Good question. I'm not quite sure I follow though--doesn't it need to get through that concrete level one way or the other?

I am not sure why you want the concrete plank floor system.

You might consider open web trusses instead. You could pour an inch of concrete on the floor and add insulation for soundproofing.

Another option, if you stay with the concrete floors is to have a separate heating / cooling system downstairs from the one upstairs.

We preferred a concrete over wood floor because of soundproofing, termites, and moisture. Our neighbors wood floor got all rotted out. With that said, it was just our initial thought. We are amateurs and are not sure.

I believe my architect said the plank system was more economical than the cast-in-place slab. Is that the same or similar to pouring as you suggested? After a quick google, open web trusses look interesting! Also, good idea on the separate systems. Thank you.

In Florida, you have very hot humid summers. A single system in a two story house typically can not cool the upstairs and downstairs equally (upstairs is usually hotter - hot air rises). Two smaller separate systems is a very good idea in this setting. More expensive initially but MUCH more comfortable when the downstairs is freezing and the upstaitrs is sweaty! In theory, you can accomplish this with a zoned system but in practice, few contractors know how to do this properly and they tend to be headaches down the road.

As I mentioned earlier in this post you could use an all metal open web system. The floor itself could be approximately one inch of concrete poured over a one inch thick board that is similar to plywood but made with a material that is impervious to termites.

the open web allows for ducting.

I would think that this would be less expensive then the concrete methods.

Concrete floors add a lot of weight to the structure, so your structural costs would be less with web joists.

Using two HVAC systems could also be less costly in the long run.

I would not install HVAC equipment in an attic. See if you have space for a utility closet on the second floor. If you use floor trusses you may be able to run the duct work in the first floor ceiling.

Like others have said, 2 units.

Was the plan 1 unit and where was it?

I would gather best practice in Florida is spray foamed attic with 2 units in attic. Then a chase down to an open web floor system or in your case, soffits. The reason is that construction costs are high (concrete, wind) and basements are a no go. Since the construction costs are high, the HVAC in closets costs some money - plus who wants the noise.

I don't see any big issue with that plan. There is zero difference in efficiency in that setup between open web trusses and soffits. There is an aesthetic difference of course but with an architect designing on the front end, it should be fine.

An alternative is HVAC units in an upstairs closet since most people want more downstairs space. So you usually have some extra upstairs space. Then you don't have to spray foam the attic - just build something above the 2nd floor ceiling plane for the ducts. Much cheaper. I have something similar in a NC slab situation. But I would work hard to isolate the noise.

Research is good. Unless you're an engineer. Then it just annoys your contractors and you pay for that annoyance factor. But you have to realize the the costs of specialty construction and specialty contractors. Do you really want to pay 2.4M for a house that you could build for 1.5M? That's where you're headed with this. With no builder wanting to take the job.

foam seal the attic and put ductwork equipment etc in the attic.

better building practices advocates unvented

attics for our climate & hurricane resistance.

invest in a company that will do a load calc. stay away

from rule of thumb hvac contractors that use 400-500

sq ft per ton.

look at a mid range SEER heat pump.

invest in a variable speed air handler.

a correctly sized & installed hvac system will

add comfort, rather than oversized systems

that don't run long enough to remove humidity.

best of luck.