What is used to transfer flue gases from the boiler stack to the indicator unit?

The correct answer involves utilizing a tube and hand bulb to transfer flue gases from the boiler stack to the indicator unit. This method is particularly effective because it creates a direct pathway for gas transportation without interference from external factors, allowing for accurate measurement and observation.

A tube allows the flue gases to flow through it efficiently, while the hand bulb serves as a mechanism to facilitate the movement of gases into the indicator unit, ensuring that the gases reach the unit in a defined and controlled manner. This setup is essential for precise readings or alarm systems that monitor flue gas emissions, contributing to the overall safety and efficiency of the boiler system.

In contrast, the other options do not serve the purpose of transferring flue gases in the same effective or practical way. A fan and duct system may be used for broader air movement but would not be as targeted as a tube and hand bulb setup for indicator measurement. A pressure gauge typically measures pressure rather than transferring gases. A flexible hose could allow for gas movement but may lack the necessary control and precision provided by a tube and hand bulb.