What is the primary purpose of an O2 sensor in a boiler stack?

• A. To measure boiler stack temperature.
• B. To ensure the boiler is running efficiently and there is a good fuel-to-oxygen ratio.
• C. To monitor flame color.
• D. To regulate water level.

Answer: (B)

An oxygen sensor in a boiler stack provides feedback on how much oxygen remains in the flue gases after combustion. This information lets the burner control adjust the air supply (and sometimes fuel) so the fuel-to-air ratio stays at the desired level. When the oxygen level is right, combustion is efficient—you use less fuel and reduce emissions. If the stack O2 is too high, there’s too much excess air; if it’s too low, there may be incomplete combustion. So the main purpose is to maintain the proper fuel-to-oxygen balance for efficient, clean combustion, not to measure temperature, monitor flame color, or regulate water level.