The passage of combustion gases along the length of the boiler is known as a:

The correct term for the passage of combustion gases along the length of the boiler is "combustion gas pass." This refers specifically to the designated pathway within a boiler through which the gases produced from burning fuel travel before being expelled into the atmosphere. This pathway is critical in the overall efficiency of the boiler system, as it allows for heat transfer to take place effectively, aiding in the generation of steam or hot water.

In a typical boiler design, the combustion gases absorb heat as they pass through various components like the tubes or heat exchangers, which is fundamental for the process that converts water into steam. The design and configuration of this pass can significantly impact the boiler's performance, emissions, and the energy utilized from the combustion of fuels.

Other terms presented in the options refer to more specific components or functions within the boiler system or are less commonly used definitions. "Stack pass" generally refers to the area where gases exit the boiler into the stack or chimney but does not cover the entire pathway through the boiler itself. "Combustion air pass" describes the intake process of air required for combustion, while "chimney pass" is not a standard term associated with the flow of combustion gases in boiler operation. The focus on the combustion gas pass highlights its