Why does a chimney develop draft?

A chimney develops draft primarily due to the principle of buoyancy, which is influenced by the temperature difference between the flue gases and the surrounding air. When the combustion gases rise within the chimney, they tend to be warmer and, thus, less dense than the cooler outside air. This creates a situation where the lighter, warm air inside the chimney wants to rise, resulting in a draft that pulls in cooler outside air to replace it.

The option that states the air inside the chimney is lighter than the air outside directly encapsulates this principle, emphasizing the role of temperature and density in facilitating draft. When the inside air is lighter due to being heated by the combustion process, it rises, and this action generates the necessary draft to draw in fresh air for combustion. This process is vital for efficient and safe fireplace or furnace operation, as it helps ensure adequate ventilation and proper functioning.

While other factors like wind pressure can influence draft in specific scenarios, the foundational principle remains that the air within the chimney is heated and lighter, which establishes the necessary conditions for effective draft.