Hello. I am having a contractor install CAC in my home in the near future using a system that includes a Mitsubishi Mr. Slim combination ducted/ductless system. I had previously posted on this board the following thread based on contractors original proposal of running two air handlers for through a single compressor: http://hvac-talk.com/vbb/showthread....ghlight=hob333. That proposal has been scrapped and I'm now doing the following:

My home is a traditional 1930 center hall colonial, approx. 2100 sq ft (probably 2300-2400 including finished attic). My complicating factor is that the attic was finished with lots of nice built-ins so it is impossible to run traditional ductwork across the whole attic to reach each of the rooms. There are however accessible knee walls that run along the front and rear of the 3rd floor. The second floor has the following: a master bedroom and bath that takes up one "whole side" of the center hall, a hall bath, and 2 bedrooms on the "other side" of the center hall. So this is the plan:
- traditional Luxaire CAC with compressor outside and air handler in my basement and traditional ductwork to come up through the first floor of house.
- Use a 3 zone Mr. Slim system to air condition the 2d floor and 3rd floor. There will be a ductless unit placed to air condition the 3rd floor. There will be two ducted units: one in the front knee wall and the other in the rear knee wall. I don't believe the contractor has completely figured out where he's going to place the actual ducts in the ceiling of the second floor.

What questions/concerns do I need to have regarding this system?
- Can sufficient ducts be run from the two ducted units to air condition the entire 2d floor?
- Are two of these ducted units adequate to air condition the 2d floor of a home?
- He's mentioned possibly doing a "rear zone" and a "front zone". Is it better to tie it more to each room? For example, one of the bedrooms is not used nearly as much. Or in the interest of cooling the whole house would the whole thing be running anyway?
- What will the noise and/or vibration level be like with basically 3 units all running on the third floor?

Any other thoughts regarding this type of system and any questions I should be raising with my contractor are greatly appreciated.