Ducted heatpumps are a thing here in NZ and make sense to me, not that I have used one. The heatpump sits in the roof and air is pushed into three or so rooms through ducts.
If building a new house I think ducts would be the way to go.
Most roofs in Australia still aren’t sealed. The air barrier and insulation barrier is the ceiling. The roof space itself is not insulated, so the ducts are exposed to extreme temperatures, thus destroying the efficiency.
Here is just one article on it: https://newenergythinking.com/2018/10/20/dont-use-ducts/
My argument with split systems, at least how they are installed in NZ, is that they are usually installed in the main living area and if there is a second unit, in a corridor.
So corridors are heated or cooled far hotter or colder than they need to be in order to heat/cool bedrooms.
One has mini splits. You definitely don’t generally put mini splits in every single room (like bathrooms or interior hallways) because each mini split requires tubing, is large, and expensive. No, I think it’s way more likely that you would have them in the bedrooms rather than the hallway
But we have many very toasty hallways leading to cold single glazed bedrooms!
In the last few years the central govt has brought in legislation requiring landlords to heat and insulate homes to a certain standard.
Edit: not to mention the huge difference in surface area. Ducts can expose your conditioned air to a huge, poorly insulated surface. Bad for efficiency.
They also recently undid a long held myth about ducts buried in insulation leading to condensation issues. Most areas now allow ducts to be buried as long as you have R19 above and below, or R30 above. When I just installed a ducted mini for 3 upstairs bedrooms, I surrounded my R8 flex duct with R30 insulation.
Like you said, it’s physics.
We have a two-level, five bedroom new build in Auckland which came with a single-zone ducted system. So outputs are the bedrooms plus upstairs and downstairs lounges. The vents are tidy and unobtrusive - much less space than a wall unit in each location.
We're having major issues balancing the temperature across the different rooms as there is one thermostat.
Think setting the temperature overnight for one room with a couple plus a cat vs another room with one of the kids. Basically the wife and I are always far too hot because otherwise we're freezing all the other rooms. Or setting a reasonable temperature in the lounge makes all the rooms icy in short time.
I'm now thinking about forking out the couple of thousand to get a small heat pump installed directly into our room so we can run it separately from the rest of the house.
So if building from scratch, either look into a multi-zone system or separate heatpumps. If you can get separate systems but hidden in the walls/roof that would be the best.
https://www.amazon.com/AC-Infinity-Register-Thermostat-Contr...
Appropriately placed In-line duct fans and register fans will pull the heated/cooled air where you want.
I have the fan programmed to run 10 or 15 minutes an hour regardless of if the heat pump needs to run. It keeps things pretty even.