How to optimize HVAC ducting
I will be building a new home in Idaho in an area not serviced by natural gas. Most new homes there use electric heat pumps. In brief, super insulated and well sealed 2400 SF with 900 SF attached garage, 750 SF bonus room, conditioned crawl space, vented attic. I’ve never worked with heat pumps or ducting before. I’m uncomfortable turning the design over to an HVAC contractor without specific instructions. Two have told me the air handler will be placed in the garage and return will be through the (unconditioned) attic. Most homes I’ve seen have a single air return vent in the center of the house, which does not sound like a good idea to me. And I’ve seen a lot of ducting that is flexible, probably easy to install but doesn’t seem efficient. Would it be unreasonable to specify placement of the air handler in the crawl space and also specify smooth wall return lines from each room in the crawl space? (Yes, it will cost more.) Also the garage and bonus rooms only need to be kept above freezing and below melting so would it be appropriate to service those with a mini split? I don’t mind paying more up front for long term savings.
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Putting the air handler in unconditioned space and running the ducts in an unconditioned attic is common, but absolutely the WRONG way to go for a high-performance house.
Even if you have to build interior soffits to run the ducts and build out a mechanical room reducing the size of the garage, all of that stuff should live inside the pressure & insulation boundary of the house!. This is more than just an HVAC contractor problem- it has to be designed-in.
But since you have a crawl space, it's probably pretty straightforward to make it a sealed-insulated unvented crawlspace, and run the ducts in there.
Sheet metal ducts with mastic-sealed joints & seams are better than flex ducts, since the have lower duct impedance(they resist air flow less.) Simply not oversizing the heat pump by the typical 2-3x (often necessary to overcome the efficiency problems of having the ducts & air handler outside of conditioned space) means the ducts don't have to be as big either.
Any HVAC plan should come with an ACCA Manual-J compliant room-by-room heat load analysis, and any ducted solution should follow ACCA Manual-D. (This is required by code in CA, but not most other places.) Oversizing the heat pump by more than 1.5x begins to cut into comfort, and oversizing by more 2x begins cut into equipment life. So be aggressive rather than conservative on the assumptions entered into the heat load calculation.
In most ID locations an insulated garage that's reasonably air tight would normally stay above freezing without a dedicated heat source. It's hard to rationalize the cost of a mini-split for freeze control. There may be pretty good ductless solutions for the rest of the house if the loads are low enough, but that can only be determined from the Manual-J.