What does white smoke from the flue of an oil burner indicate?

White smoke from the flue of an oil burner typically indicates a lean air-oil mixture. In this scenario, the air supply to the burner is inadequate relative to the amount of oil being burned, leading to incomplete combustion. As a result, unburned oil droplets are carried away in the exhaust, creating the appearance of white smoke.

A lean air-fuel mixture means that while there is enough oil for combustion, there isn’t enough air to fully utilize that oil efficiently. This condition not only produces white smoke but can also lead to longer-term issues, such as soot buildup or reduced efficiency of the burner.

In contrast, other conditions like excess oil, a rich fuel mixture, or a clogged burner would produce different symptoms in the exhaust. Excess oil would usually lead to poor combustion, resulting in black smoke or soot, while a rich fuel mixture often also leads to black smoke. A clogged burner might cause a variety of issues including backpressure and could result in smoke or soot, but would not typically produce just white smoke alone.