Going by yhe way most gas burners behave I would have to go for the blue cone effect!. I always thought (rightly or wrongly ) that the orange flame was too much fuel or shortage of air.

The combustion is too rich if there is a yellow flame. It is less efficiencient with partially burnt products being produced which, in large quantities, will do you harm. In practise, the radiation from the yellow, carbon rich, flame could overheat the burner.

The ideal is a translucient blue flame.

I note that Mike mentions an orange glow on the ceramic. I guess that this is with a blue flame. If so, does it only occur at low gas settings? I don’t see it as a direct combustion event but the overheating could damage the burner.

In the best engineering tradition the question has not been answered.

JA

It depends on what thermal transfer the boiler is designed for. Most will be for conduction from hot gases. A glowing red element is primarily a radiative heat source. Most of the radiation will be absorbed by the fire box and some of that conducted outside and lost.
So a plain flame is probably best.
You want hot gas going through the boiler tubes, not radiation in the firebox.

Robert.

Taking Robert’s point, it rather depends on the design of firebox. If it is water jacketed as in steam locomotives, a radiant flame is considered very desirable as the heat transfer per unit area is much higher than for convection. Can’t put any numbers to it I’m afraid.