What does an O2 sensor in a boiler stack do?

• A. Measures stack flow rate
• B. Detects emissions levels
• C. Maintains boiler pressure
• D. Ensures the boiler is running efficiently, and ensures there is a proper fuel to oxygen ratio

Answer: (D)

A stack O2 sensor uses the oxygen content in the flue gas to judge how cleanly and efficiently the fuel is burning. By sensing how much oxygen remains after combustion, the boiler control can adjust air and fuel input to reach the right amount of excess air. If too much oxygen is detected, it signals too much air for the fuel, wasting energy and cooling the flame; if oxygen is low, the mix may be fuel-rich, risking incomplete combustion and higher pollutants. The goal is to keep the flame burning efficiently with the proper fuel-to-oxygen ratio, which improves efficiency and helps control emissions. This sensor isn’t measuring how fast the exhaust flows, nor is it a direct emissions meter, and it doesn’t set boiler pressure.