We installed the Unico system in our previous home, built in 1927, and kept the radiators. Unico is a high-velocity system that uses small air tubes instead of larger rectangular "ducts," so the ductwork is easier to run and less intrusive. It is more expensive than if your home already has ducts. Also, it is sometimes advertised as being "silent" -- it isn't! You cannot push air at high velocity and not make some noise!

All in all, we were very satisfied with the system.

When we bought a two story colonial with baseboard radiators a number of years ago, we solved the A/C problem by installing an air handler in the walk up attic and putting ceiling registers in only the upstairs room and installed a return in the upstairs hallway. Not a perfect solution, but it cooled the hottest part of the house well, and the cool air naturally moved down to the first floor. It was cost effective and avoided having to install ductwork in the living space.
If you have a basement, you can add a second air handler with floor or baseboard registers and returns on the first floor.

I have a 1915 4-square with radiators. We installed central a/c when we took possession of the house 2 years ago. Luckily, there were enough places to run the ducts with the a/c unit in the attic. We lost a little bit of space in some closets but not so much as to have impacted them greatly.

Each bedroom (3), the main bath, living room and kitchen now have a/c. Total of 4 upstairs, 2 downstairs. We were a concerned that only 2 downstairs may not be enough but the system installed handles everything fine - house is only about 1500 square feet if that which includes the basement which has no heat or cooling in it.

Installation was very easy and we're pleased. We were able to get the government rebate at the time, so I believe the total cost only came out to about $6500 or so. Well worth it.

I believe it's a Trane but I don't remember off hand exact model or capacity. Not something I would anyway.

We installed a high velocity system in our 1920 four square (with radiator heat) this past April in New Jersey. It did zero damage to the plaster walls and ceilings, so we are happy. We have one unit in our third floor that heats the 3rd and 2nd floors (approximately 1500 sq. ft). Even without anything on the 1st floor, the hottest the 1st floor got was 78 degrees with no humidity.

I will echo everyone else's comments--it is expensive. For the 3 ton unit in the 3rd floor with all new equipment and installation, the cost was $12,500. All the prices ranged from $11,500 to $15,000. The traditional AC was running between $9,000 and $12,000 with estimates. Remember that was for New Jersey. You should have cheaper numbers. Also, it is pretty loud--similar to white noise. The noise has been good for the little kids and I am already used to it.

I'm in NJ also.

I can the air handler in the attic from our 2nd floor, although it's most prominent in the main bathroom so it really doesn't seem to bother us much. It's a low hum, background noise. The one return is upstairs in the hallway.

We also had no damage to any walls or ceiling - all plaster. They were exceptionally careful with installation.

Also, we still have our radiators. A few people mentioned getting rid of them but they heat the house really well so it seemed unnecessary to go tearing up the house further.

It's great to hear that AC can be added without wrecking plaster walls! That was my main concern. The Unico system sounds like it would be worth looking into, as we don't have closets in all of the bedrooms and our attic is only a small crawlspace, due to having a high-pitched mansard roof. Thank you all very much for sharing your experiences with me, it's helped a lot.

Getting central A/C in our old house was one of the BEST renovations we have done to date! We have a large, two-story home with all plaster walls and orignal hardwood floors, so we wanted to be careful with our choices. We have a big enough attic that we put all of the ductwork up there and made the system large enough to cool both upstairs and downstairs, but only put vents upstairs--most in the ceilings, which doesn't bother me in the slightest. The cool air drops through both the front and back stairwells (they put big vents over each stairwell), so it's perfectly comfortable downstairs. If the upstairs is set at 68 for sleeping at night, it might be 72 downstairs. If we wanted to, we could put a mini-split in the wall of the downstairs living room, but we're going on year three and it's worked out fine.
We are in upstate, NY and use it basically mid-May through mid-September.
I *think* it was $9,000 to have it installed three years ago. To do the downstairs would have meant cutting vents into the hardwood floors and a lot of extra $$. :(

Hope that helps!
Sarah

You can put an air handler in a crawlspace attic, as long as it's tall enough for the unit, and it runs above the rooms in which you want to put registers.

I was lucky enough to have moved into a cape cod home that was already retro-fitted with central a/c. Whoever did the installation was clever and had a well executed design. The air handler was placed in the basement, and the main floor vents are in the floor with the duct work in the basement. The clever bit is they positioned the air handler under a large upstairs closet. Half of this closet was sacrificed to run duct up to the uppermost level of the house, and the duct work was run in the ceiling, I believe it was installed at the same time this space was converted from an attic to the master bedroom. It all works very well and I am very happy the previous owner was the one who paid the expense! To tie in with the OP question, the house also has a radiator oil boiler system, this was not altered or removed in any way.