I wanted to get an opinion on how to resolve the issues we have.
Our home is a two story 2600 sq ft home in Albuquerque, NM which isn't high humidity or extreme temperatures either way.
Our upstairs bedroom (above the north facing 3 car garage with non insulted doors but insulated walls) is cold in the winter and hot in the summer. The air handler is a Rheem Criterion II 5 ton downdraft unit mounted upstairs. The Return in the attic in insulated 16" flex into the side of a box in which air enters in at 90 degrees. the grill on the box is 20x20 and is at the top of the stairs in the ceiling. Home has no 2nd story vaults. The master bedroom has 2 vents with the attached bathroom/closet combo with one vent, about 450sq ft total. Downstairs gets hot fast in the winter (and we have a nice AFUE-rated gas fireplace that can put out 24k btu/h) and in the summer will freeze into the mid 60s before upstairs gets in the mid 70s.
The filters are inside the unit, which i believe the installer did a bang up job of making the filters worthless since the attached ducting for the return has a ledge that causes a 1" gap in all but the lip where the ducting attaches. The Filters are mounted in a V shape on the return entrance and is 2 16" filters.
I called an A/C contractor that I already know, and he told me over the phone it was probably poor duct design and the home should have had 2 smaller units installed with one upstairs and one downstairs. I agree, but that sounds like reconstructive surgery. When he came out, he stated the same results and didnt think another return (like I thought) would help. He took no measurements and in fact never had the unit turn on - just asked if the vents near the curtains would make the curtains move when air was blowing out. Also, a worthy note it that when the unit turns on there is a howl in the return vent, which disappears with the blower door off.
While I don't doubt that the ducting design may be at fault, my own tests of running the unit for a few minutes without the blower door on increased air flow and comfort quite a bit. I know that's not a good way to do this - that's why after that I called a pro. His answer was a small mini-split for that bedroom. I'm not sure I want to pay for that and the additional maint. cost of having 2 units.
Are there other options I should think about, or what else can I do when selecting my next HVAC pro to give me more input.