Carbon monoxide at home: signs, alarms and what to do

Why carbon monoxide is a heating-season risk

Carbon monoxide (CO) is produced when gas, oil, coal or wood burns without enough air. A boiler, gas fire, cooker or wood burner that is working properly vents its fumes safely through a flue. When the appliance is faulty, the flue is blocked or the room is short of ventilation, CO can build up indoors.

Autumn and winter are when this matters most. Boilers and fires that have been idle or barely used over summer are switched back on, windows stay shut, and flues and chimneys can be partly blocked by nests, leaves or debris. Anyone using a portable heater, a barbecue or a generator indoors or in a garage during a power cut adds to the risk. None of these should ever be used inside.

Symptoms of carbon monoxide exposure

The NHS lists the symptoms of carbon monoxide poisoning as a headache, dizziness, feeling or being sick, feeling weak, tiredness and confusion, chest and muscle pain, shortness of breath, a hot red face and, in serious cases, loss of vision. They are easily mistaken for flu, food poisoning or a hangover, which is why CO is often missed.

The pattern is the clue. The NHS notes that symptoms can come and go, getting worse in an affected room or building and better when you go outside. Be suspicious too if several people in the household feel unwell at the same time, if pets seem ill, or if symptoms are worse when the heating is on.

This page is general safety information, not medical advice. If you think someone has been exposed to carbon monoxide, get them into fresh air and seek medical help: NHS 111 for advice, or 999 if they are finding it hard to breathe, have suddenly become confused, feel weak, have chest or muscle pain or have lost consciousness.

Warning signs on boilers, fires and cookers

You cannot smell carbon monoxide, but faulty appliances often give visible signs. Treat any of these as a reason to stop using the appliance and have it checked by a Gas Safe registered engineer (or a competent engineer for oil and solid fuel).

• A lazy yellow or orange flame on a gas hob, fire or boiler where you would expect a crisp blue one
• Soot, black marks or brown staining on or around the appliance or its flue
• A pilot light that keeps going out
• Much more condensation than usual on windows in the room with the appliance
• A boiler flue that is damaged, loose, boxed in with no inspection hatch, or blocked outside
• Smoke building up in a room when a fire or wood burner is lit

What to do if your CO alarm goes off

Treat a sounding carbon monoxide alarm as real until proved otherwise. Do not assume it is a fault because nobody feels ill.

• Open doors and windows to ventilate the property.
• Turn off gas appliances if you can do so safely, and turn off the gas at the meter if you know where it is.
• Get everyone, including pets, out into fresh air. Do not go back in to find the source.
• Call the National Gas Emergency Service on 0800 111 999. The line is free and open 24 hours.
• Get medical advice for anyone with symptoms, and mention possible carbon monoxide exposure.
• Do not use the appliance again until a registered engineer has checked it and confirmed it is safe.

Carbon monoxide alarms: choosing and placing them

Buy an audible carbon monoxide alarm marked to BS EN 50291. Smoke alarms do not detect CO, and the old colour-change spot detectors give no audible warning, so they are no substitute. You need an alarm in any room with a fuel-burning appliance, and it is sensible to have one in bedrooms and in rooms next to a boiler cupboard.

Placement matters. Follow the instructions supplied with the alarm, which normally give a distance from the appliance and a height on the wall or ceiling. Avoid fitting it right next to a window, a cooker hood or an extractor fan, where air movement can delay it. Test it monthly with the button, and replace it when the end-of-life warning sounds or by the date printed on it.

Landlords in England have specific duties. Since 1 October 2022, a CO alarm must be fitted in any room of a rented home that has a fixed combustion appliance (other than a gas cooker), and landlords must repair or replace alarms once told they are faulty. Landlords also need a gas safety check on every gas appliance and flue each year.

Preventing carbon monoxide in the first place

An alarm tells you something has gone wrong. The prevention is regular servicing. Have your boiler and any other gas appliances serviced every year by a Gas Safe registered engineer, ideally before the cold weather, and have chimneys swept before you light an open fire or wood burner for the season.

Keep air vents and air bricks clear, never block a room vent to stop draughts, and keep the area around the boiler flue outside free of plants, bins and anything stacked against it. If you are having building work done, check the boiler flue is not being boxed in or enclosed by a new extension or loft conversion without proper access. Our guides on boiler servicing versus gas safety checks and on planning a boiler flue explain what an engineer looks at.

Getting your appliances checked

Once the emergency service has made the situation safe, the appliance still needs a proper repair before it is used again. Loyal Plumbers' gas work is carried out by Gas Safe registered engineers, across London and everywhere inside the M25, with 10 vans working 24 hours a day. We service and repair boilers, carry out landlord gas safety checks and replace boilers that cannot be made safe.

When you contact us, tell us which appliance was involved, what the emergency engineer said or left in writing (for example a warning notice), and whether the appliance has been capped off. That lets us plan the right visit.