health effects of CO depend on the concentration and length of exposure (carbon monoxide concentration is measured in parts per million). According to respected CO alarm manufacturer Kidde, “a carbon monoxide detector is a time-weighted alarm that measures the buildup of carbon monoxide in a house.

“For a person to begin feeling the effects of carbon monoxide poisoning, they would need to be exposed to a carbon monoxide level of 50 parts per million (ppm) for eight hours. An alarm’s response time will vary depending on the level of carbon monoxide in the air. For example, an alarm will sound after three and a half hours of continuous exposure at a level of 50ppm, yet will sound after eight minutes of continuous exposure at a level of 400ppm.”

Carbon monoxide sensors have a finite lifespan of between five and seven years, although a few manufacturers now claim that their sensors will last a full ten years. Generally speaking, it’s worth playing it safe and replacing your alarm a year or two before its life is supposed to run out. Tests in the US have shown that alarms over eight years old can fail to detect CO at levels of above 400ppm.

It’s also worth noting that the test button on these devices doesn’t actually test the sensor – it only makes sure that the circuit’s working.

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What levels of CO are considered dangerous?

Most people will not experience any symptoms from prolonged exposure to CO levels of under, say, 40ppm. It depends on the length of exposure, but the real danger level starts at around 400ppm (two to three hours of exposure at this level can be life-threatening). To give you some idea of how dangerous higher levels of concentration can be, 1,500ppm could result in death within an hour and 6,000ppm within 15 minutes.

All of these concentrations are bad enough when you’re awake and realise that something isn’t quite right – especially if others are feeling a similar effect – but if you’re asleep, you may wake up with a severe headache and nausea and be incapable of even crawling for fresh air. Worse, you might not even wake up at all.

Why should I be concerned?

According to the NHS, 25 people die every year in the UK from accidental CO poisoning. For extra peace of mind, you’re advised to have your boiler, gas fire or wood/coal fire's flue checked at regular intervals. And never use a gas-powered lamp or cooker inside a tent.

Where should I install my CO alarm?

Most manufacturers suggest locating your alarm next to or near a gas appliance, and it’s worth having one in the same room as any fire or wood-burning stove. It should be around 1.5m from the ground and at least 30cm away from any ceiling, and most can be mounted on a worktop, shelf or table – or simply mounted on the wall.

Don’t think that CO alarms are just for your home, either. They’re essential for caravans and motorhomes, and you might even want to take one on holiday if you’re staying in a cottage or self-catering accommodation.

Are there any features worth looking out for?

Generally speaking, CO alarms are pretty basic devices. Only the new and more expensive smart or app-connected alarms have any real features to speak of. The most important thing to look for is British Standards certification. CO alarms should be kitemarked to EN50291-1:2018 for use in domestic premises, and BS EN 50291-2:2010 for use in the home or a boat or caravan. The second standard mostly ensures that the alarm is tough enough for use on the move.

Otherwise, the key features come down to whether there’s a display and how the alarm is powered. A growing number of CO alarms now have a display which gives you the sensor’s current reading on CO particulates in the air, though – to save batteries – you’ll only see this after a test or if the reading goes above safe levels.

Until recently most CO alarms ran from a set of removable AA or AAA batteries, which may need to be replaced every three to five years. However, many now use a sealed-in Lithium battery designed to last for the same lifespan as the sensor; you buy the unit and just replace it when the lifespan’s up. This is arguably safer, and a new Scottish law will require from 2022 that homeowners and landlords either fit tamper proof alarms with a long-life Lithium battery or a mains-wired CO alarm if there’s a carbon-fuelled appliance – like a boiler, woodburner or open fire – on the property.  

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The best carbon monoxide alarms to buy

1. Nest Protect: The best all-in-one, app-connected CO and smoke detector

Price: £93 | Buy now from Argos

If you have a smartphone and can afford it, the Nest Protect is our top choice of carbon monoxide alarms, not least because it also detects smoke and works with an Android or iOS app. The Nest Protect is by far the most aesthetically pleasing alarm and the most polite when it comes to false alarms. It also comes with more tech than most, including a three-level LED lamp that lights the way in an emergency or when you get up in the middle of the night.

The Nest Protect is very easy to set up: simply remove the battery tab, launch the app, log in to your Wi-Fi, give it a name (kitchen, lounge and so on) and mount it in a suitably elevated place on a wall.

Unlike other CO detectors, the Nest notifies you (and anyone else who has registered) with a notification on your mobile device, even if you’re in another country. It also does the same for smoke. The main unit also speaks a warning in a calm English female voice when it detects smoke or CO in a room. If the CO levels are slowly increasing, the voice will warn you before setting off the alarm. But if CO levels become dangerously high, the centre lamp glows red and a very loud, high-pitched beep is emitted along with a spoken warning letting you know which room is affected. You can temporarily cancel any alarms by holding the “Stop” button on the app or by pressing the unit’s central button.

The Nest Protect passed our sealed incense test with flying colours: the smoke alarm went off first, followed by the CO alarm five minutes later; according to the app, the CO level was 414ppm and increasing. We can’t think of a better CO currently alarm on the market, particularly as it also detects smoke.

