Boiler efficiency is increased by:

Boiler efficiency relates to how effectively a boiler converts fuel into usable energy, and several components can significantly enhance this process. When analyzing the listed options, each plays a crucial role in improving boiler efficiency in different ways.

Open feedwater heaters increase the water temperature before it enters the boiler, thus reducing the energy required to convert water into steam. This decrease in energy demand translates to improved efficiency since less fuel is used during operation. Air preheaters serve a similar purpose by recovering waste heat from flue gases and using it to preheat the combustion air entering the boiler, which enhances combustion efficiency and minimizes fuel consumption. Closed feedwater heaters function by using steam from the boiler to heat the feedwater before it enters the boiler, further reducing the energy required for steam generation.

Each of these components contributes uniquely to the overall efficiency of the boiler system. Therefore, the correct answer encompasses all of these options as they collectively enhance the efficiency of the boiler, making "all of the above" the right choice. Understanding how these various systems work together takes into account their individual benefits to the boiler operation and highlights the interconnected nature of energy efficiency technologies.