What does a boiler's efficiency rating indicate?

A boiler's efficiency rating provides critical information about how effectively it converts fuel energy into usable heat. This rating is calculated as a percentage of how much of the energy contained in the fuel is transformed into heat that can be used for heating water or space. A higher efficiency rating indicates that more of the energy input is being utilized effectively, which not only contributes to better performance but can also lead to lower operational costs and reduced environmental impact due to less fuel consumption.

The other options focus on different aspects of a boiler's operation. While the speed of heating water, size considerations, and maintenance frequency are all important in evaluating boiler systems, they do not directly relate to how efficiently the boiler operates in terms of energy conversion. Therefore, the efficiency rating distinctly stands out as it deals specifically with the relationship between fuel input and usable heat output.