Which type of furnace requires the largest size?

The choice of a coal furnace requiring the largest size is primarily due to the characteristics of coal as a fuel source. Coal has a lower energy density compared to gas and oil, meaning that a larger volume of coal is needed to produce the same amount of heat. This necessitates a more substantial physical structure for the furnace to accommodate the larger fuel supply and its combustion process.

Additionally, coal furnaces typically have to incorporate more extensive ash handling systems, along with larger combustion chambers to ensure complete combustion of the fuel. This adds to the overall size of the furnace. In practical applications, coal needs to be stored and handled in larger quantities, and the design of the furnace must reflect that, subsequently leading to a need for a larger footprint.

Gas, oil, and wood furnaces generally have more compact designs since their fuel types can be stored and burned more efficiently, often requiring less space for both the fuel supply and combustion processes. Therefore, the inherent requirements of a coal-fueled system dictate that a coal furnace is typically the largest among the options provided.