What defines system efficiency in a heating system?

The correct choice identifies system efficiency by taking into account the energy entering the boiler and then subtracting any losses that occur in the system. In a heating system, efficiency is crucial because it measures how effectively the system converts input energy (from the fuel used in the boiler) into useful heating output for the space or water needing heat.

When you think about boiler input, it refers to the total energy supplied to the boiler, while system losses can include heat that escapes through flues, piping, or even radiation losses in poorly insulated environments. Therefore, calculating the boiler input minus these system losses provides a clear understanding of the actual energy that can effectively be utilized for heating, thus offering an accurate measure of the system's efficiency.

Other options do not apply effectively to defining efficiency in this context. For example, boiler output plus system gains is not a standard measure since it does not appropriately consider losses. Heating demand minus environmental factors is more of a consideration of the requirements for space heating rather than efficiency itself. Similarly, boiler capacity minus operational overhead fails to directly address how well the system utilizes input energy relative to output heating, thereby not providing a focus on efficiency.