What does thermal efficiency in a boiler measure?

Thermal efficiency in a boiler is a crucial measure that indicates how effectively the boiler converts the energy contained in the fuel into usable heat. This efficiency is typically expressed as a percentage, representing the ratio of useful heat output to the total energy input from the fuel. A higher thermal efficiency means that more of the fuel's potential energy is being utilized for heating purposes, leading to reduced fuel consumption and operational costs.

The other choices focus on different aspects of boiler functionality. Holding water under pressure pertains to the design and operational safety of the boiler, while the frequency of maintenance relates to the maintenance practices necessary to ensure proper operation and longevity. The total amount of steam generated is a measure of output capacity rather than efficiency in energy conversion. Therefore, option B accurately identifies thermal efficiency as assessing the conversion effectiveness of fuel energy into heat output.