What does the term "stack effect" refer to?

The term "stack effect" refers to the phenomenon where warm air rises in a vertical structure, such as a building or a chimney. This occurs because warm air is lighter than cold air, leading to a difference in pressure. As the warm air rises, it creates a slight vacuum at the lower levels, which can draw in cooler air from outside or lower levels.

Understanding the stack effect is crucial in the context of building ventilation and air quality, as it influences how air exchanges occur within a space. When warm air rises, it can promote better distribution of fresh air and help in the natural circulation of air through a building. This principle is particularly relevant when considering heating systems and appliance venting, as it assists in the effective removal of combustion gases from gas appliances, ensuring safety and efficiency.

In contrast, the other options relate to different concepts in heating and air movement. Airflow through appliances examines how air circulates within specific devices, negative building pressure pertains to a situation where the pressure inside a building is lower than outside, and the external building envelope refers to the exterior materials and surfaces of a building that separate the interior from the outside environment. Each of these concepts is important, but they do not specifically define the stack effect.