Carbon Monoxide From a Boiler: Warning Signs and What to Do
How to tell if a boiler, gas fire or cooker could be leaking carbon monoxide: the signs on the appliance, the symptoms of exposure, what to do straight away, and how to prevent it as the heating goes back on.
Carbon monoxide (CO) has no colour, smell or taste, so you can't rely on your senses to detect it. Warning signs on an appliance include a lazy yellow or orange flame where it should be crisp and blue, black or brown sooty marks on or around the boiler or fire, a pilot light that keeps going out, and more condensation than usual in the room. In people, symptoms include headaches, dizziness, nausea, tiredness, confusion and breathlessness that ease when you leave home. If you suspect CO, open doors and windows, turn the appliance off if you can, get out, call the Gas Emergency line on 0800 111 999 and get medical advice.
Key Takeaways
- CO has no smell. A working, audible CO alarm is the only reliable way to know it's there.
- Appliance warning signs: yellow or orange flames, soot or staining, a pilot light that keeps going out, extra condensation, a damaged or blocked flue.
- Symptoms to watch for: headaches, dizziness, nausea, tiredness, confusion and breathlessness, especially if they ease when you're out of the house.
- If you suspect CO: fresh air, appliance off if safe, get everyone out, call 0800 111 999, and seek medical advice.
- Don't use the appliance again until a Gas Safe registered engineer has checked it. An annual service is the best prevention.
Carbon monoxide (CO) has no colour, smell or taste, so you can't detect it with your senses. The warning signs are indirect. On the appliance, look for a lazy yellow or orange flame where it should be crisp and blue, black or brown sooty staining, a pilot light that keeps going out, and more condensation in the room than usual. In people, CO causes headaches, dizziness, nausea, tiredness, confusion and breathlessness, which often ease when you leave home. If you suspect CO, open doors and windows, turn the appliance off if you can, get everyone out, call the Gas Emergency line on 0800 111 999 and get medical advice. Don't use the appliance again until a Gas Safe registered engineer has checked it. Autumn is a good time to be alert, as boilers, gas fires and flues come back into use after months off.
Why boilers can produce CO
Gas, oil, coal and wood all give off carbon monoxide when they burn without enough oxygen. A boiler that's working properly burns gas cleanly and sends its fumes safely outside through the flue. CO becomes a risk when something goes wrong with that process:
- the burner is dirty, worn or badly adjusted, so the gas doesn't burn completely
- the flue is blocked, damaged, disconnected or badly fitted, so fumes don't get out
- on older open-flued boilers and gas fires, the room doesn't get enough fresh air because a vent has been blocked
- a chimney or shared flue is blocked by debris or a bird's nest
- the appliance was installed or repaired by someone who isn't Gas Safe registered
Modern condensing boilers are room-sealed, which means they take their air from outside and are much safer than older models. They can still leak CO if the flue or its seals fail. Our guide to a blocked boiler flue covers flue problems in more detail.
Warning signs on the appliance
| What you might see | What it can mean |
|---|---|
| Yellow, orange or floppy flame instead of crisp blue | Incomplete combustion. On modern boilers you can't see the flame, but you can on gas fires, hobs and older boilers |
| Black, brown or yellow staining on or around the appliance | Soot from poor combustion or fumes escaping |
| A pilot light that keeps going out | Can be a draught, a faulty part or a combustion problem. See pilot light keeps going out |
| Much more condensation than usual on windows in that room | Can point to fumes and moisture not being carried away |
| Damaged, rusted or loose flue pipe, or a flue terminal outside that's blocked | Fumes may be escaping into the home |
| Soot falling into an open fireplace | Chimney may be blocked |
None of these proves there's CO in the air, and a boiler can leak CO without any of them. That's why a CO alarm matters. But if you see any of them, stop using the appliance and have it checked.
Symptoms of CO exposure
The early symptoms of CO poisoning are easily mistaken for flu, a cold or a hangover. The common ones are:
- a tension-type headache
- dizziness
- feeling or being sick
- tiredness and confusion
- stomach pain
- shortness of breath
Higher levels can cause loss of balance, blurred vision, chest pain, seizures and loss of consciousness, and can kill. Patterns that suggest CO rather than a bug:
- symptoms ease when you go out and return when you come back
- several people in the home feel ill at the same time, and pets seem unwell too
- symptoms are worse when the heating, gas fire or cooker is in use
- symptoms started after the heating came back on for the season
What to do right now
Get fresh air in
Open doors and windows as you go.
