Carbon Monoxide From a Boiler: The Warning Signs

A boiler can produce carbon monoxide (CO) when gas does not burn properly, for example because the boiler is poorly maintained, the flue is blocked or damaged, or there isn't enough ventilation. You can't see, smell or taste it, so the warning signs are indirect: a flame that burns yellow or orange instead of blue, soot or brown staining on or around the boiler, a pilot light that keeps going out, and headaches, dizziness or nausea that get better when you are away from home. If you suspect carbon monoxide, turn the appliance off if you can, open windows, get everyone out, and call the National Gas Emergency number on 0800 111 999.

How a Boiler Produces Carbon Monoxide

When a gas boiler is working properly, it burns gas cleanly and sends the waste gases out through the flue. Carbon monoxide is produced when combustion is incomplete. That can happen when the burner is dirty, a seal has failed, the flue is blocked or has come apart, or the air supply the boiler needs has been restricted. A problem with the flue is especially serious, because it can send exhaust gases back into the room instead of outside.

Warning Signs Around the Boiler

• A yellow, orange or lazy flame. On boilers and appliances where you can see the flame, it should be a crisp blue.
• Soot, scorch marks or brown or yellow staining on the boiler casing, the wall above it or around the flue.
• A pilot light that keeps going out on older boilers that have one.
• More condensation than usual on the windows in the room where the boiler is.
• A flue that looks damaged, disconnected or blocked, inside or where it leaves the building.

None of these proves there is carbon monoxide, but each one means the boiler should not be used until a Gas Safe registered engineer has checked it.

Symptoms to Take Seriously

The NHS lists the most common symptoms of carbon monoxide poisoning as headache, dizziness, feeling or being sick, tiredness and confusion, stomach pain, and shortness of breath or difficulty breathing. They are easy to mistake for a cold, flu or food poisoning. The clues that point to carbon monoxide are:

• Symptoms that ease when you leave the house and come back when you return.
• More than one person in the household feeling unwell at the same time, sometimes pets too.
• Symptoms that are worse in the room where the boiler or another gas appliance is.
• Symptoms that started around the time the heating came on for the season.

What to Do If You Suspect Carbon Monoxide

1. Turn the appliance off if it is safe to do so.
2. Open doors and windows to let fresh air in.
3. Get everyone out of the property, including pets.
4. Call the National Gas Emergency number on 0800 111 999. The line is open 24 hours a day.
5. Get medical advice. Contact NHS 111, or call 999 if someone is seriously unwell, and tell them you think carbon monoxide is involved.
6. Don't use the appliance again until a Gas Safe registered engineer has inspected it and confirmed it is safe.

Carbon Monoxide Alarms

An audible carbon monoxide alarm in any room with a gas appliance is the only way to be warned about CO you cannot otherwise detect. Fit it following the manufacturer's instructions, test it regularly, and replace it when it reaches the end of its life. Remember that an alarm tells you there is a problem. It does not tell you the boiler is safe when it is quiet, and it is no substitute for regular servicing.

Landlords in England have a legal duty to fit a carbon monoxide alarm in any room used as living accommodation that has a fixed combustion appliance, excluding gas cookers, and to repair or replace alarms that are reported faulty. That sits alongside the annual Landlord Gas Safety Certificate (CP12), which requires every gas appliance to be checked by a Gas Safe registered engineer each year. Our landlord gas safety guide covers the full set of obligations.

How an Annual Service Checks for It

A proper annual boiler service includes a combustion analysis. The engineer uses a flue gas analyser to measure what the boiler is producing, including carbon monoxide, and confirms it is burning safely and efficiently. They also check the flue along its route, look at ventilation, and test the safety devices that shut the boiler down when something is wrong. That is how the gradual problems behind carbon monoxide, such as a dirty burner, a failing seal or a flue joint coming loose, get found before they become dangerous. It is also one of the most common reasons an appliance fails a gas safety check.

Our article on what a Gas Safe engineer checks during a service goes through each step. If your boiler shows any of the signs above, don't use it. Once the property is safe and the emergency service has been, or if you simply want the boiler checked, contact us to book a Gas Safe registered engineer.