Most of us never think about the colour of a gas flame. You turn the hob on, it lights, dinner gets cooked. But the colour of that flame is one of the few things about your gas supply you can actually see, and it tells you a fair bit about whether the appliance is burning properly.
Here is what to look for, what is normal, and when it is worth picking up the phone.
Why a healthy gas flame is blue
Gas needs the right amount of air to burn completely. When it gets that air, the gas burns hot and clean, and the flame sits crisp and blue with a sharp inner cone. You will often see a small yellow tip at the very top, and on most appliances that is nothing to worry about.
That blue colour is basically a sign of efficiency. The fuel is being fully converted into heat, carbon dioxide and water vapour, and very little is going to waste. A well set up hob burner, a gas fire pilot light, or a boiler burner should all look steady, blue and controlled.
What a yellow or orange flame means
A lazy, floppy, yellow or orange flame usually means the opposite. The gas is not getting enough air, so it is not burning completely. That is called incomplete combustion, and the by-product is carbon monoxide.
Carbon monoxide has no smell, no colour and no taste. You cannot detect it without an alarm. That is exactly why a flame changing colour matters so much, because it is one of the few visible warnings you get.
Common causes include:
- Blocked or dirty burner ports, often from food spills, grease or dust
- Blocked ventilation, so the appliance cannot draw enough fresh air
- A partly blocked or damaged flue, so combustion gases are not clearing properly
- An appliance set up for the wrong gas, for example a natural gas appliance running on LPG or the other way round
- A worn or dirty injector, or an air shutter that has drifted out of adjustment
- General wear and tear on an appliance that has not been serviced in years
When yellow is completely normal
Not every yellow flame is a problem, and it is worth knowing the difference before you panic.
Decorative and living flame gas fires are designed to produce yellow flames. That is the whole point of them. They mimic a coal or log fire, and the appliance is built and flued to handle it. A yellow flame on one of these is expected.
A quick orange flicker on the hob is often just dust, a splash of water, or salt burning off. Sodium in salt gives off an orange colour when it burns. If it happens for a second and then settles back to blue, that is fine.
A brief yellow flare on ignition as the burner settles is also common.
The thing to watch for is a flame that is persistently yellow when it should be blue, especially on a hob, a boiler, a water heater or a pilot light.
Other warning signs to look for alongside the flame
Flame colour rarely shows up on its own. If an appliance is burning badly, you will often notice:
- Black, sooty or brown scorch marks on or around the appliance
- Yellow or orange flames instead of crisp blue
- A pilot light that keeps blowing out for no obvious reason
- Excessive condensation on the windows in the room where the appliance sits
- A lazy flame that seems to float rather than sit firmly on the burner
Any of these on their own is worth a look. Two or more together and you should stop using the appliance.
Carbon monoxide symptoms worth knowing
Carbon monoxide poisoning is easy to mistake for a cold, a hangover or general tiredness. Symptoms include headaches, dizziness, feeling sick, breathlessness, tiredness and stomach pain.
The pattern is the giveaway. If the symptoms ease when you leave the house and come back when you return, or if more than one person in the home feels the same way at the same time, treat it seriously. Pets are often affected first.
What to do if your flame has turned yellow
- Stop using the appliance. Turn it off at the appliance and do not use it again until it has been checked.
- Open windows and doors to ventilate the room.
- If anyone feels unwell, get everyone out into fresh air and seek medical help. Tell the doctor you suspect carbon monoxide.
- If you suspect an immediate danger, call the National Gas Emergency Service on 0800 111 999. It is free and staffed 24 hours a day.
- Book a Gas Safe registered engineer to inspect the appliance, the ventilation, and the flue before you use it again.
What you should not do is take the appliance apart, adjust the air shutter yourself or try to clean the injector. Working on gas appliances without being Gas Safe registered is illegal in the UK, and getting the gas and air mixture wrong is exactly how carbon monoxide problems start in the first place.
Fit an alarm, do not rely on your eyes
Flame colour is a useful clue, but it is not a safety system. Plenty of appliances hide the burner behind a sealed casing, so on a modern boiler you will never see the flame at all.
Fit an audible carbon monoxide alarm that meets BS EN 50291 in every room with a gas appliance. Check it regularly and replace it at the end of its stated lifespan. The black spot patches you sometimes see are not a substitute for a proper alarm.
Get it checked properly
The most reliable way to keep flames blue is an annual service by a registered engineer. During a service the burner gets cleaned, combustion is tested with an analyser, the flue is checked and the ventilation is confirmed. It catches the slow drift into incomplete combustion long before you would spot it yourself.
At The Gas Repair Company, our Gas Safe registered engineers cover homes and businesses right across Scotland, with 24/7 cover for urgent problems.
If you have noticed a yellow flame, sooting or any of the warning signs above, book a gas safety inspection or get in touch for a free quote. If you think you have an immediate problem, our emergency gas services are available around the clock.
Frequently Asked Questions
Is a yellow flame on my gas hob dangerous? It can be. A persistently yellow flame on a hob usually means incomplete combustion, which produces carbon monoxide. Stop using the hob, ventilate the room and have it checked by a Gas Safe registered engineer. A brief orange flicker from a spill or dust is not the same thing.
Why is my gas fire yellow? If it is a decorative or living flame fire, yellow flames are by design. If it is a radiant or convector heater that used to burn blue and has turned yellow, that is a fault and needs checking.
Can a yellow flame produce carbon monoxide? Yes. Incomplete combustion is the main way carbon monoxide is produced by gas appliances, and a yellow flame is one of its visible signs.
Can I fix a yellow flame myself? No. Adjusting burners, air shutters or injectors is gas work, and only a Gas Safe registered engineer can legally carry it out.
How often should gas appliances be serviced? Once a year for most domestic appliances, and in line with the manufacturer’s guidance. Landlords have additional legal duties, which we cover in our post on what a Gas Safe certificate actually covers.