The primary function of the induced draft fan in a boiler system is to:

The primary function of the induced draft fan in a boiler system is to facilitate the exit of combustion gases. This fan creates a negative pressure, or vacuum, in the boiler combustion chamber, which effectively pulls the exhaust gases out of the system through the flue or stack. This process is essential for ensuring that combustion gases are efficiently removed, helping to maintain optimal combustion conditions within the boiler.

By promoting the proper flow of flue gases, the induced draft fan helps to ensure that there is sufficient air for combustion, which contributes to the efficient operation of the boiler. It also plays a pivotal role in preventing the accumulation of unburned fuel or combustion by-products in the combustion chamber, thereby enhancing safety and performance.

The other options, while related to boiler operation, do not accurately describe the essential role of the induced draft fan. For instance, it does not primarily focus on increasing heat transfer efficiency, nor is it designed to reduce flue gas temperature directly. Thus, the correct answer centers on the vital function of aiding in the removal of combustion gases from the system.