I think you are correct concerning the ductwork. I've never done HVAC work in Florida but I heard through the grapevine that humidity issues will kill you these. One question though? How come the ductwork wasn't calculated to begin with for a five tons of air conditioning? That's a major thing when installing new units to make sure the ductwork will handler the CFM's. I don't have my duct-u-lator available but 26x8 or a little over 15 inches round is for 1300 CFM's. Why would you install a five ton system on ductwork like this? And why would another company say the ductwork was correct for five tons? Something doesn't add up here? Keep us informed as to what happens and I hope the problem has a good resolution.