What a healthy gas flame looks like, which flame and soot signs point to incomplete combustion, and what to do when you see them.
What a healthy flame looks like
A correctly burning, properly aerated burner, such as a natural-gas hob burner, shows stable, mostly blue flames with clearly defined inner cones. Some appliances are designed differently: a fuel-effect fire may deliberately have yellow flames. The appliance instructions set the expected pattern.
Signs that point to incomplete combustion
On a burner designed to burn blue, investigate:
- persistent lazy yellow or orange flames;
- soot on the burner, heat exchanger or pans;
- flames lifting off the burner ports, or light-back, where the flame burns back towards the injector;
- flame contact with a cooler heat-exchanger surface or pan base: the flame is chilled, combustion can stop short, and carbon monoxide and soot can form.
Usual causes are too little combustion air, a blocked flue, or an appliance that is poorly maintained or wrongly adjusted. Appearance points to a problem but does not identify its cause.
What to do
- Treat lazy yellow flames with sooting as a possible sign of carbon monoxide production.
- Turn the appliance off and arrange a competent assessment before it is used again.
- If the analyser shows CO continuing to rise and the CO/CO₂ ratio over its accepted limit, stop the appliance. Find the cause of the unsafe combustion before it is used again.
What a flame cannot prove
A blue flame, including one at a different setting, does not prove complete combustion or a safe CO level: carbon monoxide cannot be seen or smelt. A CO alarm self-test checks the alarm, not the appliance. Flame supervision is a separate function: a flame that stays alight does not prove the gas will shut off after flame loss, so test it by the prescribed method.