What problem may occur if an oil burner is left in the furnace when not in use?

When an oil burner is left in the furnace when not in use, one significant problem that can occur is the accumulation of carbon. This happens because, during combustion, incomplete burning of the oil may produce carbon soot. If the burner remains in the furnace for extended periods without operation, this carbon can build up on the burner components and inside the combustion chamber.

The presence of carbon accumulation can lead to various operational issues, including reduced efficiency of the burner, increased maintenance requirements, and potential safety hazards. It can obstruct airflow and heat transfer, reducing the overall effectiveness of the heating system. Furthermore, the buildup can create hotspots that increase the risk of overheating certain components, potentially leading to mechanical failure or hazardous situations.

In contrast, increased efficiency and overheating are generally not concerns when an oil burner is simply left unused; rather, they are results of improper operation or maintenance. Oxidation of the oil could occur over time, but this is not directly related to the burner being left in the furnace. Thus, the phenomenon of carbon accumulation is a direct consequence of leaving the oil burner inactive in the furnace, making it the most pertinent answer to the question.