I'm finalizing a design on a 2600 sq ft ranch located in Iowa. In the end it will have a full mostly finished basement. ICF wall's with roof trusses. The basement and garage will have radiant heat in the floor. I'm leaning towards using an above floor radiant option on the main floor. My initial thought was to use wall mounted mini split's for cooling and not install any duct work other than for a couple HRV's. I'm a little concerned that the wall mounted minisplits would be way oversized for the bedrooms which is leading me towards a combination of ducted mini's for the bedrooms/bathrooms and wall mounted for the open living room/kitchen area. If the mini split idea was going to work, I was going to use an 8' basement and I joist floor system. Where is the best place to run AC only ducts? I know it is commonly done in the attic. But I'm wondering if it would be wise to spend the extra money on floor trusses and run it below the floor.

I would recommend avoiding ducts in an unconditioned attic at all costs. It is always best to keep the ducts in the conditioned space and you can use soffits or plenum trusses to accomplish that. They also make HRV units that don’t require any duct work.

Yes, I would recommend using an above-floor HR floor heating emitter and avoid anything that puts the PEX below the subfloor. Better performance HR floor heating emitters get the PEX as close to the floor surface as possible. The lowest cost and best performance HR floor heating emitter is a standard concrete slab that you are already pouring and just place PEX in it. Next best is a thin lightweight concrete slab (e.g., Gypcrete, etc.), but the floor structure needs to be capable of supporting the additional weight. Performance is further improved by keeping the R-value above the PEX to a minimum (e.g., avoid carpet, wood floor, etc.) and/or placing proportionally more insulation and R-value below the PEX.

You will first need to accomplish a room-by-room heat gain/loss analysis (e.g., ACCA Manual J8 or ASHRAE) before you can really design a HR floor heating system or sort out the size and location of mini splits.

What sailawayrb said- run the ducts in the basement, but run a careful but aggressive Manual-J on it first. A single 1.5 ton Fujitsu 18RLFCD may be adequate for cooling the whole house, not just the bedrooms. They have the beefiest mini-duct cassette in the industry and can handle much longer duct runs than the competition. The cassette can also be mounted vertically (unlike the competition), which can also make for a compact, easy to service "utility cabinet/closet" if you can give up 10 square feet of floor area.

An outfit in the Atlanta area compiled this chart of square feet of conditioned space against cooling load from several dozen Manual-Js they had done for clients:

https://www.energyvanguard.com/sites/default/files/styles/panopoly_image_original/public/square-feet-per-ton-air-conditioner-sizing.png?itok=vsJxOobH


The center of the cluster in the 2600' range is 1200-1300' of conditioned space per ton, so odds are you'll come in around 2 tons, maybe a bit less, maybe a bit more. The 18RLFCD is "rated" as a 1.5 ton, but is good for about 20,000 BTU/hr or 1-2/3 tons. They make 3/4 ton and 1 ton versions too, if it needs to be zoned or a single isn't quite enough. A pair of the 3/4 tonners is good for 24,000 BTU/hr (=2 tons).

http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/18RLFCD.pdf

http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/12RLFCD.pdf

http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/9RLFCD.pdf

Another factor where Fujitsu has a leg up on the competition in mini-ducted solutions is heating capacity. Even though they don't have the pan heaters and associated controls for dealing with potential defrost ice potential, this series uses the same type of compressor that their cold climate units have, and are quite efficient with a lot of capacity even at 0F. At their minimum temperature in the extended temperature capacity charts, at 70F indoors they can put out over 14,000 BTU/hr, over 15,000 BTU/hr, over 18,400 BTU/hr for the 3/4 ton, 1 ton, and 1.5 ton respectively. The capacity charts are in chapter 6 (starting with cooling capapcity charts on p.13, PDF pagination.)

http://bangorwinsupply.com/wp-content/uploads/2014/02/Single-Ducted-Design-Technical-Manual.pdf

Unless you're building the crummiest new ranch house in Iowa a pair of any of those, or one xxRLFCD and a wall or floor mounted cold-climate mini-split heat pump (not a cooling only model, not a warmer climate heat pump model) for the open section would be able to heat your place too if the boiler ever failed (or is running off pricey propane.) I know of at least one high-performance house in northern Minnesota using a single 18RLFCD for space heating & cooling, as well as a handful of houses in more temperate California. (I know of several houses in northern New England heated & cooled solely with cold climate ductless mini-splits in locations that regularly hit negative double digits F.)

These pics are from a house in Berkeley California with a vertically mounted 18RLFCD built into a "utility cabinet" by the back door:

https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg

https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg?w=800&h=1200

The big grille is the common return plenum rather than ducting the returns separately, using jump-ducts into the hall for the returns. With floor registers on the opposite side of the room as the supply registers you could use joist bays as jump ducts to a common return if installing it similarly in your basement.

In this retrofit they soffited the ducts running down the hall rather than poking it up into the attic. In a new build with a full basement you could run it between joists, as needed, keeping the head-bonker soffited sections to a minimum. Using hard duct rather than flex you could also go shallow and wide on any ducts that had to run below the joists rather than between them.