What is the primary difference between internally and externally fired boilers?

The primary difference between internally and externally fired boilers lies in the location of the combustion chambers. In an internally fired boiler, the combustion chamber is located within the boiler shell itself. This means that the heat generated by the burning fuel is transferred directly to the water contained in the boiler. Conversely, in an externally fired boiler, the combustion chamber is situated outside the boiler shell, and heat is transferred to the boiler water through walls or heat exchangers. This distinction affects both the design and operation of the boiler, including aspects such as efficiency and the overall space required for installation.

The other choices, while they may pertain to various boiler features, do not define the fundamental difference between internally and externally fired boilers. The size of the boiler can vary widely regardless of the internal or external design, efficiency ratings can be influenced by numerous factors beyond the combustion chamber placement, and both types can utilize different fuels depending on the application. Thus, the located combustion chamber is the key differentiator between these two types of boilers.