What defines an externally fired boiler?

An externally fired boiler is characterized by having its grate and combustion chamber located outside of the boiler shell itself. This design allows for better heat transfer and can enhance the efficiency of the boiler since the combustion gases do not have to pass through the boiler shell directly, which minimizes heat loss.

The other options describe different aspects of boiler design or features. The mention of higher efficiency ratings may apply to certain boiler designs, but it's not a defining characteristic of externally fired boilers. Maintenance is always required for any boiler, as regular upkeep is essential for safe and efficient operation. The option that states all parts are enclosed in one shell describes an internally fired boiler, which is a completely different design compared to the externally fired boiler, confirming that it does not align with the defining features of this type.