Boilers and carbon monoxide: warning signs and safe action

Carbon monoxide can be produced when a fuel-burning appliance is faulty, poorly maintained or does not receive enough air for correct combustion. A gas boiler is one possible source, but fires, cookers, heaters, solid-fuel appliances and portable generators can also produce it.

Carbon monoxide has no colour, taste or smell. You cannot confirm that a home is safe by sniffing the air or looking at the boiler. This guide explains the warning signs, the action to take and the role of alarms and professional checks.

If a carbon-monoxide alarm sounds

Treat an alarm as real. Do not assume it is a flat battery or silence it and carry on using the boiler.

1. Stop using suspected fuel-burning appliances if you can do so without delaying your escape.
2. Open doors and windows if they are beside you and it is safe, then get everyone into fresh air.
3. Do not go back into the building until you have received safety advice.
4. Get medical advice. Call NHS 111 if you think you have symptoms.
5. Call 999 for severe symptoms such as difficulty breathing, sudden confusion, weakness, chest or muscle pain, or loss of consciousness.
6. If you suspect a gas appliance or gas leak, call the National Gas Emergency Service on 0800 111 999 in Great Britain.

Do not spend time trying to identify which appliance is responsible. Fresh air and medical advice come first.

Symptoms of carbon-monoxide poisoning

The NHS lists symptoms including:

• headache;
• dizziness;
• feeling sick or being sick;
• weakness, tiredness or confusion;
• chest or muscle pain;
• shortness of breath;
• loss of vision;
• loss of consciousness in serious cases.

Symptoms can resemble flu, food poisoning or fatigue. A particularly important clue is that they appear or worsen inside a building and improve after going outside. More than one person, or pets, becoming unwell at the same time also raises concern.

Do not wait for every symptom or for an alarm to sound. If you suspect exposure, leave and obtain medical advice.

Can you smell carbon monoxide?

No. Carbon monoxide itself is odourless. A smell of gas is a different emergency, although both situations require immediate action.

If you smell gas, do not operate electrical switches, smoke or use a naked flame. Leave the building and call the gas emergency number from a safe place. Never search for a leak yourself.

Warning signs around a boiler or other gas appliance

Gas Safe Register advises people to watch for signs that an appliance may not be working correctly, including:

• dark, sooty staining on or around an appliance;
• a lazy yellow or orange flame where the flame should normally be crisp and blue;
• increased condensation inside nearby windows;
• a pilot light that repeatedly goes out;
• blocked or covered ventilation openings.

Many modern room-sealed boilers do not have a user-visible flame, so you may not be able to perform a flame check. Never remove the case to look. The absence of staining, smells or unusual noises does not prove that an appliance is safe.

What can cause a boiler to produce carbon monoxide?

Correct combustion and safe removal of flue gases depend on the appliance, gas supply, ventilation and flue all working as designed. Potential problems can include poor maintenance, an installation defect, an obstructed or damaged flue, inadequate combustion air, or a failed component.

These are matters for a Gas Safe registered engineer with the correct appliance qualifications. Do not open a Baxi, Worcester Bosch, Ideal or any other gas boiler or gas appliance, alter its flue, block a vent or attempt internal cleaning.

Carbon-monoxide alarms: essential backup protection

An audible carbon-monoxide alarm provides an important warning if CO reaches a dangerous level. Gas Safe Register recommends an alarm marked to BS EN 50291 and regular checks that it works.

Follow the alarm manufacturer’s instructions for location, mounting, testing, battery replacement and end-of-life date. Positioning is important and varies with the room and alarm design, so do not rely on a generic distance found online when the supplied instructions say something different.

Test the alarm using its test button at the interval stated by the manufacturer. A test button confirms the alarm and electronics respond; it does not test the safety of the boiler. Replace the unit when it reaches its printed end-of-life date or gives the specified end-of-life signal.

An alarm is not a substitute for correct installation, servicing or gas-safety checks. It is a final layer of protection.

What if the alarm chirps rather than sounding continuously?

Alarm patterns vary. A periodic chirp may indicate a low battery, fault or end-of-life condition, while a full alarm pattern can indicate detected CO. Read the label and manufacturer instructions from fresh air if there is any uncertainty.

