Replaced several sections of deteriorated flex duct in a 20-year-old system. The inner liner had collapsed in two branches, causing airflow restriction. Measured and cut new R-8 insulated duct, sealed all connections with UL-181 mastic tape, and strapped them with reinforced supports. Balanced airflow at the supply registers afterward. Duct system is now fully functional, and static pressure readings confirmed improved efficiency and comfort in all zones.