Which of the following best describes the purpose of an economizer?

The primary purpose of an economizer is to enhance boiler efficiency by recovering waste heat from flue gases prior to their exit into the atmosphere. This recovered heat is typically used to preheat the boiler feedwater, which decreases the amount of fuel needed to reach the desired steam temperature or pressure. By utilizing this otherwise wasted energy, the economizer improves the overall efficiency of the boiler system, resulting in reduced fuel consumption and lower operating costs.

In the context of the other options, thermostat control of heating elements pertains to regulating temperature within a system and does not directly contribute to the efficiency of boiler operation. Reducing water temperature might be an aspect of other systems but is not the primary function of an economizer. Separating flue gas from air describes a different process and doesn't convey the operational benefits provided by an economizer in optimizing boiler efficiency. Thus, the choice that accurately reflects the economizer's role in enhancing boiler efficiency is indeed the correct response.