In the context of boiler operation, what does the term "draft" most commonly refer to?

In boiler operation, "draft" refers specifically to the movement of air and combustion gases within the boiler system, particularly in the firebox where combustion occurs. Draft is essential for maintaining the necessary flow of air needed for combustion, as well as for effectively removing combustion gases from the boiler and venting them outside.

Proper draft ensures that there is sufficient oxygen for the fuel to burn efficiently, which affects both the efficiency of the boiler and the emissions it produces. A controlled draft system helps in creating the right vacuum or pressure to facilitate this airflow, making it crucial for safe and optimal boiler performance. Therefore, option A is the accurate definition of "draft" in this context. The other choices pertain to different aspects of boiler operation that do not relate to the airflow or combustion process in the firebox.