Hi, we recently moved to a new home. Right now we have forced hot air and a 2.5 ton AC unit. The duct coming off the furnace is 18x8 and then reduces to 14x8, off this there are 12- 6" flex duct supplies. A couple of these are very long runs through my unheated garage. Needless to say the system is not working very well for our needs.
We are thinking about putting in a high efficiency modulating gas furnace with all new ductwork. I have had several people come out for estimates and none of them have done a load calculation.

The house is about 1500 sq', one level Ranch with finished basement area. We live in CT. From what I can figure out I believe we need a 60,000 btu furnace. The one I am looking at is a coleman echelon rated at 1200 cfm.
I have a few questions:
1. With my current set up having 12- 6" flex ducts was the main duct undersized?
2. Should the main duct for my new setup be sized for 1200 cfm?
3. Can I have three zones, which would be about equal in size?
4. If so can I split the main duct into 3 ducts sized for 400 cfm, and then have my supplies come off those?