What is typically expressed in square meters (m²) of heat receiving surface area?

The capacity of a boiler is typically expressed in square meters (m²) of heat receiving surface area because this measurement indicates the amount of surface available for heat transfer between the combustion gases and the water or steam inside the boiler. A larger heat receiving surface area generally allows for more efficient heat exchange, leading to higher boiler capacity and improved thermal performance. By quantifying this area, engineers can assess how effectively a boiler can generate steam or hot water, thus determining its operational capabilities.

In contrast, the efficiency of a boiler pertains to how well it converts fuel energy into usable heat, which is expressed as a percentage rather than an area. Output temperature refers to the temperature of steam or water produced by the boiler and is not measured in square meters. Fuel consumption indicates the amount of fuel the boiler uses over time, typically expressed in mass or energy units, not in surface area. Therefore, the heat receiving surface area is directly linked to the capacity of the boiler, making it the correct choice.