Looking to optimize the HVAC design of a house in NC near beach on pilings. House is 2580' tight insulation design, inverted floor plan with all closed-cell foam (attic, walls, below 1st floor) and had planned for 2 zones (one for each floor) with an additional zone on the 2nd floor for the master suite. Independent engineering with Manual J says 1 ton needed for 1st floor 1.2 ton for 2nd floor - so initial plan is to have two 1.5 ton units with matched 2-ton air handlers (as they don't make anything smaller) with an ERV for each floor. Also planned to integrate a dehumidifier into the mix for the shoulder seasons, but not sure the best way to do this.  After looking at all this I am wondering if this is best design from a cost, energy use and duct-integration considerations.  I am now considering dumping the multi-zone idea and having a single zone for the whole house with a single right-sized system (2.5 ton) with single ERV and dehumidifier.  Obviously need to decide on the final concept before following up with the Manual D, S and T.  There seem to be no shortage of ideas out there on these topics, but for this application any one have suggestions, and the best way to integrate the ERV and dehumidifier?

Have you looked into a mini-split ductless heat pump? They can take care of the heating, cooling and the dehumidification, including in the shoulder season. You still need an ERV for air exchange and for reducing some humidity.

Yes, initially; and may still consider as an option for the 1st floor, but concerned about cost, reliability, noise, etc.; and might have to get additional technical advice as to best product and installation / layout, as not sure if the HVAC contractor is familiar with them. Any suggestions in this area would be welcome.

as not sure if the HVAC contractor is familiar with them.

Find one that is.

'Tis the season for homeshows. Go to your local homeshow. A quick walkthrough will reveal the HVAC contractors displaying the ductless minis. One of them is going to be able to demonstrate a unit in operation so you can judge for yourself on the noise, but the benefit of the "split" is that the compressor is outside. I have numerous quotes for installation of a 2-1/2 or 3 ton system with 3 indoor units that range from $6,800 to $8,000. These have been in use outside the US for decades.

Why only the first floor?

Seems to me that a single zone would be less expensive and could be properly balanced to keep temperatures correct everywhere. Don't some AC units have features to improve dehumidification performance? (multistage/variable fans/adding back heat/coated coils). A mini-split might pay for itself if you have a situation where it would allow the main system to be shut off (say cooling only a master bedroom at night).

Why couldn't he do away with the "main system" entirely?