Carbon Monoxide and Your Boiler: What Every Homeowner Needs to Know

The Gas You Can't Sense Until It's Too Late

Carbon monoxide (CO) has no colour, no smell, and no taste. It doesn't irritate your eyes or throat to warn you it's there. By the time most people realise they've been exposed, they're already too unwell to act — which is why it kills around 40 people in UK homes every year and sends over 4,000 to A&E.

The leading domestic source of carbon monoxide is a faulty or poorly maintained gas boiler. The good news is that the combination of an annual boiler service by a Gas Safe registered engineer and a working CO alarm makes CO poisoning almost entirely preventable in a modern home.

How Boilers Produce Carbon Monoxide

A correctly operating gas boiler burns natural gas in a controlled combustion process that produces carbon dioxide (CO₂) and water vapour — both harmless in the quantities produced and safely vented through the flue to outside. Carbon monoxide is produced when combustion is incomplete — when the gas doesn't fully react with sufficient oxygen.

Incomplete combustion can occur when:

• The heat exchanger is cracked or corroded, allowing combustion products to mix with the water circuit or the room air
• The flue is blocked, damaged, or incorrectly fitted — preventing combustion gases from escaping outside
• The burner is dirty or damaged, producing an incomplete burn
• Ventilation to the boiler cupboard is inadequate, restricting the oxygen supply needed for complete combustion
• The boiler is significantly older and has never been properly maintained

This is why combustion analysis — testing the CO/CO₂ ratio in the flue gases — is a mandatory part of every boiler service. It's the only way to confirm that the combustion process is safe and that no CO is escaping into the living space.

Symptoms of Carbon Monoxide Poisoning

CO poisoning symptoms are often mistaken for flu, a virus, or a bad headache — because CO deprives the body of oxygen in a way that mimics those conditions:

• Headache — often the first symptom, particularly in the morning or after time in a room with the boiler running
• Dizziness or lightheadedness
• Nausea or vomiting
• Breathlessness or shortness of breath
• Confusion or difficulty concentrating
• Chest pain

A crucial distinction from flu: CO symptoms typically improve when you leave the property and return when you go back inside. If multiple members of a household have similar symptoms at the same time — including pets, which are often affected first — this is a serious warning sign. Get everyone out of the property and call 999.

Carbon Monoxide Alarms: Where to Fit Them and Which Type to Choose

Since 1 October 2022, the Smoke and Carbon Monoxide Alarm (England) Regulations 2022 require a CO alarm to be installed in every room of a rented property that contains a fixed combustion appliance (including gas boilers). For owner-occupiers, CO alarms are strongly recommended but not yet legally mandated in England outside rented properties.

Where to position CO alarms:

• In every room where there is a gas appliance — including the boiler cupboard if it opens into a habitable space
• In the main bedroom — CO poisoning is most dangerous during sleep when you can't respond to symptoms
• On each floor of the property

Choose an alarm certified to British Standard BS EN 50291. Battery-powered alarms are available from £15–£30; mains-wired combined smoke/CO detectors from £40–£70. Alarms have a limited lifespan — typically 5–7 years — after which the sensor degrades and must be replaced, regardless of whether it has ever triggered.

What to Do If Your CO Alarm Sounds

If your carbon monoxide alarm activates:

1. Get everyone out of the property immediately — including pets. Do not stop to collect belongings.
2. Leave the door open as you leave — this helps ventilate the property.
3. Call 999 from outside the property — do not go back inside. The emergency services will ventilate the building and assess whether it's safe to re-enter.
4. Seek medical attention — even if you feel well. CO binds to haemoglobin for hours after exposure. A blood test at A&E can confirm whether you've been exposed.
5. Do not re-enter until the emergency services have declared the property safe.
6. Call a Gas Safe registered engineer — before the boiler or any gas appliance is turned on again. Our emergency team can carry out a safety inspection and combustion analysis before you return to normal use.

The Role of Annual Boiler Servicing

An annual boiler service by a Gas Safe registered engineer is the most effective measure available against CO risk from your boiler. During every service our engineers:

• Carry out full combustion analysis using a calibrated flue gas analyser — measuring CO and CO₂ levels in the flue gases against safe operating parameters
• Inspect the heat exchanger for cracks or corrosion
• Check the flue and terminal for damage, blockages, or incorrect clearance from openings
• Verify adequate ventilation to the appliance
• Follow the Gas Industry Unsafe Situations Procedure (GIUSP) if any unsafe condition is found — including immediately advising on making the appliance safe before leaving the property

Don't wait for a fault code or a symptom. Book your annual boiler service now — it takes around an hour and is the single most important thing you can do for your household's safety. Call 02039514510 or book online.

Landlord Obligations

Landlords in England are legally required under the Smoke and Carbon Monoxide Alarm (England) Regulations 2022 to install a working CO alarm in every room with a fixed combustion appliance, and to repair or replace any alarm that is reported as faulty. This applies to both private rentals and HMOs. Read our landlord gas safety guide for the full picture on annual gas safety obligations and CP12 requirements alongside CO alarm duties.

Frequently Asked Questions

Can a new boiler produce carbon monoxide?

Yes — a new boiler can produce CO if the flue is incorrectly installed, if there is inadequate ventilation to the appliance, or if a manufacturing defect causes incomplete combustion. This is why commissioning by a Gas Safe registered engineer (not just installation) is essential, and why the first annual service — typically within 12 months of installation — is required by most manufacturers to validate the warranty.

How do I know if my CO alarm is working?

Press the test button monthly to confirm the alarm and sounder are functioning. Note the manufacture date on the back of the unit — if the alarm is more than 5–7 years old (check the manufacturer's stated sensor lifespan), replace it regardless of whether it has ever triggered. An alarm that has passed its sensor expiry date may not detect CO reliably even if the sounder still works.

What colour is a healthy boiler flame?

A healthy gas boiler flame is crisp and predominantly blue, sometimes with a small inner cone of lighter blue. A yellow, orange, or flickering flame indicates incomplete combustion and is a warning sign that requires immediate attention from a Gas Safe engineer. Do not continue to use a boiler with a yellow or orange flame.

Is carbon monoxide only produced by boilers?

No — any gas appliance can produce CO if faulty: gas fires, gas cookers, gas water heaters, and portable gas heaters. Boilers are the most common source in UK homes because they run for long periods and are often in enclosed spaces. A CO alarm protects against all combustion sources, not just the boiler.