Use flame appearance as one observation within the appliance combustion and safety checks.
Compare the flame with the appliance design
An aerated natural-gas hob burner normally shows stable, mainly blue flames with defined inner cones. A fuel-effect fire may deliberately have yellow flames. The appliance instructions establish the expected pattern; one flame-colour rule does not fit every burner.
Recognise a reason to investigate
On a burner intended to have blue flames, persistent lazy yellow flames, sooting, abnormal lifting or unstable operation warrant investigation. Burner assembly, gas supply, air supply and flue conditions can affect operation. Appearance points to a problem but does not identify its cause by itself.
Complete the combustion assessment
A blue flame does not prove complete combustion or a safe CO level. Carbon monoxide cannot be seen or smelt, and a dangerous condition need not produce an obvious visual warning. Carry out the checks required for the appliance, including suitable combustion measurements where specified and the applicable checks of gas input, air supply, flue performance and safe operation.
Flame supervision is a separate function. A flame that stays alight does not prove the gas will shut off after flame loss; test that function by the prescribed method. Neither a normal flame nor one satisfactory analyser value replaces the full assessment.
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