If anyone has symptoms, a fuel-burning appliance is running or you cannot confidently distinguish the signal, leave the property and treat it as a potential CO alarm. Do not remove the battery and forget about it.

Does a boiler service check for carbon monoxide?

A proper service follows the boiler manufacturer’s instructions and includes checks relevant to safe combustion and operation. A landlord gas-safety check has a different legal purpose and records prescribed safety checks at the time of inspection.

Neither replaces an audible CO alarm, and an alarm does not replace servicing. Ask the engineer what was checked and keep the service or safety record.

Homeowner responsibilities

Homeowners should arrange regular servicing and safety checks with an appropriately qualified Gas Safe registered engineer. Check the business and engineer on the official Gas Safe Register and look at the back of the engineer’s ID card to confirm they are qualified for the type of appliance and gas.

Keep permanent vents clear, never alter a flue or appliance casing, and act on recurring lockouts, soot, unusual staining or alarm signals. A boiler that appears to run normally can still need professional maintenance.

Landlord and tenant responsibilities

In England, relevant private and social landlords must provide a carbon-monoxide alarm in any room used as living accommodation that contains a fixed combustion appliance, excluding gas cookers. Landlords must repair or replace a reported faulty alarm as soon as reasonably practicable.

HSE also states that landlords must maintain gas appliances, fittings and flues they provide and arrange an annual gas-safety check by a Gas Safe registered engineer. Existing tenants must receive a copy of the record within 28 days, and new tenants before moving in.

Rules differ across England, Scotland, Wales and Northern Ireland, so landlords should follow the current requirements for the property’s nation. Tenants should report alarm faults, appliance concerns and symptoms immediately; they should not disable alarms or attempt boiler repairs.

Can a boiler leak carbon monoxide when it is switched off?

A switched-off boiler is not actively burning gas, but the source may be another appliance, fumes may remain in the property, or an appliance may not be fully isolated. Do not re-enter simply because someone has pressed the boiler’s off button. Follow emergency advice and have the property assessed.

Can a new boiler produce carbon monoxide?

Any fuel-burning appliance can be unsafe if it is incorrectly installed, damaged or operating with a flue or ventilation problem. Newness is not proof of safety. Correct commissioning, manufacturer instructions and Gas Safe registration all matter.

Will the boiler display a fault code?

It may lock out if its controls detect a fault, but a display cannot be relied upon as a carbon-monoxide detector. Fault codes are model-specific and often describe operating conditions rather than the presence of CO in the room.

Never wait for a Baxi, Worcester Bosch or Ideal code before responding to symptoms or an alarm.

What an engineer may inspect

A qualified engineer may check the appliance condition, combustion, ventilation, flue integrity, safety devices and installation against the manufacturer’s requirements. The precise work depends on the boiler and situation. It cannot be replaced by a visual homeowner check or a consumer CO meter reading.

If an appliance is classified unsafe, follow the engineer’s warning and do not reconnect or use it until the required work has been completed.

A simple household carbon-monoxide plan

• Fit suitable audible alarms and record their replacement dates.
• Test them as the manufacturer instructs.
• Make sure everyone recognises the alarm sound.
• Keep the gas emergency number available: 0800 111 999 in Great Britain.
• Know how to leave the home quickly without searching for the source.
• Book regular appliance servicing and required landlord checks.
• Never block ventilation or attempt internal gas-appliance work.

Frequently asked questions

Does opening a window solve a carbon-monoxide problem?

Ventilation may reduce the immediate concentration, but it does not make a faulty appliance safe. Leave the property and obtain professional and medical advice.

Can pets be affected?

Yes. Pets can be exposed too. Several occupants or animals becoming unwell together is an important warning sign. Get everyone into fresh air.

Should I reset the boiler after a CO alarm?

No. Do not reset and reuse a suspected appliance until the property and appliance have been assessed and declared safe.

Is an annual service enough?

Regular servicing reduces risk but cannot guarantee that a later fault, blockage or damage will not occur. Maintain working alarms and respond immediately to warnings.

Related boiler-help guides

• Boiler smells unusual: which smells need urgent action?
• Boiler service or gas-safety check: what is the difference?
• How often should a boiler be serviced?