Energy Efficient Envolopes & HVAC

Since we all burn coal not all my clients want the same.
We are building a energy efficient envelope and have had much debate with HVAC.
Current project is 3400 SF Ranch lots of low e glass
In a nut shell R-30 plus walls and R-60 ceilings with a floor R of 48+, we have a tight home.
Air exchange is planned in multiple ways. With that basic knowledge, here is my quest for knowledge.
We have had a couple different approaches to HVAC, 1 the standard with 2 gas furnaces with ducting all over the place and the 2nd high SEER mini split heat pumps with 2 slim duct units and 3 ceiling cassettes.
What my needing to understand is why all this duct work when a ceiling cassette will do the same job.

Thank you for your input.

Without going into a lot of detail, what you are planning is a super insulated home. Humidity and indoor air quality must be dealt with. If the House doesn't have sufficient outdoor air for the occupants, a sick building syndrome could occur.

Ususally these type of homes have an air to air heat exchanger that takes air from the Bathrooms and kitchen and preheats incomming outdoor air to save energy.

On top of that there still needs to be heating and cooling. Ductless split systems are nice but expensive. Furnaces with ductwork are more standard. 5 ductless split systems will no doubt cost alot more than two typical furnace/split ac systems with ductwork.

If equipment cost was not the primary consideration, I would choose a ducted central HVAC with room zoned damper controls. The compressor would be variable speed and would have a de-superheater to heat or pre-heat domestic hot water. The whole house ducts permit distribution of preheated outdoor air to the entire house and to humidify the air during the heating season. Be sure to have the system designed by an engineer using methods defined in the ASHRAE design manuals. What you need is more than many HVAC contractors can design.

30 years ago my brother built a super energy efficient home. 2 x 4 walls with another 2 inches of foam insulation on the inside, sheetrock was hung with 4 inch screws. It almost rained in the basement! He said he only needed a 30000 BTU funrnace. He is weird and got divorced so I do not have much more info. He aslo had an air to air heat exchanger in the kitchen dining room area to help with humidity and air exchanges, but the basemnent still sweated like crazy.