I recently bought a 2 story home that had forced hot air system from the 60's until the 90's. The owners converted to baseboard heat but left all of the ductwork and registers from the original forced hot air system. The ducts are 4" in diameter. My question is...can the old ducts be used with a new AC unit? We had one person tell us that it could be done. Another told us that there are no returns up high and a completely new AC system needs to be installed which can be done through the attic. Meaning that all of the old ducts are useless. Can anyone help with a definative answer before I hire someone to do the work? I appreciate any advice and/or knowledge.

You could probably use the old duct work assuming there is enough air flow, but there wouldn't be a guarantee of even air flow at each vent since it probably wasn't designed right in the first place. But, like the other contractor said, you should have returns up high for AC, if possible(up high assuming you have high ceilings and not normal 8' ceilings). If you have high ceilings(that go up to the 2nd floor, except for the normal staircase) and there isn't a return air somewhere up there then the heat will build up and you would probably have to close the doors to each room to keep it cool.