Performed semiannual maintenance on two heat pump cooling systems. Inspected all Performance, Mechanical and Safety Systems. While inspecting the upstairs system, I noticed the refrigerant charge to be low. Homeowners mentioned that the previous company that came out recharged the system but was unable to locate the leak. I used my electronic leak detector and located a significant leak inside the indoor fan coil. Also used bubble leak detection to confirm leak. Showed the homeowner the leak as well and we agreed the leak needs to be resolved. Flushed and treated all drain lines and drain pans. All air purification systems are on and operational at the time of departure. Changed two 20x25 F100 filters provided by homeowner. We decided to move forward with replacement of the indoor coil and install surge protection devices on both air handlers and the downstairs heat pump. Replaced the indoor coil on the upstairs unit. Removed the old refrigerant and old inline filter dryer.