Turn the appliance off
Only if you can do it quickly and safely. If you can reach the gas emergency control valve by the meter, turn it off.
Get everyone out
Including pets. Don't stay inside to investigate.
Call 0800 111 999
This is the free Gas Emergency number, open 24 hours. They'll advise you and send someone to make the supply safe.
Get medical advice
See a GP or call NHS 111, and tell them you think you've been exposed to carbon monoxide. Call 999 if anyone is seriously unwell.
If you smell gas as well, the steps are similar but there's more to avoid, such as switches and naked flames. See our guide on what to do if you smell gas.
After the emergency
The emergency service will make the supply safe. They don't repair appliances. Before the boiler, fire or cooker is used again, it needs to be inspected and repaired by a Gas Safe registered engineer. Ask for the fault to be explained and written down. If you rent, tell your landlord straight away, as the repair is normally their responsibility.
If you're left without heating or hot water while waiting for a repair, keep warm safely. Never use a gas cooker or oven to heat a room, and never bring a barbecue or outdoor heater indoors.
How to prevent it
- Fit an audible CO alarm in every room with a fuel-burning appliance. Choose one marked to the BS EN 50291 standard, test it regularly and replace it when it reaches the end of its life. Our carbon monoxide detector guide covers where to put it.
- Have the boiler serviced every year by a Gas Safe registered engineer. A service checks combustion and the flue. See our boiler service page, and how long a boiler service takes.
- Check the engineer's Gas Safe ID card before any gas work, and check the card covers the type of appliance. Our guide to checking a gas engineer explains how.
- Keep vents and flue terminals clear. Don't block air vents, and keep plants, bins and sheds away from the flue outside.
- Don't ignore warning signs. Soot, yellow flames or a pilot light that won't stay lit are reasons to stop using the appliance, not to keep an eye on it.
In England, landlords must fit a CO alarm in any room used as living accommodation that has a fixed combustion appliance such as a gas boiler or fire (gas cookers are excluded), and have gas appliances checked every year. See our gas safety certificate guide.
Booking a safety check
If you've noticed any of the warning signs, the heating has come back on after months off and the boiler hasn't been serviced for over a year, or you've just moved in and have no service history, book a check with a Gas Safe registered engineer. We arrange boiler services, boiler repairs and gas safety certificates across London. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before any work starts. If an alarm is sounding or anyone feels unwell, call 0800 111 999 first, not us.
If your detector is chirping rather than sounding a full alarm, our guide to a carbon monoxide alarm beeping explains how to tell a CO warning from a low battery or end-of-life signal.
Want your boiler checked?
Tell us what you've noticed and when the boiler was last serviced. We'll confirm the price before starting.
📞 Call 020 7870 3200Or see our boiler servicing and pricing.
Frequently Asked Questions
How do I know if my boiler is leaking carbon monoxide?
You can't tell by smell, because carbon monoxide has none. Look for a yellow or orange flame instead of a blue one, black or brown staining on or around the boiler, a pilot light that keeps going out, and heavy condensation in the room. The only reliable warning is an audible CO alarm. If anyone in the home has headaches, nausea or dizziness that ease when they go out, treat it as a possible CO leak.
What are the symptoms of carbon monoxide poisoning?
Common symptoms include a headache, dizziness, feeling or being sick, tiredness, confusion, stomach pain and shortness of breath. They are easily mistaken for flu or a cold. Clues that point to CO are several people or pets in the home feeling ill at once, and symptoms that ease when you leave the house and return when you come back.
What should I do if my CO alarm goes off?
Open doors and windows, turn off the gas appliance if you can do so safely, and get everyone out into fresh air. Call the Gas Emergency number, 0800 111 999. If anyone has symptoms, get medical help. Call 999 if someone is seriously unwell. Don't go back in or use the appliance until it has been checked by a Gas Safe registered engineer.
Can a new boiler leak carbon monoxide?
It is less likely, as modern condensing boilers are room-sealed and draw air from outside, but it can still happen if the boiler or flue is installed badly, the flue is damaged or blocked, or a seal fails. That is why every gas boiler should be fitted and serviced by a Gas Safe registered engineer and every home with one should have a CO alarm.
Do landlords have to fit carbon monoxide alarms?
In England, yes. Since October 2022, landlords must fit a CO alarm in any room used as living accommodation that has a fixed combustion appliance, such as a gas boiler or gas fire. Gas cookers are excluded, although an alarm is still a good idea. Landlords must also have gas appliances checked every year by a Gas Safe registered engineer.
Sources and further reading
Source links checked 27 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.