What component of the boiler system directly affects the combustion process by providing air?

The forced draft fan is a crucial component of the boiler system that plays a vital role in the combustion process. It provides the necessary airflow to maintain an optimal combustion environment within the boiler. By introducing a controlled amount of air into the combustion chamber, the forced draft fan ensures that the fuel burns efficiently and completely, which is essential for maintaining performance and minimizing emissions. The pressure differential created by the fan helps in overcoming resistance in the combustion system, allowing for effective mixing of air and fuel.

In contrast, other components like the return line are primarily involved in returning water from the heating system back to the boiler, the header functions to distribute steam or hot water to various parts of the system, and the waterleg is responsible for the collection of condensate and maintaining water levels but does not influence the combustion process directly. Therefore, the forced draft fan specifically impacts the combustion process through its role in air supply, making it the correct choice in this context.