Carbon monoxide has no colour, smell or taste, so without an alarm you would never know it was there. The good news is that CO poisoning is largely preventable. This guide covers where CO comes from, the symptoms to watch for, the warning signs on appliances, how to fit and look after alarms and exactly what to do in an emergency. Our Gas Safe registered engineers test appliances for safe combustion every week across Surrey, Sussex and Kent.
“A carbon monoxide alarm is your safety net, but an annual service is what stops the problem in the first place. We would always rather find a fault on a routine visit than in an emergency.”
1. Know what carbon monoxide is — the silent risk
Carbon monoxide, or CO, is a poisonous gas produced when fuels such as gas, oil, coal and wood do not burn completely. You cannot see it, smell it or taste it. That is why it is often called the silent killer.
When you breathe it in, CO stops your blood carrying oxygen properly. Low levels over time can leave you feeling persistently unwell. High levels can cause collapse and can be fatal within a short time.
Any appliance that burns fuel can produce CO if something goes wrong. In the home, that includes:
- Gas, oil and LPG boilers
- Gas fires and gas cookers
- Wood burners, open fires and multi-fuel stoves
- Oil-fired range cookers
- Portable generators and barbecues, which should never be used indoors or in a garage
Problems usually start when an appliance is badly fitted, poorly maintained or has a blocked or damaged flue or chimney. Poor ventilation makes things worse.
2. Spot the symptoms — and the clues that point to CO
CO symptoms are easy to mistake for flu, food poisoning or simple tiredness. According to the NHS guidance on carbon monoxide poisoning, the main signs are:
- Headaches. Often the first and most common symptom.
- Dizziness. Feeling light-headed or unsteady.
- Nausea. Feeling or being sick.
- Breathlessness. Shortness of breath or difficulty breathing.
- Collapse. At higher levels, collapse and loss of consciousness.
Certain patterns should make you suspect CO rather than illness. Symptoms that ease when you leave the house and return when you come back are a classic warning. So is several people, or pets, feeling unwell at the same time, or symptoms that get worse when the heating or a particular appliance is running.
Flu usually brings a high temperature. CO poisoning does not. If you feel ill without a fever, and the pattern fits, treat it seriously.
3. Check your appliances — the visible warning signs
Appliances often show clues before an alarm sounds. Keep an eye out for:
- Lazy flames. Yellow or orange flames on a gas appliance that normally burns crisp blue. Some decorative gas fires are designed to burn yellow.
- Soot. Black or brown staining on or around the appliance.
- Pilot problems. A pilot light that keeps going out.
- Condensation. More than usual on the windows of the room.
- Lockouts. A boiler that keeps locking out or showing fault codes.
If you notice any of these, stop using the appliance and book an inspection. Do not remove panels or casings to look for yourself. Our Gas Safe boiler repair engineers can find the cause and make the appliance safe.
4. Fit the right alarms — in the right places
An audible CO alarm is your best early warning. Choose one marked as meeting BS EN 50291 and carrying a recognised safety mark. Many modern alarms have sealed batteries that last the life of the unit.
Fit alarms in every room with a fuel-burning appliance, such as the room with your boiler, gas fire or wood burner. Add them in bedrooms or close to where people sleep, so the alarm will wake them, and in living areas where you spend a lot of time.
Follow the manufacturer's instructions on height and distance from the appliance, as these vary. Avoid enclosed cupboards, spots directly above sinks or cookers, places next to extractor fans or open windows and damp rooms such as bathrooms.
If you have a wood burner or open fire, make sure the alarm in that room is not hidden behind furniture or curtains. In a home with a loft conversion or an annexe, think about each sleeping area separately, as an alarm two floors away may not wake anyone.
Smoke alarms do not detect carbon monoxide. You need a dedicated CO alarm, or a combined unit designed to detect both. Press the test button once a month, check the expiry date, and never remove batteries to silence an alarm.
5. Act fast — what to do if you suspect carbon monoxide
If your CO alarm sounds or you suspect carbon monoxide, act straight away. Do not wait to see if symptoms get worse.
- Turn off the appliance. Switch off fuel-burning appliances if you can do so quickly and safely.
- Get into fresh air. Open doors and windows on your way out, and take everyone, including pets, outside.
- Call the emergency line. For gas appliances, call the National Gas Emergency number on 0800 111 999. It is free and open 24/7.
