I'm thinking of using ductless mini-splits in a new home built with ICF walls and concrete roof. I've been told that radiant heat is not necessary in a home such as this in S. California. The 3600 sqft, one level home will have central HVAC system in one half the home and ductless in the other half. If i over size the 2 zone ductless mini splits (2 air handlers in separate rooms on opposite ends of the conditioned area), would it be possible to cool/heat smaller adjoining spaces (a bathroom or hallway)? Or am i better off just going with 2 central ducted systems?

Run a room by room, zone by zone Manual-J. Size the minisplit for the zone, not just the room that it's in. In a higher-R house heat/cool distribution to adjacent rooms is fine with just an open doorway. For crummier houses it takes an open archway, but still works pretty well.

Fujitsu's ducted xxRLFCD mini-splits are very flexibel and fairly easy to design with. There are 1200-1500' ranch houses in the SF Bay area heated/cooled with a single 1-ton or 1.5 ton Fujitsu.

Thanks Dana. Ducted mini-splits seem like the perfect solution. I've only got about 12" of joist space to work with so hopefully a ceiling unit will work.

Posted By minotto disegno on 16 Dec 2018 02:02 AM
Thanks Dana. Ducted mini-splits seem like the perfect solution. I've only got about 12" of joist space to work with so hopefully a ceiling unit will work.

Ceiling cassettes need custom framing to fit- even 24" on center joists are too close.

A mini-ducted Fujitsu can be mounted vertically into a tiny 8 square foot "mechanical room" closet for easy access, and ducts can be run between joists for most or all of the run with minimum soffited runs below the ceiling gypsum/insulation level. On upper floors a "plenum truss" could be used to mount both the ducted cassette & ducts, using long-throw ceiling registers for better air mixing.

https://www.energyvanguard.com/blog/73245/Go-Up-Go-Small-and-Get-Those-Ducts-Inside-Conditioned-Space

The plenum truss approach works pretty well in better-than-code construction such as ICF.

ok thanks again.

I will echo what Dana1 said about minisplit sizing in ranchers. I was ready to go geo but at the same time renovating my house. I am not even done with the energy efficient retrofit and the high efficiency LG ductless I put in my living room (we were getting rid of the gas logs due to the people only wanting to deliver for a certain size tank that we had to rent on a set interval) with the intent to minimize electric heat strip backup. In reality we have pretty much eliminated any other heat source needed when it is above 40f outside. In the summer the thing easily cools the entire first floor which is about 1000sft. This winter I tried something different and switched the downstairs 2 ton heat pump on several degrees lower than the split. If the split gets overwelmed the first floor heat pump picks up the slack. This is helpful because the second floor heat pump hardly cuts on now since the heat flows up the master stairway. We are going solar as soon as the permits are approved and we broke even with the projected solar production and KWH usage in December. That means solar would have saved us $40 in the month of december.