How a healthy natural-gas flame looks and the visual warning signs of incomplete combustion.
A large share of CCN1 marks come from recognising what a healthy flame and a problem flame look like, so it is worth training the eye.
Complete combustion of natural gas gives a mainly blue flame that sits steadily on the burner with a sharp inner cone. It tells you the burner is getting enough air and the appliance is working as designed.
Incomplete combustion looks different: the flame turns yellow, lazy or it lifts and flickers, and you may see soot, a sooty smell, or yellow or brown staining around or above the appliance case. These are the classic visual clues that the air supply, burner or flue needs investigating.
The reason this matters is carbon monoxide. Incomplete combustion can release carbon monoxide, a gas you cannot see, taste or smell, so an appliance can be dangerous long before anyone notices a problem by instinct.
Treat the flame picture as a first indicator that points you towards a fuller check. Confirm combustion quality with a flue gas analyser and follow the complete procedure rather than judging safety on appearance alone.