Performed semiannual maintenance on one heat pump cooling system. Inspected all Performance, Mechanical and Safety Systems. Did note that the indoor coil is extremely rusted and corroded. I used my electronic leak detector to inspect the coil for any refrigerant leaks and at this time none were present. This will require close monitoring to ensure refrigerant levels stay within the manufacturer’s specifications. Flushed and treated the primary drain line and changed one 16x25x4 Honeywell filter. Verified everything was operating within manufacture’s specifications for operation. Everything is working at this time.