What causes all ventilation of a space and movement of air in a duct?

The movement of air in a duct and the ventilation of a space are primarily caused by differences in air pressure. When there is a pressure differential, air will naturally flow from an area of higher pressure to an area of lower pressure. This principle is fundamental in the design and functionality of HVAC systems, as it allows for the effective circulation of air throughout a building. For instance, mechanical systems, such as fans, can create zones of varying pressures, encouraging the flow of air to ventilate different areas.

While temperature differences can influence pressure, as warmer air expands and creates lower density (thus lower pressure), the actual driving force for airflow in duct systems is the pressure differential itself. Therefore, understanding the role of pressure differences is vital when analyzing and designing ventilation systems in order to ensure proper air movement and indoor air quality.