Which of the following is NOT typically a type of boiler system?

The correct answer is C, air-tube boiler, because there is no recognized boiler system specifically named "air-tube boiler" in standard engineering practice. The most common types of boiler systems are water-tube and fire-tube boilers.

Water-tube boilers use water-filled tubes that carry water and steam away from the boiler itself, which allows for high pressure and large capacity applications. Fire-tube boilers, on the other hand, have hot combustion gases passing through tubes that are surrounded by water, making them efficient for smaller applications.

Combination boilers, which can provide both heating and hot water, also fall within the scope of recognized boiler types, as they combine features of various systems to meet multiple needs.

Thus, the absence of a well-defined "air-tube boiler" in conventional boiler technology classifies it as the option that does not belong in a list of typical boiler systems.