What type of boiler features include external combustion chambers?

Externally-fired boilers are specifically designed with combustion chambers located outside the boiler shell, which allows for better heat transfer and efficiency. This design helps to separate the combustion process from the water or steam that is being heated, leading to improved safety and control over combustion conditions. By locating the combustion chamber externally, these boilers can utilize various fuel types more effectively and be adapted to larger or more complex systems where traditional designs may not perform as efficiently.

In contrast, boilers such as Scotch Marine, forced draft, and packaged boilers typically have integral combustion chambers, where the combustion reaction occurs within the confines of the boiler itself. This fundamental difference in design is the key characteristic that defines externally-fired boilers, setting them apart from other types in the choices provided.