HVAC (heating, ventilation, and air conditioning) duct installation begins with the routing of main supply and return ducts from the central equipment. Ductwork sizing is based on air flow requirements calculated from the building's heating and cooling load. Main ducts are typically 12 to 24 inches in dimension and may be rectangular, round, or flexible ducting. Supply ducts route from the unit to supply registers (vents) located in each room. Register locations are planned during design to provide adequate ventilation and comfort. Supply ductwork slope should promote air flow and prevent low spots where condensation can accumulate. Return air ducts carry air back to the unit for recirculation. The return duct size is typically larger than supply ducts since air velocity should be lower to minimize noise. A centralized return air plenum may be used in drop-ceiling areas instead of individual return ducts. Flexible ducting is used for short connections from rigid main ducts to registers, reducing noise and allowing routing around obstacles. Duct sealing with mastic sealant and duct tape prevents air leakage, improving efficiency. Leaks in ductwork are common and reduce system efficiency significantly. Proper support of ductwork prevents sagging and stress on connections. Ducts must be insulated in unconditioned spaces to prevent heat loss or gain. Fiberglass duct insulation is typically 1 to 2 inches thick. After installation, the system is balanced by adjusting dampers in each supply branch to deliver the correct amount of air to each room. Manual balancing adjusts dampers based on measured velocity at each register. Commissioning (system verification) confirms proper operation.