What is the main characteristic of natural draft furnace operation?

The main characteristic of natural draft furnace operation is that it relies on temperature difference. This type of operation uses the principle that warm air rises, creating a draft that pulls cooler air into the furnace from outside. As the air inside the furnace gets heated, it becomes less dense and rises, which in turn creates a vacuum that brings in fresh air. This process is entirely driven by the natural movement of air due to temperature variations, rather than mechanical means or external power sources.

In contrast, other options involve characteristics that are not applicable to natural draft systems. For example, requiring external power or mechanical fans indicates forced draft or powered systems, which utilize fans or blowers to create air movement. Additionally, stating that a furnace can work without fuel does not fit the definition since all furnace operation fundamentally needs a fuel source to generate heat.