Yellow Tip vs Blue Flame: What Flame Colour Means for Combustion and CO

Why a Correct Flame Is Blue

Gas needs oxygen to burn fully. A Bunsen-type burner entrains primary air at the venturi, and the flame gets additional secondary air around it. With the correct ratio, the hydrocarbon burns in a well-defined, mostly blue inner cone with no yellow body. The blue colour comes from excited molecular radicals during complete combustion, not from the gas itself. Once the air supply falls short, the gas cannot oxidise fully; glowing hot carbon particles form and emit the yellow light, which is the same effect as a candle flame. That visible yellow is already evidence of incomplete combustion and of the CO that comes with it.

What the Flame Appearance Tells You

Root Causes of a Yellow Flame on the Line

A frequent production error is a mixed orifice batch — LPG and natural-gas orifices look almost identical but pass different flow rates, so a single swapped spud produces a yellow, sooting flame on an otherwise correct cooker. Positive control of the orifice part and a combustion check at the line catch this before it ships.

What a Yellow Flame Costs

• Carbon monoxide: incomplete combustion raises CO in the flue gas, the poisoning hazard covered in F5: CO risk;
• Soot: carbon deposits on pots, burners and heat-exchange surfaces, worsening over time;
• Efficiency: unburned gas carries energy away, lowering thermal efficiency measured in A6;
• Safety interlock: a yellow or lifting flame can move off the FFD sensor, causing nuisance shut-downs or unreliable detection.

Testing Combustion in Production

1. Inspect each burner for a steady blue flame with no yellow tips, lifting or flashback at high and low rate;
2. Sample flue gas and measure CO against the standard’s limit, using the correct test gas and pressure (EN 437);
3. Adjust the primary-air shutter to the point of a stable blue flame, then re-confirm CO;
4. Record the visual result, CO value and setting against the serial number.

Visual inspection is a first screen, not a measurement: a flame can look blue and still produce CO, and a subtle yellow tip is easy to miss under factory lighting. A gas cooker test bench supplies the correct gas and pressure, ignites and stabilises the flame, supports a flue-gas CO measurement, and fails the unit on either visual or CO criteria, interlocked with the tightness and FFD checks.

High Rate, Low Rate and Gas Conversion

Combustion should be checked at more than one operating point, because the air-to-gas ratio is not constant across the range. A burner that burns cleanly at full rate can show yellow tips at the minimum simmer rate, where the gas flow is small and the injector pulls less primary air; a fault that only appears at low rate is the one most often missed when the line checks only the full flame. Dual- or multi-ring burners and inner/outer rings should each be checked separately, because a single mis-set ring can soot the whole appliance.

Gas conversion deserves the same care. When an appliance is changed from one gas family to another, it is not enough to change the cylinder or supply pressure: the injectors, and sometimes the pressure setting and air adjustment, must be changed together, and the flame and CO must be re-verified after conversion. A cooker that was correct at the factory can be converted incorrectly in the field, producing a yellow flame; clear conversion instructions and a marked, matched injector kit are part of preventing that. Where an appliance is certified for several gas categories, the line must confirm the unit is assembled for the category it will actually be sold under, rather than assuming one setting covers all of them. Keeping the assembly, injector part and gas category together on the same serial record removes the single most common cause of a yellow flame reaching an end user, and it lets the factory prove, appliance by appliance, which category and setting were verified. That record is the first thing an importer or a market-surveillance inspector will ask to see when a flame or CO complaint is raised.

Common Mistakes

• Treating a yellow flame as cosmetic rather than a combustion and CO fault;
• Swapping LPG and NG orifices because they look the same;
• Adjusting air by eye without measuring flue-gas CO;
• Testing only at full rate and missing a yellow low-rate or simmer flame;
• Ignoring blocked ports, caps or venturi before changing parts;
• Not confirming the gas category and pressure the appliance was actually built for.