Carbon Monoxide: The Symptoms, the Signs on Your Appliances, and What to Do

What carbon monoxide is

Every gas appliance burns fuel with air. When it burns properly the waste is mostly water vapour and carbon dioxide, and it goes up the flue. When the burn is incomplete, because the appliance is dirty, the air supply is restricted or the flue is blocked, it produces carbon monoxide instead. Boilers, gas fires, cookers and water heaters can all do it, and so can solid fuel stoves, open fires, and petrol generators or barbecues used indoors.

The reason it is dangerous is the reason it is hard to notice. Carbon monoxide has no smell, no colour and no taste. It binds to the blood in place of oxygen, and at high enough levels it kills. At lower levels, over weeks, it makes people ill in ways that are easy to put down to something else.

Symptoms of carbon monoxide poisoning

The early symptoms look like flu without the temperature, or a hangover that will not go:

• Headache. The most common symptom, and often the first.
• Dizziness and feeling sick, sometimes with vomiting.
• Tiredness and confusion, or finding it hard to concentrate.
• Breathlessness and, at higher levels, chest pain.
• Collapse or loss of consciousness at high levels. This is an emergency.

Three patterns point to carbon monoxide rather than a bug. The symptoms come on or get worse when the heating or an appliance is running. They ease when you are out of the house and return when you come back. And more than one person in the household, including pets, is affected at the same time. If that sounds like your home, treat it as carbon monoxide until it is proved otherwise.

Signs an appliance may be producing carbon monoxide

Carbon monoxide itself is invisible, but an appliance burning badly usually leaves clues:

• A lazy yellow or orange flame where it should be crisp and blue. On a gas hob or fire you can see this directly.
• Black, sooty marks or yellow-brown staining on or around the appliance, or on the wall above a gas fire.
• A pilot light that keeps going out.
• More condensation than usual on the windows of the room the appliance is in.
• A smell of fumes. You cannot smell carbon monoxide, but the other products of a bad burn often do smell, and any smell of fumes from a gas appliance means stop using it.

None of these prove there is carbon monoxide in the room. A room-sealed boiler, which most modern ones are, takes its air from outside and sends its fumes outside, so it can burn badly without the fumes reaching you. But an appliance showing these signs is one to have checked, because the seals and the flue that keep the fumes out are the next things to fail.

What to do if you suspect carbon monoxide

1. Get everyone out into fresh air, and leave the doors and windows open behind you.
2. Turn the appliance off if you can do it on the way out. Do not go back in to do it.
3. Call the National Gas Emergency Service on 0800 111 999. It is free and answered 24 hours a day, and they will make the supply safe.
4. Get medical advice. Call 999 if anyone has collapsed, is confused or is struggling to breathe. Otherwise call NHS 111, and tell them you think it is carbon monoxide so they test for it.
5. Do not use the appliance again until a Gas Safe registered engineer has checked it. We can attend the same day. Details are on our gas leak and emergency page.

Carbon monoxide alarms

An audible carbon monoxide alarm is the only thing that will warn you before symptoms do. Buy one that carries the BS EN 50291 mark, which is the standard for home alarms, and fit one in every room that has a gas or solid fuel appliance. Follow the instructions in the box for position: as a rule, one to three metres from the appliance, at about head height, and not in a cupboard or behind furniture. Test it every month with the button, and replace it when it reaches the date printed on it, usually seven to ten years.

A good alarm costs less than £20. If you do not have one, we will position it for you free on any visit. Bring it to the door and we will tell you where it goes.

For landlords in England, the rules changed in October 2022: a carbon monoxide alarm is required in every room with a fixed combustion appliance, gas cookers excepted, and it must be repaired or replaced when a tenant reports it faulty. The detail is in our landlord gas safety guide.

How a service prevents it

Carbon monoxide comes from an appliance that is burning badly, and an appliance burns badly for reasons a service finds: a heat exchanger fouled with debris, a blocked or split flue, perished seals, a burner that needs cleaning. Every boiler service we do starts and ends with a flue gas analyser in the flue, which measures exactly how the boiler is burning. A high reading is the boiler telling us there is a problem before it becomes yours.

We have seen how far it can go. A Vokera we serviced this year read over 999 parts per million of carbon monoxide, the top of the analyser's range, because its heat exchanger was choked with debris. A full strip-down service brought it down to 112 ppm. You can read that job, with the before and after photos, in our Vokera strip-down case study.

The practical advice is short. Have every gas appliance serviced once a year by a Gas Safe engineer, and ask them for the combustion reading. Keep air vents and flue terminals clear. Never use a barbecue, patio heater or generator indoors, or in a garage with the door shut. And fit an alarm.

What we charge

A basic boiler service is £95, an intermediate service £125 and a full strip-down £185, all including a combustion check on the analyser. A gas fire service is £120. If you suspect a fault, an emergency gas trace and check is from £99 in office hours. A landlord gas safety certificate, which checks every appliance and its flue, is £90 for a boiler and cooker. Full details are on our boiler servicing and gas safety certificate pages.

Written by Saeed Iqbal, the engineer who leads Synergy Gas & Plumbing (Gas Safe registered 594385). Last reviewed September 2026. This guide is general safety advice; if anyone is unwell, medical advice comes first.