- Get medical advice. Call NHS 111 and say you suspect CO. If anyone is seriously unwell or has collapsed, call 999.
- Keep it off. Do not use the appliance again until a qualified engineer has checked it and declared it safe.
Never try to repair a gas appliance or open a boiler casing yourself. Any work on gas appliances must be carried out by a Gas Safe registered engineer. If you smell gas, leave the property and call 0800 111 999 straight away.
National Gas lists its official gas emergency contact numbers online, and it is a good idea to save the number in your phone now.
6. Call the right engineer — and check their credentials
Any appliance that may be producing CO needs inspecting by the right specialist. That means a Gas Safe registered engineer for gas and LPG, an OFTEC registered technician for oil, or a HETAS registered installer for solid fuel stoves and fires.
Always check a gas engineer is genuine. Ask to see their Gas Safe ID card and check the 7-digit licence number using the official Gas Safe Register engineer search, or by calling 0800 408 5500. The card also shows which types of gas work they are qualified for.
Our certified heating engineers carry out flue gas analysis to measure exactly how an appliance is burning, then put right whatever is causing the problem.
7. Prevent problems — annual servicing is the best protection
The most effective way to protect your family is to have every fuel-burning appliance serviced once a year. During a service, the engineer checks combustion with a flue gas analyser, inspects the flue and confirms that fumes are being safely removed. Our guide to what happens during a boiler service explains each step.
Between services, a few simple habits help:
- Keep vents clear. Never block air bricks, vents or the gap under a door that an appliance relies on.
- Protect the flue. Keep the flue terminal outside free of plants, sheds and stored items.
- Sweep chimneys. Have chimneys for open fires and stoves swept regularly.
Book your annual check with our boiler servicing and maintenance team. The Gas Safe Register's carbon monoxide safety advice is also worth sharing with older relatives.
“Book your boiler service for late summer or early autumn. If we find a fault, it is fixed before you rely on the heating every day, and before the winter rush.”
8. Know the rules for rented homes — landlords and tenants
Landlords must have every gas appliance and flue checked by a Gas Safe registered engineer every 12 months. Tenants must receive a copy of the record before moving in, or within 28 days of the check, and landlords must keep records for 2 years. Our guide to landlord gas safety certificates covers the details.
Rules on CO alarms in rented homes have also been updated in recent years and differ between England, Wales and Scotland, so landlords should check the current requirements for each property.
The HSE's gas safety guidance for landlords sets out the legal duties in full. If you are a tenant and have not seen a current gas safety record, ask your landlord or letting agent for a copy. Report any appliance warning signs straight away rather than waiting for the next annual check.
We carry out landlord gas safety checks and servicing right across the towns and villages we cover, with clear paperwork issued after every visit.
Frequently asked questions
Can I smell carbon monoxide?
No. Carbon monoxide has no smell, colour or taste, which is why an audible CO alarm is so important. The smell many people associate with gas is added to the supply to warn of a leak and is a separate danger. If you smell gas, leave the property and call the National Gas Emergency number on 0800 111 999.
What should I do if my CO alarm goes off?
Turn off any fuel-burning appliances if it is safe to do so, open doors and windows, and get everyone into fresh air. For gas appliances, call 0800 111 999. If anyone has symptoms such as headaches or dizziness, call NHS 111, or 999 if they are seriously unwell. Do not use the appliance until an engineer has checked it.
Do heat pumps produce carbon monoxide?
No. Heat pumps do not burn fuel, so they do not produce carbon monoxide. Switching your heating to a heat pump removes that risk from the heating system itself. You still need CO alarms in any rooms with other fuel-burning appliances, such as a gas cooker, gas fire, wood burner or oil-fired range cooker.
How often should I replace my CO alarm?
Check the expiry date printed on the alarm and replace it when it reaches the end of its life, following the manufacturer's guidance. Many modern alarms have sealed batteries that last for the life of the unit, often several years. Test it monthly with the button, and replace it straight away if it chirps or shows an end-of-life warning.
Written by the NW Heating Services team
NW Heating Services installs, services and repairs boilers, heat pumps and underfloor heating for homes and landlords across Surrey, Sussex and Kent, with more than ten years of local experience. Our guides are written from the jobs our engineers carry out every week. More about our team.