Video by FireAngel Safety Technology. This guide draws on the installation walkthrough "How to fit a FireAngel Carbon Monoxide Alarm — CO-9B" from FireAngel, one of the UK’s leading fire and CO safety manufacturers. Their video covers the full fitting process clearly, including battery installation, LED indicator meanings, and testing the alarm correctly before relying on it.
1. Choose a British Standard – approved alarm
Look for a CO alarm marked to BS EN 50291-1. This is the UK standard for carbon monoxide alarms and ensures the sensor responds reliably to dangerous CO concentrations. Most major brands sold in UK hardware and DIY shops meet this standard — check the packaging if you are not sure.
Alarms with a 10-year sealed battery are worth the extra cost for a permanent fitting. Replaceable battery models are fine but need checking and swapping regularly. Check the alarm's expiry date too — CO sensors degrade over time and the whole unit needs replacing, not just the battery.
2. Pick the right position in each room
Carbon monoxide is almost exactly the same density as air, so it disperses evenly through a room rather than rising or sinking. This means CO alarms should be fitted at roughly head height, between 1 and 3 metres from the fuel-burning appliance you are protecting.
Do not fit CO alarms inside cupboards, directly above or beside the appliance, or in areas with poor air circulation. Bedrooms are particularly important to protect — CO poisoning often happens at night when symptoms go unnoticed.
3. Check the mounting instructions before you drill
Most CO alarms come with a backplate that screws to the wall or ceiling. Read the instructions for the specific model before drilling — some prefer wall mounting, others ceiling mounting. The FireAngel CO-9B, for instance, mounts to the wall at the recommended height.
Mark the screw positions using the backplate as a template. Use a stud detector and cable detector to check there are no cables or pipes hidden behind the wall at that point. CO alarms are lightweight — standard brown wall plugs and screws are perfectly adequate for plasterboard or masonry.
4. Fit the backplate and install the batteries
Drill and plug the holes, then screw the backplate securely to the wall. Do not overtighten into plasterboard — snug is enough. Insert the batteries in the correct orientation as marked inside the battery compartment.
The alarm should chirp or flash an LED to confirm it has powered up. Check your specific model's instructions for what the startup sequence looks like — a red LED, a green LED, or a short beep pattern each mean something slightly different.
5. Clip the alarm onto the backplate
Once the batteries are in and the backplate is fixed, clip or twist the alarm unit onto the plate. It should lock with a gentle click. Give it a light tug to make sure it is seated securely — the last thing you want is the alarm dropping off the wall if it ever does trigger.
Keep the instruction manual somewhere accessible — in a kitchen drawer, say. You will want it if the alarm sounds and you need to know whether you are dealing with a genuine emergency or a sensor fault.
6. Test it immediately, then monthly
Press and hold the test button for the number of seconds shown in the instructions. The alarm should sound clearly. If it does not, check the batteries and their contacts, and try again.
Test the alarm monthly as a matter of habit — use the test button, not a flame or a cigarette lighter near the sensor. CO alarms test the electronics, not the sensor, so a monthly button test is good practice. If the alarm sounds when there is no combustion appliance in use, open windows and doors immediately and contact the Gas Emergency Helpline on 0800 111 999.
7. Know the alarm sounds
Different alarm patterns mean different things. A continuous alarm (typically four beeps repeated) means elevated CO has been detected — evacuate immediately, get into fresh air, and call 999. A single beep or chirp every minute usually means the battery is low. Refer to your alarm's manual for the exact patterns for your model.
Never assume a beeping CO alarm is just a battery warning without checking. If you are not certain which pattern you are hearing, treat it as an emergency first and investigate second.
When to call a handyman
Fitting a battery CO alarm is a simple DIY task that does not involve any electrical work. Call Richard if you need a mains-wired alarm fitted, if you want alarms installed throughout a property, or if you have concerns about a fuel-burning appliance that might need a Gas Safe engineer rather than a handyman.
Need safety alarms fitted?
The Sandwich Handyman can help with smoke alarms, CO alarms, and other basic safety fittings around the home in Sandwich and nearby East Kent.
Contact Richard