I have a 3 story cabin in the North Georgia mountains. I'm pretty sure the unit was sized correctly (not there now so I can not report the size). House is around 3k sq. ft. Problem has always been the top floor (3BR - 700-800 sq ft) is always hot, while the second floor (where the thermostat is) is great and the first floor is a little too cool. I have tried shutting off most of the supply air on the bottom two floor to force more air to the 3rd floor and close returns on 1st floor - did not help much.
I'm now wanting to add a 2nd system. My wife does not like the look of the ductless mini-splits so it will either be a ducted mini split or a small unit. Any advice? My real question is should I close off the supply air from the main unit and just add a second unit or should I make the second unit supplemental. In either case what size should I be looking at. My current thought was the Mitsubishi ducted mini-split with 24k BTU as a supplement but I really have no idea what I'm doing. And it is hard to find anyone in the service area that is good at design issues.
Thanks for any guidance/suggestions.