Key specs – Lifespan: 10 years; Battery type: 6 x AA; Display: None (uses smartphone app)

2. X-Sense SC01: The best combination smoke and carbon monoxide alarm

Price: £30 | Buy now from Amazon

This CO and smoke alarm combo gives you up to ten years of protection at a bargain price, thanks to a CR1 lithium battery sealed into the unit. This means you’ll be ditching the whole shebang once its lifespan is up, but as CO sensors degrade over time this isn’t necessarily a bad thing. It’s easy to install, while powering it on takes nothing more than sliding a switch on the base. After that, pressing the Test button wakes up the LCD panel to confirm the current CO level of the room.

This alarm combines a photoelectric smoke sensor with a Figaro electrochemical sensor for CO, and the 85dB alarm tone shouldn’t have any problems waking you from sleep. Like the sound of an effective, easy-to-use alarm? Hate the bother of changing batteries? This is the one to buy.

Key specs – Lifespan: 10 years; Battery type: CR1 lithium (sealed in); Display: 3cm LCD

3. FireAngel Pro Connected FP1820-W2-R: The best Wi-Fi-connected home safety system

Price: £56 (CO detector only), £158 (4-piece home kit) | Buy now from Robert Dyas

This carbon monoxide alarm forms part of FireAngel’s new Pro Connected Range of smoke, CO and heat detectors that interconnect via FireAngel’s dedicated Wi-Safe2 mesh network system to provide app-connected protection for an entire house. The idea is that if one unit detects a problem (CO, smoke or heat), all the other installed units will sound their 85dB alarms at the same time. This means that no matter where you are in the home, there should always be at least one alarm within earshot, whether it’s for smoke, CO or sudden extreme heat. It will also send notifications to the iOS/Android app telling the user which unit’s sensor has been activated. The system can accept up to 50 devices on a single Wi-Fi circuit.

All Pro Connected products can be bought separately and will work as standalone detectors as well. We received a package of all three detectors (CO, heat and smoke, each fitted with a sealed-for-life lithium battery) and, while they were fiddly to set up using the supplied Ethernet-connected wireless gateway box, they did all sound their alarms when testing the CO device using a few puffs of smoke.

FireAngel is a major player in the field of CO, fire and smoke detection and this unique system is clearly a clever, albeit pricey, way to protect the entire household in one fell swoop.

Key specs – Lifespan: 10 years; Battery type: Built-in lithium; Display: On app

4. FireAngel CO-9D: A super-sensitive, keenly-priced carbon monoxide alarm

Price: £20 | Buy now from Amazon

This excellent battery model from FireAngel comes with a 4.5cm LCD screen that displays both the temperature in the room and any concentrations of carbon monoxide, even if it’s as low as 10ppm.

This is a sealed unit with a seven-year lifespan/warranty and non-replaceable batteries. We would suggest replacing the whole unit a bit earlier, just to be on the safe side; you might want to set a calendar reminder in advance.

The FireAngel performed exceedingly well in our test, sounding the alarm when CO levels reached 357ppm. However, its 85dB alarm didn’t sound quite as loud as some others on this page and that could be an issue for anyone with hearing difficulties.

FireAngel also produces a non-screen version (model CO-9X) that’s a bit cheaper but only by a few pounds. Nevertheless, it’s a worthwhile option if you’re not bothered about an LCD screen or if you’re installing several units in different rooms around the home.

Key specs – Lifespan: 7 years; Battery type: Sealed; Display: 4.5cm LCD

5. Kidde 7DCOC: The best CO detector for camping and travel

Price: £30 | Buy now from Plumbworld

Kidde produces a wide range of carbon monoxide detectors but we like this one because, while slightly ugly, it has a small LCD screen that displays CO readings from 10ppm upwards. The Kidde’s sensor takes CO readings every 15 seconds and is a very sensitive thing indeed: it’s set to sound its 85dB alarm when carbon monoxide levels remain at 41ppm for 60 minutes.

The 7DCO comes with a ten-year lifespan and is BS EN 50291-2 certified for camping, caravanning and travel, or even use on a boat. As with all CO detectors, the Kidde provides end-of-life and battery warnings, along with the obligatory test/reset button for peace of mind. The only thing to watch out for is that it’s powered by a pair of AA batteries which need replacing roughly every two years We had no issues whatsoever during tests, but it means this alarm won’t be compliant with the new Scottish regulations. 

Key specs – Lifespan: 10 years; Battery type: 3 x AA; Display: LCD

6. Kidde 5CO: The best cheap CO alarm

Price: £20 | Buy now from Argos

Replacing the old Lifesaver 5CO, this wallet-friendly Kidde alarm now has an upgraded ten-year lifespan. Its sensor is set to sound the alarm after 60-90 minutes of exposure to at 50ppm, at which point you can expect a rude awakening from its 85dB siren. There’s no screen or anything fancy, but it’s simple and easy to use, with clear red/green LED indicators and a noisy warning when its batteries fail or the sensor reaches end of life. 

The one downside with this model is that it runs off a pair of AA batteries, which need to be switched out roughly every three years, making three changes during its lifespan. This isn’t a huge issue, but it means this model isn’t suitable for use in Scottish homes. It is, however, suitable for use in a boat or caravan, making it a good option if you need an alarm on your next boating or caravanning trip.

Key specs – Lifespan: 10 years; Battery type: 3 x AA; Display: None

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