What causes natural draft to occur in a furnace?

Natural draft in a furnace is primarily caused by the movement of air created by differences in air density, which is directly influenced by temperature. When the warm air inside the stack rises, it creates a pressure difference. The warm air, being less dense than the cooler air outside, will buoyantly rise, leading to a reduction in pressure at the bottom of the chimney or stack. This difference in pressure then pulls cooler air from the surrounding environment into the furnace to replace the rising warm air.

This principle of natural draft relies on the density of the air being inversely related to temperature—warm air is lighter than cool air. Thus, as the warm air rises, the cooler air around it will move in to fill the void, establishing a continual airflow. This is crucial for maintaining the combustion process as fuel sources require a specific amount of oxygen to burn effectively.