Today I replaced an outdated 3.5-ton AC unit with a high-efficiency two-stage model. Removed the old condenser and air handler, brazed new copper lines, pressure-tested with nitrogen, and vacuumed the system down to 500 microns. Verified proper refrigerant charge using superheat/subcooling method. Installed a new digital thermostat and programmed schedules for optimal efficiency. Unit is operating quietly and efficiently with a balanced airflow profile.