If the adjustable blades of a supply register are moved closer to the closed position, what happens to the static pressure?

When the adjustable blades of a supply register are moved closer to the closed position, the static pressure within the duct system increases. This occurs because the airflow is being constricted as the blades are closed. As the space for air to pass through becomes narrower, the resistance to airflow increases, which in turn raises the static pressure upstream of the blades. Essentially, this is a result of the principles of fluid dynamics, where increasing resistance in a flow path leads to an increase in pressure at the point of constriction.

While some might consider that the pressure could decrease or remain the same, that does not take into account the impact of restricting airflow, which is a fundamental concept in HVAC systems. The phenomenon is influenced by the design of air distribution systems and their responses to changes in airflow through various components. It's also important to note that fluctuations in pressure can occur during system operation, especially if there are additional variations in demand or fan speed, but the immediate effect of closing the blades is a clear increase in static pressure.