First I want to thank everyone on here for all the advice you have given me. Last fall I upgraded from my old 80% 100k furnace to a 70k Goodman 95% VS 2 stage unit and it is amazing. *some problems have popped up and I'll discuss.
However, now is time to upgrade the AC.
Location: Kansas City, MO.
House Properties: 1960 build, R48 attic insulation, R13-19 wall insulation, Double pane argon low E windows, Cedar shake siding, Professionally air sealed, split level 1200 sq ft home, 2 car garage 3 bed, 2 bath.
Current ducting: Main truck is 16x14 to lower level and 16x10 to upper level. lower level ducts run along floor joists in basement (conditioned), upper level run along floor joists of garage. (Garage is uninsulated, both walls and ceiling.[will add R30 insulation in joists this spring) All branches are 6" round ducting, hvac tape sealed to room exterior walls then a mixture of vents, mostly 10x6 ones. Return has two lower wall mounted 24x6 vents into a 22x8 truck feeding a 20x25 filter.
Current system: Goodman 70k VS 2 stage LP furnace, 2.5ton 1990 Nordyne AC *10ish SEER.
* House is currently zoned via Honeywell zoning control board and two 2 stage thermostats. Both areas heat and cool 600ish sq ft with the Zone board only allowing 2nd stage if both zones are calling. The lower level allows some bleed off when only upper level is calling.
Current problems: Upper level vents have high velocity, thus are somewhat noisy (I was told by you and the contractor this would happen). Currently blower is set to lowest cfm limit of 850ish for heat (no AC need yet). This spring when I replace the AC I'm getting 2 new supply branches installed in upper level (master bath doesn't have a vent and 3rd bedroom only has 1 vent) and 6" rounds will be upsized to 7" rounds.
Goal: I am finishing the basement and want HVAC down there. I would really like to just get ducts ran off the main branch that runs along the joists so I'm wondering what needs to happen. The space is concrete walls with R6 foam board then drywall (originally built so 2x2 framing and drywall). There are 3 windows each 3'x2', the ceiling is uninsulated but I will have R19-21 installed before floor joists then drywall. There is a completely closed off utility room that will not be included in this HVAC area. Thus basement sq ft is 450ish with small bathroom (has fan vent to outside). Ceilings are 8'. I want a low velocity system with 1 duct in the game room area 13'x14' and 1 duct in bar area 7x17, and 1 duct in small bath 5x8ish. Because the game room will be able to get closed off I think I'll need a return in there. Currently there is a 8x8 return in bar area of basement which ties into return trunk.
Tradeoffs: I DO NOT want a mini split. Although this may be the BEST option, I do not want it. I do not care if the basement has it's own zone, it can share with the lower level.
Questions: (1) Can I get branches off the main duct to feed this basement area, with minimal negative impact?
(2) What size branches to get lowest velocity, joists are 16" on center?
(3) What size AC should I get, contractor said up size to 3 ton 2 stage?
(4) What size return for game area?
(5) Will up sizing from 6" rounds to 7" rounds + 2 new 7" rounds resolve my upper level high velocity problem?
Sorry so long and if you need more information let me know. I'm asking because my contractor has told me A or B and I want to verify before I pay more money, especially if it isn't going to solve my problems or make me happy.