What are the warning signs of carbon monoxide from a boiler?

Carbon monoxide (CO) has no colour, taste or smell, so you can't notice it on its own. The warning signs come from two places instead: the boiler, such as a lazy yellow or orange flame, sooty or brown staining around it, or more condensation on the windows than usual, and the people in your home, with headaches, sickness or tiredness that ease when you go out. A working CO alarm is the only reliable way to know it's there.

With the heating going back on for winter, this post covers the signs to look for, what to do if your alarm goes off, and the checks you can make safely. You can look at the boiler, the flue outside and your alarm, but never remove the boiler casing or try to adjust the flue yourself.

Why a boiler can make carbon monoxide

When the gas in your boiler burns properly, with plenty of air, it makes mostly carbon dioxide and water vapour, which leave through the flue, the pipe that carries the fumes outside. When something stops the gas burning properly, it makes carbon monoxide instead. Engineers call this incomplete combustion.

The usual causes are:

• a flue that is blocked, damaged or has come apart at a joint
• a burner that is dirty or not set up correctly
• not enough fresh air reaching the boiler, for example because a vent has been covered
• worn seals that let fumes escape from the boiler's sealed case

Most of these build up slowly and out of sight.

Warning signs on the boiler

None of these proves carbon monoxide is present, but each one means the boiler should be checked by a Gas Safe registered engineer before you rely on it:

• A yellow or orange flame. On older boilers where you can see the flame, it should be crisp and blue. A floppy yellow or orange flame suggests the gas isn't burning properly. Most modern boilers have a sealed case, so not seeing a flame is normal.
• Soot or staining. Black sooty marks, or yellow or brown staining, on or around the boiler, the flue or the wall and ceiling nearby.
• A pilot light that keeps going out on an older boiler.
• More condensation than usual on the windows of the room the boiler is in.

Symptoms to watch for in your household

The NHS lists the main symptoms of carbon monoxide poisoning as a headache, dizziness, feeling or being sick, feeling weak, tiredness and confusion, chest or muscle pain and shortness of breath. They are easily mistaken for flu or a stomach bug.

The clues that point to carbon monoxide are:

• symptoms that ease when you are away from home and come back when you return
• symptoms that are worse in one room, often the one with the boiler
• more than one person in the home feeling unwell at the same time

What to do if your alarm goes off or you suspect CO

1. Turn off the boiler and any other gas appliances, if you can do it safely.
2. Open doors and windows to let fresh air in.
3. Get everyone out of the house, pets included.
4. Call the free 24-hour National Gas Emergency number on 0800 111 999.
5. Get medical advice. Call NHS 111, or 999 if someone is struggling to breathe, suddenly confused, has chest pain or has collapsed. Don't drive yourself to A&E.
6. Don't use the boiler again until a Gas Safe registered engineer has checked it and confirmed it's safe.

The emergency service will make things safe, which can mean disconnecting the appliance. Fixing the fault is then a job for a Gas Safe registered engineer.

Getting your carbon monoxide alarm right

An alarm only protects you if it works and is in the right place.

• Choose one that meets the British standard BS EN 50291-1 and carries the Kitemark.
• Fit it in the room with the boiler, following the maker's instructions. As a guide, that's roughly at head height, one to three metres from the boiler, and not next to a window, door, vent or inside a cupboard.
• Press the test button once a month, and replace the alarm by the expiry date on it.

There are rules too. In England, a carbon monoxide alarm has been required in the room whenever a new or replacement boiler is fitted since October 2022. Landlords in England must also fit one in every room used as living space that has a fixed appliance burning fuel, such as a gas boiler, and repair or replace it once they're told it isn't working. Our landlord gas safety certificate checklist covers the rest of a landlord's yearly duties.

Safe checks you can make yourself

None of these involve touching anything inside the boiler:

• Look at the flue where it comes out of the wall or roof. Make sure leaves, ivy, a nest or a new shed aren't blocking it. Don't poke anything into it.
• Check that air vents in walls or doors near the boiler aren't covered, and that the boiler cupboard isn't packed with stored items.
• Test your alarm and check its expiry date.
• Find the date of your last boiler service.

Never take the casing off, block a vent to stop a draught, or try to refit a flue joint. By law, any work on a gas appliance must be done by a Gas Safe registered engineer, so always ask to see the engineer's Gas Safe ID card.

How a yearly service and gas safety check help

Because most of these faults develop out of sight, a yearly check is the best protection. On a boiler service, our engineer checks the flue and ventilation, cleans the boiler, tests the safety controls and measures the fumes it gives off with a calibrated flue gas analyser. A gas safety check also tests for gas leaks, the burner pressure and how well the flue is working. If an appliance isn't safe, we label it At Risk or Immediately Dangerous, explain why, and advise you to keep it switched off until it's put right.

If you've noticed any of these signs, or your boiler hasn't been serviced in the last year, book a gas safety check with TwinSun Heating. Our Gas Safe registered engineers will check how your boiler is burning and tell you plainly if anything needs doing.