If cost effective is a consideration,I would go for a mini-split now and deferr any change to the downstairs system until it requires replacement or major expense for repair. If cost is not a factor,new is always better and more dependable,so changing out downstairs would be a go but I would still use a mini-split for the office. A ceiling heat lamp will be suffecent in cold weather and a small through wall fan can be added if bath isn't cool enough in hot weather. Zoning is something 85% of technicians don't fully understand. Having a zoned system leads to technicians wasting time trying to deal with it every time they are doing any thing to system from now on.

moving air handler from crawlspace to attic is a big undertaking.

hotter in the attic, dirtier in the crawlspace.

moving units is always a pita & to replace
all ducts...that is a big job, with a hefty cost.
you posted that the unit as is operates almost
flawlessly...kinda hate to mess with that!

you don't know that once you take ducts out of
cool crawlspace & put them into the hot
attic that you'll have the same performance
as before. and its doubtful...given the extreme
heat in the attic.

is there anyway to put ducts into the conditioned space
by building furdowns? that would be ideal, but
depends upon layout of the house as to if it
can be done or not.
can you locate the ahu in a central closet inside
the conditoined space? the r/a would be at the
bottom of the closet.

kudos for putting the sheathing on the walls of the
room above the garage. did you diy it, or hire it out?

mini split would be faster and probably cheaper than
moving equipment & ducts.

its a tough call...if you could put it all inside the
living/conditioned space instead of the attic...
it would be worth the extra cost because of
energy savings.
personally...I think I'd look at that option over
putting system & ducts in attic.but layout of
house is what will determine if you can
do this.

best of luck.

If the office space is going to be used while the rest of the house is not occupied, I think that a mini split is a good thing to do. Why condition the whole house for use of the office only?

You can install a mini spit ducted air handler and run ducts to two rooms. They will probably cost more and are not as efficient as the high wall units. The question is, do you have a place to put it inside of the house envelope?

I have mini splits with seven indoor units. I wrung my hands over whether I would miss having the ducted system running to the baths. I am in a predominantly cooling environment, Gulf of Mexico coast. I don't miss the cooling in there in hot weather. It is a bit cool in cold weather.

Thanks for suggestions folks....

klem1, I agree that it seems that a lot of techs do not truly understand a zoned system.

energy rater la, thanks for your words of wisdom. I guess I could install fur downs (didn't even know what a fur down was...had to Google it), but it seems like it would be quite a hassle. I do have a place I could move the ahu to, inside the house envelope. It'd require me to purchase a new ahu however since my current one is not designed for a vertical application (needed for the space I've got). As far as installing the polyiso, I did that myself...man what a pita!

ionized, would you happen to have a link to a mini split ducted air handler somewhere online? You mentioned that you have mini splits with seven indoor units (that must be one heck of a big house you have there). Did you get the ceiling mounted condensers or the ones that mount on the wall? Also, how do you like this option?

If you do a search on mini split (mini split, mini-split) and Sanyo or Mitsubishi, you will easily find info about alternatives to high wall mounted equipment. I have 2000 sq feet near New Orleans so it is hot and humid here. I have seven rooms with one unit, 6K or 9K, per room. I have high wall, but I wish I had put a ceiling recessed in the smallest room (dining room) and it is obtrusive in there. All of the associated plumbing and cables are run in the stud space and under the house rather then trough exterior walls. All but one are mounted on interior walls.

I like the way my system works for the most part. I would not go back to ducts. There are a couple of drawbacks. I can not do a true set-back with the remotes, only a timed start/stop. Can't do a hard-wired t-stat either. For the heating season, I would like a real set-back. We don't do much heat here so it is not such a big deal. I believe both of these things have been changed for Mitsubishi high wall and you always had those options for ceiling recessed and ceiling hidden.

One thing that came out of the sun that I should have thought of is that maintenance is more time-consuming. I have to keep seven filters clean rather than swapping out one dispo filter now and again. It is not a big deal though.

In heating season they high-wall units turn into good ceiling heaters in the default programming. The blower slows way down as they approach set point. A ceiling fan or setting the air handler manually to a higher speed fixes it. Keep in mind that I have an open crawlspace and a poorly-insulated house that I am working on.

Some of the benefit is lifestyle-related. It is just my wife and I so we just condition the rooms that we are in. I like the way it throttles back. Humidity control is great.

It is easy to run parts of the system on portable generators. At low speed inverter-driven motors are very efficient compared to other types of variable speed. I think that the start-up surge is way less too. I was running two outdoor units with one indoor unit each plus a refrig and chest freezer and a couple of lights on a Rigid Rd6800 genset. I went for a couple of days and I was considering trying to add an additional indoor unit to see if it sagged, but the power came back on.

Sounds awesome ionized....I'm definitely going to look into these. Thanks so much....