Adding central air without forced air heat
Typically, the method you are describing is for multi-zone temperature control. To complete the picture, another unit would be mounted in the basement to supply the first floor, with the unit you mentioned in the attic for the second floor. This eliminates the need for running ducting between floors (and the associated cost for keeping it all hidden) as, presumably, all first floor rooms are easily accessible from the basement, and all second floor rooms from the attic.
This design may not necessarily save you money, but it does improve the comfort level as you can control each floor‚Äôs temperature separate from the other. You probably already have that ability with your hot-water heat.
Keep I mind that you will need ample electric supply lines to take on the 30+ amps the units will each need. If your home only has 100-amp main service, you will need to consider upgrading to 200 amp or live with blown fuses or tripped circuits.
You could save money by just putting central air for the second floor for now. Since cool air drops, the first floor will cool off somewhat as well.
I have heard of two versions of this design.
In one, an outside chiller, similar to the typical outside unit a normal home may have, chills a liquid, similar to antifreeze in a car. This liquid is brought down to a very cold temperature, where it is circulated around to inside units which are simply radiators with fans ‚Ä¶ sort of the reverse of hot water heat. I guess the individual units can be zoned the same way. Although new to residential homes, this technology has been used in grocery stores and other businesses for years.
The other design utilizes a ground water source, similar to geothermal, which is pumped inside the home where different a/c units extract the heat in the rooms and pass it into the water.
Although I have never worked with either or even seen one, I would imagine, since it is new technology, that the cost would be more than more traditional systems, at least until the idea catches on and mass-production kicks in. If someone goes that way, I would like to see a posting about it. It would be very interesting.