Supply and Return
HVAC systems have two main types of duct lines: supply and return. Supply lines are either classified as plenum or extended plenum. In plenum systems, all branch ducts that make up the line start at the plenum, which is a box-like structure usually located above an HVAC unit and the structural ceiling. In this configuration, the duct lines look like spokes on a wheel. Plenums are also sometimes located under a raised floor.
Extended plenum duct lines branch out from a long plenum that runs to the end of the building. Branch duct lines are attached to the main plenum duct and are usually smaller in size than plenum ducts. Return lines provide a path from individual rooms back to the central system. Registers connecting to the return duct lines are often located in common areas such as hallways and foyers.
Pressures and Velocities
Duct lines are classified either high pressure or low pressure, or high velocity or low velocity, depending on static pressure and air speed design. Supply duct and return duct lines usually operate at low velocity and low pressure unless design indicates a need for higher pressure. This type is common in extended plenum systems. Return ductwork is usually low velocity and low pressure. High-velocity duct systems have smaller ducts that allow them to fit into existing ceiling, floor or wall cavities.
Materials and Shape
Most duct lines are made of galvanized steel. Glass fiber insulation board is also commonly used to construct low-pressure systems. Flexible ducts are gaining in popularity in residential systems. These are made of a spiral metal framework covered by a plastic, wrap-like material.
Supply duct lines, particularly those that run through spaces without air flow such as crawl spaces, are often insulated. Older duct lines are rectangular, while newer ones are round or oval flexible designs.
Accessories and Components
Though not technically part of a duct line, fan blowers, located in the central air conditioning or heating unit, move air through duct lines. Dampers are louvers within duct lines that control air flow. Splitters and turning vanes reduce turbulence, noise and friction losses within the system. Risers and branch ducts extend duct lines to room registers.