Your boiler has just been serviced, the invoice sits on the table, and the radiators tick back to life. The house feels sorted. There’s one last job no one talks about loudly: testing the carbon monoxide alarm. That small plastic box on the wall decides whether a silent problem stays silent—or gets heard.
You notice the faint smell of warm metal and a trace of dust your eye didn’t catch this morning. The cat stretches by the airing cupboard. In the kitchen, the carbon monoxide alarm watches like a small, patient lighthouse.
Your finger hovers near the test button. You tell yourself everything’s fine—new seals, fresh notes on the service sheet, tidy tools. Then you remember a neighbour’s story about a flue that shifted after a routine visit, and the alarm that wouldn’t shut up later that night. A quiet room isn’t always a safe room.
One press could change what you do next. Or reassure you completely. Strange how a beep can carry so much weight. Try it.
Why testing after a service still matters
Boiler servicing is meant to make things safer, and it usually does. Yet a service can disturb parts you never see—flue joints nudged, casings opened, burners adjusted by a fraction. The system settles again over the next few cycles. That’s normal. It’s also exactly when a carbon monoxide alarm earns its keep.
Testing right after a service checks two things at once: your alarm still works, and the system behaves under real heat. It’s a small ritual that closes the loop. Think of it like checking your car’s seatbelt after a garage visit. You don’t expect trouble. You just respect what could happen if there is.
There’s a story that sticks with me from a semi in Leeds. A couple had their boiler serviced on a Thursday; the engineer left at tea time. Later, their CO alarm chirped, then screamed. A flue elbow had shifted by a few millimetres after the case went back on. The fix was quick because the warning came early.
UK figures sketch the quiet risk behind tales like this: around 20–30 accidental deaths a year from carbon monoxide, with thousands heading to A&E with suspected exposure. Those aren’t figures designed to scare. They’re just the world as it is. One alarm that actually works turns them into someone else’s problem.
Carbon monoxide has no smell, no colour, no taste. It builds quietly when combustion stumbles: a blocked flue, starved air, imperfect mix. A service can change the balance in good ways, but it is still a change. Testing your alarm post-visit confirms the sensor, the sounder, and the location are doing their job.
Look for a device meeting BS EN 50291, with a clear expiry date. Most sensors last 5–10 years, not forever. When you press test, you’re checking the brain and the voice. When you position it correctly, you’re checking its eyes. That’s the logic of safety: one action, three answers.
How to test your CO alarm the right way
Start simple. With the heating running for a few minutes, press and hold the test button on the alarm. You should hear a loud, pulsing tone and see lights flash in a set pattern. That checks power, sounder, and circuits. Do it the same day as your service. Then again monthly. After every boiler service, test your carbon monoxide alarm the same day.
If you want to test the sensing element, use a CO test aerosol made for alarms and follow the can’s instructions. Slip the supplied hood over the unit if your kit includes one. Short, controlled bursts are enough—never flood the sensor. Step back, let it react, then ventilate and reset as the manufacturer advises. Never try to create carbon monoxide to ‘see if it works’.
Placement makes or breaks performance. In rooms with a fuel-burning appliance, mount the alarm 1–3 metres from it, at breathing height on a wall, away from corners and behind curtains. In bedrooms, put one near where you sleep. Don’t fit it in bathrooms, right above sinks, or on top of cupboards. Heat and steam lie to sensors. Dust does too.
Replace batteries when low, but also watch the end-of-life date on the label—when it’s done, the whole unit goes. If your alarm chirps with a fault code, check the manual, not your guesswork. We’ve all had that moment when we delay a tiny job because life gets in the way. Let’s be honest: nobody does this every day. Set a reminder on your phone for the first of the month and tie it to a daily habit—kettle on, button press, job done.
If the alarm sounds continuously during normal use, switch off appliances, open windows, leave the property, and call 0800 111 999. Testing is different: it’s planned, short, and controlled. Real alarms don’t negotiate. They mean “out”.
“Testing isn’t about doubting your engineer. It’s about catching the tiny things that life and gravity change after he leaves.”
- Call a Gas Safe registered engineer if alarms trigger without a test, flames burn yellow, or you see sooty marks.
- Symptoms to take seriously: headaches, dizziness, nausea, breathlessness—especially if they improve outside.
- Press the test button monthly; sensor checks with CO aerosol once or twice a year are a smart extra.
- Gas Emergency number (UK): 0800 111 999. Save it. You’ll forget it in a panic.
A small habit that protects the quiet moments
You don’t test a CO alarm to prove danger. You test to prove calm. The odd thing about prevention is that a good day ends with nothing to report, and still you feel lighter. After a service, your system has changed a little, like a guitar that’s been restrung. It needs a quick tune. The alarm is your ear.
You test to find nothing, and that’s the point. Houses remember the small rituals we keep. A button press. A short beep. The sort of magic that comes from equipment doing what it promised. Maybe that’s what safety really is—turning “what if” into “not today.” Share the habit with the people you live with. Tell a friend who just moved. Quiet pays forward.
| Key points | Detail | Reader Interest |
|---|---|---|
| Test the same day as servicing | Button test with heating running; optional sensor test with certified aerosol | Actionable step that takes under a minute |
| Place alarms where they can “see” | 1–3 m from appliance, breathing height, away from steam and corners | Quick win without buying anything new |
| Know what to do if it sounds | Shut appliances, ventilate, evacuate, call 0800 111 999 | Clarity in a stressful moment |
FAQ :
- How soon after a boiler service should I test my CO alarm?Same day is best, once the heating has been on for a few minutes. Then set a monthly reminder so it becomes routine.
- Is the test button enough to prove the sensor works?The button checks power, sounder, and circuitry. For the sensing element, use a purpose-made CO test aerosol as per the alarm maker’s guidance—once or twice a year is a sensible rhythm.
- Where should I put a CO alarm near my boiler?Mount it 1–3 metres from the appliance, at breathing height on a wall, not behind curtains or in dead air spaces. In bedrooms, place an alarm near the bed so you’ll hear and benefit from it while sleeping.
- My alarm went off after servicing. What should I do?Turn off gas appliances if safe to do so, open windows, leave the property, and call the Gas Emergency Service on 0800 111 999. Do not re-enter until told it’s safe, then book a Gas Safe engineer to investigate.
- How long do CO alarms last, and when should I replace one?Most last 5–10 years. Check the expiry date on the unit. When it reaches end-of-life, replace the whole alarm, not just the battery.
