Carbon monoxide (CO) is sometimes called the "silent killer" because you can't see it, smell it or taste it. A well-maintained gas boiler is very safe, but a faulty or poorly serviced one can produce CO without any obvious warning. This guide explains what carbon monoxide is, the signs to watch for on your boiler and in your household, and the calm, sensible steps to take if you ever suspect a problem.
This article is information, not a repair manual. Never attempt to investigate or fix a suspected gas fault yourself. All work on a gas boiler must be carried out by a Gas Safe registered engineer. If you suspect carbon monoxide right now, skip to what to do.
Carbon monoxide is a gas formed when carbon-based fuels — such as the natural gas that powers most UK boilers — burn without enough oxygen to burn completely. This is called incomplete combustion. In a healthy boiler, gas burns cleanly and the by-products are safely carried outside through the flue. When something goes wrong — a blocked flue, a starved air supply, a faulty burner, or a boiler that has never been serviced — combustion can become incomplete and start releasing CO.
The danger is that carbon monoxide stops your blood from carrying oxygen around your body. Even at low levels it can make you unwell over time; at high levels it can be fatal quickly. This is why catching the early signs matters so much.
You don't need to open the boiler or touch anything to notice these. Many are visible just by looking at the appliance during normal use:
If you spot any of these, switch the boiler off if you can do so safely at the appliance or at the fused spur, stop using it, and book a Gas Safe registered engineer. Do not try to relight, clean or adjust anything yourself.
The symptoms of carbon monoxide poisoning are easy to mistake for flu, food poisoning or simple tiredness — there's no fever. The big clue is the pattern: symptoms that ease when you leave the house and return when you come home, or that affect several people (and even pets) in the household at once. Watch for:
The tell-tale pattern: if your symptoms get better when you go out and worse when you're back indoors, treat it seriously. CO affects the most vulnerable first — babies, children, older people, pregnant women and pets — so don't dismiss it because one person feels fine.
Because you can't sense carbon monoxide, an audible alarm is your most reliable early warning. Choose one that meets the British and European standard BS EN 50291 and carries the British Standards Kitemark. A CO alarm is not the same as a smoke alarm — you need both.
Fit the alarm in the same room as the boiler, following the manufacturer's instructions for positioning. Test it regularly using the test button, and replace it at the end of its stated lifespan (typically around seven years — check the date on yours). In many parts of the UK, a CO alarm is now a legal requirement in rooms with a gas boiler, and it is always recommended. If you rent, your landlord is responsible for providing one.
If your CO alarm sounds, or you notice the warning signs above, act calmly but quickly:
Do not go back inside until the gas emergency engineer or a Gas Safe registered engineer tells you it's safe. Don't try to find or fix the source yourself.
The most effective way to prevent carbon monoxide problems is a yearly boiler service by a Gas Safe registered engineer. During a service the engineer checks combustion, inspects the flue, looks for early signs of wear and confirms the boiler is burning gas safely — often long before any symptom would appear. (The Gas Safe Register replaced the old CORGI scheme back in 2009, so always check an engineer is on the current Gas Safe Register before they work on your boiler.)
This is one reason many homeowners take out boiler cover: most policies include an annual service alongside breakdown repairs, which keeps your boiler maintained without you having to remember to book it. If you're weighing up whether a policy is right for you, our guide on whether boiler cover is worth it walks through the trade-offs.
| Sign | What it may indicate |
|---|---|
| Yellow/orange flame instead of blue | Incomplete combustion — possible CO |
| Soot or staining around the boiler | Incomplete combustion residue |
| Pilot light keeps going out | Possible air supply or combustion fault |
| Headaches that ease when you go out | Possible CO exposure indoors |
| CO alarm sounding | CO detected — act immediately |
Indicative signs only — a Gas Safe registered engineer must diagnose any suspected fault.
No. Carbon monoxide is completely colourless and odourless, which is why an audible CO alarm to BS EN 50291 is so important. The smell of gas you might notice is a separate issue — natural gas has a chemical added to give it a smell — but CO itself gives no warning to your senses.
No. Smoke alarms detect smoke from fire; CO alarms detect carbon monoxide. They work in completely different ways, so you need both. Make sure your CO alarm carries the BS EN 50291 standard and a Kitemark.
Once a year by a Gas Safe registered engineer is the standard recommendation, and it's usually a condition of your boiler's warranty too. An annual service checks combustion and the flue, catching problems before they become dangerous. Many boiler cover policies include this service automatically.
Call the free National Gas Emergency Service on 0800 111 999, 24 hours a day, from outside the property. For carbon monoxide symptoms, also seek medical advice from your GP or A&E.
Cover varies by policy. Most boiler cover plans handle breakdown repairs and include an annual service, which is the key preventative measure. Always check the policy terms and exclusions before buying — you can compare boiler cover to see what each plan includes.
An annual Gas Safe service is the best defence against carbon monoxide. Many boiler cover policies include one as standard — compare plans to find cover that fits your home.
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