G2 Gas Fitter Exam 2025 – 400 Free Practice Questions to Pass the Exam

In watertube boilers, the flow of flue gases occurs outside the tubes. This design is important because the water flows within the tubes, and heat from the flue gases transfers to the water through the tube walls, facilitating efficient heating.

By having the flue gases flow around the tubes, the boiler maximizes heat exchange between the gases and the water. The presence of the tubes in which the water circulates allows for a wide surface area for heat transfer, further improving the boiler's efficiency. This configuration helps maintain a safer design as well; if there’s any kind of leak or failure in the tubes, it mainly affects the water side rather than directly exposing flue gases to the water, which could present a hazard.

The other options describe scenarios that do not accurately reflect the design and function of watertube boilers: the flue gases do not flow inside the tubes, through the boiler shell, or in any water tank as these options misrepresent the internal layout and purpose of the boiler structure.