In This Guide
What is carbon monoxide and why is it dangerous?
Carbon monoxide, or CO, is a gas produced when fuels like natural gas do not burn completely. It has no colour, no smell, and no taste, which is exactly what makes it so dangerous: without a detector, there is no way to know it is there until you feel unwell. It stops your blood carrying oxygen, and at high levels it can kill quickly. At lower levels, ongoing exposure causes headaches, dizziness, nausea, tiredness, and confusion, symptoms that are easy to mistake for something else.
A well maintained, correctly burning gas boiler produces very little CO and vents it safely outside. The risk comes when something goes wrong: incomplete combustion, a blocked flue, or poor ventilation can let CO build up indoors.
Why can a boiler produce carbon monoxide?
Any gas appliance can produce carbon monoxide if it is faulty or poorly maintained. With a boiler, the common causes are a burner that is not combusting cleanly, a blocked or damaged flue that stops exhaust gases venting outside, cracked or corroded components, and inadequate ventilation around the appliance. These are the exact things a proper service is designed to find, which is why annual servicing is the single most effective way to reduce the risk.
Why a carbon monoxide detector is essential
Because you cannot see or smell CO, a detector is your only reliable early warning. An audible CO alarm sounds when it senses a dangerous level, giving you and your family time to get out and ventilate before exposure becomes serious. It is not a replacement for servicing, it is the backup that protects you between services and catches a sudden fault. Every home with a gas appliance should have one.
Where to fit a carbon monoxide alarm
Fit an audible alarm that meets the current British Standard. Place one in the same room as the boiler or gas appliance, ideally around head height and roughly one to three metres away from the appliance, and consider additional alarms near sleeping areas so you will hear it at night. Test it regularly and replace it at the end of its stated lifespan, as the sensors do not last forever.
What to do if your carbon monoxide alarm sounds
Treat it as an emergency. Stop using the appliance, open doors and windows to ventilate, and get everyone out into fresh air. Call the National Gas Emergency Service on 0800 111 999. If anyone feels unwell, seek medical help and tell them you suspect carbon monoxide. Do not use the boiler again until a Gas Safe registered engineer has checked it and confirmed it is safe.
How 0800 Homefix can help
0800 Homefix are Gas Safe registered engineers covering London and the South East. An annual boiler service checks combustion, the flue, and ventilation, the things that keep carbon monoxide from building up, and an engineer can advise on the right alarm and where to place it. If you are worried about a specific appliance, 0800 Homefix can inspect it and confirm it is burning and venting safely.
0800 Homefix charges as the first hour of labour, paid in advance, with any parts or further work quoted and agreed with you before it begins. For current rates, see our rates page. Our labour is guaranteed for 12 months, and any parts we fit are covered by the manufacturer’s 12-month guarantee.
Frequently Asked Questions
Can a boiler give off carbon monoxide?
Does a boiler service prevent carbon monoxide?
An annual service greatly reduces the risk by checking combustion, the flue, and ventilation and catching faults early. It should be paired with an audible CO alarm as a backup, not relied on alone.
Where should I put a carbon monoxide detector?
In the same room as the boiler, around head height and about one to three metres from the appliance, with additional alarms near sleeping areas. Fit an alarm that meets the current British Standard and test it regularly.
What are the symptoms of carbon monoxide poisoning?
Headaches, dizziness, nausea, tiredness, and confusion, which can be mistaken for flu. Symptoms that ease when you leave the house and return when you go back are a warning sign. Get fresh air and seek medical help.
What should I do if my CO alarm goes off?
Stop using the appliance, ventilate, get everyone outside, and call the National Gas Emergency Service on 0800 111 999. Do not use the boiler until a Gas Safe engineer has confirmed it is safe.
Helpful resources
- Gas Safe Register: check an engineer is registered, and find carbon monoxide safety guidance.
- NHS: carbon monoxide poisoning: symptoms and what to do.
Spotted Any of These Warning Signs?
Want peace of mind that your boiler is burning safely? 0800 Homefix has Gas Safe registered engineers across London and the South East who can service and check your appliances and advise on carbon monoxide protection, seven days a week, 365 days a year.
Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
