What is the purpose of a sight glass on an oil tank?

The sight glass on an oil tank serves the important function of allowing technicians and users to visually check the level of oil in the tank. This clear view enables quick assessments of oil levels without needing to open the tank or use any other measuring tools. Monitoring the oil level is crucial for maintaining the operational efficiency of the heating system and ensuring that oil is available for consistent heating. Properly observing the oil level helps in preventing running the system on a low tank, which could lead to air entering the lines and potentially causing the burner to fail.

In contrast, the other options do not align with the specific function of a sight glass on an oil tank. Controlling oil temperature, monitoring flue gas output, and indicating air filter condition pertain to different mechanical aspects of an oil heating system that are managed through separate components or systems.