To calculate the combustion efficiency, what two factors must be known?

The correct answer involves knowing the net stack temperature and CO2 levels when calculating combustion efficiency. Combustion efficiency provides insight into how effectively a burner is converting fuel into energy, and a key component of this efficiency is understanding the relationship between combustion gases and temperature.

Net stack temperature refers to the temperature of flue gases as they exit the combustion process, which is crucial because it indicates how much heat energy is retained in the exhaust. Lower net stack temperatures generally suggest that more heat is being used for heating rather than being lost up the stack.

CO2, or carbon dioxide levels, serve as an indicator of complete combustion. Higher concentrations of CO2 in the exhaust suggest that more fuel is being completely burned, indicating higher efficiency. Monitoring CO2 allows technicians to make adjustments that optimize combustion processes, which in turn enhances efficiency.

With this understanding, it's clear that these two factors are essential for accurately assessing combustion efficiency. Other options do not directly relate to the key factors needed to gauge how well the combustion process is operating in terms of performance and efficiency.