What does "flue gas" indicate in a boiler system?

Flue gas refers to the gases that are produced as a byproduct of the combustion process in a boiler system. When fuel burns, it generates flue gas, which typically contains carbon dioxide, water vapor, oxygen, nitrogen, and other combustion products. Understanding flue gas is essential because it directly indicates the efficiency of the combustion process and the performance of the boiler. Monitoring flue gas composition and temperature can help identify issues like incomplete combustion or heat loss, which are crucial for maintaining optimal operation and safety in boiler systems.

In contrast, other options refer to different aspects of a boiler's operation. The gas used to ignite the burner relates to the initial fueling process and is not the product of combustion itself. Steam escaping from the system pertains to the operational output of a boiler rather than the byproducts of combustion. Finally, the air entering the boiler is necessary for combustion but does not represent the gases produced during that reaction.