My house is hot upstairs in the summer and cold downstairs in the winter and I can deal better with the cold downstairs but not the heat upstairs. My 3 ton condenser can't keep the upstairs cool when the outside temps are at high 80's or low 90's and my stat and return are in the upstairs hallway.
I was thinking of installing a ductless unit to help the upstairs cool when it gets too hot out for the main condenser unit. I first thought about installing (1) wall mounted evaporator / air handler in the hallway upstairs but my thermostat and return are in the hallway so I was thinking of a multi zone system for the (3) bedrooms. My bedroom space is small only (400) sf total.
And yes I've check ductwork for leaks, adjusted dampers and have added insulation in the attic.
Seems sensible. I'm not sure what exactly are you asking, to be honest.
Just wanted to know if adding a split system in the hallway with the return and thermostat would be an issue or if adding a 3 zone split in each bedroom would ok considering I already have a central AC system.
A multi-level home with hot rooms upstairs and cold rooms downstairs, is a common problem, in both seasons. It's the nature of air, hot air rises, cold air falls.
The ductless split idea is a good option for the summer, but won't help with the cold downstairs in the winter.
If you do this, it would be a good idea to move the stat to a main floor central area.
If your home does not already have one, a cold air return to the 2nd floor will help.
Many older homes only have the cold air return near the bottom of the stairs on the main floor. When I had renovations done I made sure a cold air return was put to the 2nd floor.