Hello,
KISS, the high velocity is not a common system. Experience will be difficult to find.

To: vmenroe, I am a diy-er and want to tackle this type of AC project. I was wondering if I could ask your husband a few questions e.g. did he go with seperate air handlers for each floor, did he AC the basement, if you have a primary heating system did he go with a 'back-up' heating system within the Spacepak, did he use the Spacepak sizing guide, did he end up with a well balanced system, was he able to get tech support from the factory or did he get through the local distributor, any tips based on his experience, etc.

A conventional system costs less, has more trained designers and technicians, and when done correctly produces less noise.
I have installed Unico systems many times when nothing else would work.
Plaster walls are expensive to fix, so reducing damage can help offset the Unico material cost.
Unico heating has some drawbacks. The high air flow and mixing is fine with cooling, but can create a drafty feeling with heat. Even with outlets in corners the greater air movement is a concern for some people.
The room air temps are much more uniform though.
The pre-fab square fiberboard ducts do go together easily. The Unico take offs work well in these. The take offs for flex duct are not as nice.
Making your own ducts from thicker ductboard has some real advantages when running in unconditioned spaces.
Hanging a platform from rafters helps to nearly eliminate vibration transfer from the blower motor, leaving just the outlet noise. You may need to double up rafters for the extra point loads of the hanging hardware and air handler. Vibration isolators in the refrigerant lines can further isolate the air handler.
Be sure to install a back up drain pan and run it to daylight in a visible place. You want to see any water coming from the backup pan as quickly as possible.
I have seen float switches used in backup pans instead of an actual drain, but I always install the drain. Lugging a shop vac into the attic to clean out the pan is not worth the cost of a few feet of schedule 40 drain line.
Switches also fail, gravity works all the time.
The Unico design and installation guides are all online and work well.
The biggest propblem on older houses is getting an accurate manual J calculation for cooling. There are a number of tricks that can be used based on the heating load and unit size to help bound the problem.
Infiltration is usually a bigger factor in old buildings than new, and even a small error in the R value of the exterior walls can swing the loads significantly. There are a lot of square feet there.

housebaby, after much thought I went for conventional heat pump with strip for my 1899 four square. Previously had been using window shakers and unvented gas. The comfort level is beyond all hopes and the utility bills are about half. My contractor did talk me into a variable speed air handler and multiple returns. The system is quiet and even our first winter has been very comfortable.

to: brickeye, thanks for those great tips. What advantages did you get from making your own ducts in the unheated spaces i.e. better R value, etc.? Did you put a dedicated air handler on each floor? Did you AC the basement? My house is only 15 years old and well insulated, so calculating an accurate heat loss/gain is doable. Also, I am finishing my basement therefore, do not want the conventional duct sys taking up head room. I am determining whether to put in radiant floor and AC or go with the hi velocity AC with a heat coil. Thanks for your input.

"What advantages did you get from making your own ducts in the unheated spaces i.e. better R value, etc.?"

You can use 2 inch ductboard instead of the 1 inch Unico has.
The straight sections stay 4 feet long (size of the board width). You do need to learn how to make up the turns and Ts however. You can also wrap the pre-fab ducts in another layer of insulation.

"Did you put a dedicated air handler on each floor?"

Sometimes just 1 for a 2 story cape (no AC supply to basement), one house needed 3 units. (1st floor, 2nd floor, addition).
Since there is normally not any insulation betweenthe fiorst floor and basement, basements stay cool below grade, and cold air sinks, in Virginia I can usually get away without directly ducting basements for cooling.
Be careful about using floor in a house that may not have an adequately insulated slab. The earth will suck up a lot of heat.

Follow-up question from a new poster -- we also have a 100 yr old house (rowhouse) about to undergo renovation. Architect is designing for conventional A/C with steel beams on roof to support equipment. Is there an advantage to the Unico/Spacepak model from the perspective of avoiding the costs of the steel reinforcements? Sounds like the Unico handlers just go in the attic or a closet? What am I missing? Thanks in advance for any advice. (Pls don't be too technical -- not sure I understood 3/4 of brickeyee's post above!)

The Unico system is light enough to put i an attic.
You can reduce the chance of vibration problems by hanging it on a platform from the rafters.
I place vibration isolators in both the liquid and suction lines, along with any heating lines if a hot water coil is beingused.
If the vibration isolators are vertical they take care of two axis.
A Unico (or Spacepak) system still needs a condensor though.
They can be placed on a roof or on the ground.
It is a lot easier to service ground units than units on sloped roofs, and if the roof pitch is very high you will have problems finding anyone to work on the equipment.

You will not get the same efficiency from Unico and other high velocity systems as from forced air. You will easily lose 20% or more of your energy dollar to HV.

Thanks brickeyee & airsome. This is a rowhouse with a flat roof, and in our neighborhood pretty much everyone's condensor goes on the roof, b/c the backyards are small & you can't stick the noisy condensor right under your neighbor's window. That being said, not many have central A/C.

Are condensors the same size for Unico as for conventional? So if my architect says we need steel beams to support the condensor in traditional model (I'm assuming both for weight & vibration?), would you expect we also need the steel for Unico?

Regarding efficiency, I would have thought that the bigger vents of conventional systems would have wasted more cooling, vs. smaller tubes. Why the lesser efficiency from Unico?

BTW, regarding the ground floor question from the OP, our architect is designing most cooling on the 3rd & 4th floors, some on the second (parlor) floor, and none on the garden level, which is slightly below ground. His experience is that the lower floor will stay comfortable if there's some cross-breeze. Thanks again.

I should not respond to this hijack; apologies to OP.
Sounds like Arch is on right track.
Duct should be sized for the amount of airflow required, thus typical forced air systems. When you decrease the size of the duct, you increase resistance. The manufacturer makes adjustments to compensate for this, but he can't get all the way up to the same level of efficiency.
Condensor size is determined by efficiency: the MORE EFFICIENT the condensor, the LARGER it is. This is because of the amount of surface area required to radiate heat to the surrounding air.

Apologies -- didn't think this was hijacking, as I thought my question was directly relevant to the OP's. No need to discuss further. But thanks again for very clear information.

I have a house built 1834 12" brick walls with plaster...Removed the plaster and decided to leave the brick exposed. Looks like a million dollars but have no isulation and never did...Originally going to go conventinal forced air hvac. My basement was going to become the height of a standard crawl space...Thought about going with hot water baseboard but no AC would sacrifice beacause of better heat...Next guy stops in and says the only way to go is Unico?? Anyone with some advice I'm undecided now on what to do!

I have a house built 1834 12" brick walls with plaster...Removed the plaster and decided to leave the brick exposed. Looks like a million dollars but have no isulation and never did...Originally going to go conventinal forced air hvac. My basement was going to become the height of a standard crawl space...Thought about going with hot water baseboard but no AC would sacrifice beacause of better heat...Next guy stops in and says the only way to go is Unico?? Anyone with some advice I'm undecided now on what to do!