Air is distributed in a building by means of connected individual duct sections that make up the duct system. The configuration of a duct system is often like a tree with branches connected to the terminal units and a fan located at the root. Therefore, from a topological point of view, duct or piping systems belong to a tree network. In reality ductwork forms a double tree because the fan is in the middle of supply and return/outside air parts of the system. Tees, crosses, or transitions usually connect duct sections.
One of the most important requirements for the design of a duct system is pressure balancing. The system pressure will be balanced when the fan pressure is equal to the sum of the pressure losses through each section of a branch. This is true for each system branch. Another different interpretation of pressure balancing is that pressure losses need to be balanced at each junction. If the sum of the pressure losses in a branch does not equal the fan pressure, the duct system will automatically redistribute air, which will result in air flows different from those designed.
Designing a duct system means sizing the ducts and selecting the fittings and fans. Duct sizing is not the same as making pressure loss calculations although the two are commonly confused.
