How does a high efficiency furnace utilize a high percentage of the available BTU in a fuel?

A high-efficiency furnace is designed to maximize the use of the available BTUs in fuel, achieving this primarily through advanced combustion technology. The correct answer highlights that these furnaces often use spark or hot wire ignition systems, which provide a more reliable and efficient ignition of the fuel compared to traditional methods like pilot lights.

Additionally, the inclusion of an induced fan draft plays a critical role in enhancing efficiency. This fan helps to improve airflow and ensures that combustion gases are efficiently processed through a secondary heat exchanger. The secondary heat exchanger captures additional heat from these gases before they are vented outside, allowing the furnace to extract and utilize a higher percentage of the heat energy originally contained in the fuel. This process significantly boosts energy efficiency, contributing to lower energy consumption and operational costs.

In contrast, other options either represent outdated technologies or do not specifically address how heat utilization is improved within the context of high-efficiency models. For instance, a pilot light is typically less efficient, as it continuously burns gas, but high-efficiency systems are designed to conserve energy. Conventional thermocouples are used to sense temperature and ensure safety but do not directly contribute to the efficiency of heat conversion. Lastly, while a mechanical thermostat helps regulate temperature, it does not affect