Which of the following is NOT considered a heat loss in a boiler?

The reason why energy lost through fan operation is not considered a heat loss in a boiler lies in the function of the fan itself. Fans in boiler systems are utilized primarily for air supply and to facilitate proper combustion. While they do consume energy to operate, this is not classified as a heat loss from the boiler's thermal efficiency perspective.

Heat loss typically refers to the energy that escapes from the system during the combustion and heating processes, leading to a decrease in efficiency. The other options, such as moisture from burning hydrogen, unburned carbon in ash, and heat carried away in dry flue gases, all represent actual thermodynamic losses that can decrease the effective heat generated by the boiler.

In summary, while the operation of fans does require energy, it does not directly impact the heat output and therefore is not categorized with the thermal losses that affect the overall efficiency of the boiler system.