What would cause the flame to be yellow in an oil burner?

A yellow flame in an oil burner typically indicates incomplete combustion, which is primarily caused by an insufficient air supply or incorrect nozzle size. For combustion to occur efficiently, there needs to be a proper balance of fuel and air; if there isn't enough air, the fuel cannot burn completely, leading to a yellow flame instead of the desired blue flame that signifies more efficient combustion.

When the nozzle size is incorrect, it can also affect the mixture of air and fuel. A nozzle that delivers too much fuel relative to the air supply can exacerbate this issue, causing the flame color to change as partial combustion occurs. This incomplete combustion can lead to various efficiency problems and potentially harmful emissions.

In this context, while excessive oil flow, a clogged fuel line, and low oil pressure can impact overall burner performance, they are less directly linked to the color of the flame. A yellow flame is specifically a sign of air supply issues or nozzle misconfiguration, making this option the correct identification of the cause for a yellow flame in an oil burner.