What creates natural draft in a combustion system?

Natural draft in a combustion system is primarily created by temperature differences between the air and the gases of combustion. When fuel is burned, it produces hot gases that rise due to their lower density compared to cooler air outside the combustion chamber. This rising motion creates a vacuum or lower pressure area that draws in cooler air from the surrounding environment.

As the hot gases exit through a chimney or flue, they create a flow of air that is replenished by the cooler, denser air entering from the surrounding space. This process relies heavily on the principle that warm air is lighter and will rise, which generates a continuous flow without the need for mechanical assistance like fans.

While air density differences and pressure differences can also affect draft, they are often the result of the temperature differences created during combustion. Fan systems actively induce airflow and are not a natural draft mechanism, as they rely on mechanical force rather than the inherent physical principles of temperature and density.