Red Seal Plumbing Practice Exam 2024 - Free Red Seal Plumbing Practice Questions and Study Guide.

Boiler input refers to the amount of heat energy consumed by the boiler. This means that it refers to the amount of fuel or electricity used to power the boiler and produce heat. Option A and B are incorrect because they focus on the physical size and water capacity of the boiler, which do not directly relate to the amount of heat energy consumed. Option D is also incorrect, as it refers to the temperature setting of the boiler, which is not the same as the actual energy input.