The "bonnet capacity" of a furnace refers to which of the following?

The term "bonnet capacity" specifically refers to the output BTUs of a furnace. This measurement indicates the amount of heat that the furnace can deliver to the living space after accounting for any heat lost during the combustion process and other inefficiencies. Understanding the output capacity is crucial for determining whether a furnace can adequately heat a specific area, as it directly impacts the effectiveness of a heating system in maintaining comfort levels within a home.

In contrast, the other options refer to different aspects of a furnace. The category rating pertains to the classification of the furnace based on its design and features, while the venting category deals with how the furnace exhausts gases safely to the outside. Input BTUs represent the total amount of energy consumed by the furnace but do not directly reflect the usable heating output. Therefore, "bonnet capacity" is accurately defined as the furnace output BTUs that indicate the actual heating performance.