What is a key indicator that an oil burner is not operating properly?

A strong smell of fuel oil is a key indicator that an oil burner is not operating properly because it suggests that there is an issue with the combustion process. When an oil burner is functioning correctly, it should efficiently combust the fuel, resulting in minimal unburned oil. The presence of a noticeable fuel smell may indicate incomplete combustion, which can be caused by factors such as improper air-fuel mixture, poor burner adjustment, or blockages in the system.

This strong odor signifies that fuel may be leaking or not being burned entirely, leading to both efficiency losses and potential safety hazards, such as fire risks or health concerns due to inhalation of petroleum vapors. Addressing this issue promptly is essential for ensuring the safe and efficient operation of the heating system. Other options could indicate different scenarios but do not directly highlight the crucial failure of the combustion process as clearly as the fuel oil odor does.