What happens to the air inside the chimney during combustion?

During combustion, the air inside the chimney heats up significantly as the fuel burns. The heat generated from the combustion process causes the air temperature to rise. As the temperature increases, the air expands, which results in a decrease in its density. Because warmer air is lighter than cooler air, the heated air within the chimney becomes less dense and rises.

This principle is crucial in understanding how a chimney functions. The lighter, heated air creates an upward draft that helps to draw smoke and combustion gases out of the house, maintaining proper ventilation and preventing the buildup of harmful gases indoors. This upward movement is essential for efficient operation and helps ensure that combustion occurs effectively.

While other factors may occur, such as cooling and potential moisture interactions, the main and most immediate effect during combustion is the increase in temperature and the resulting decrease in density of the air within the chimney.