Jennifer, I am a licensed HVAC technician in MA and an engineer for over 30 years. In stumbling upon your post I joined to urge you to please stop and reconsider what you are doing Monday. There are many ways to provide A/C for a house and the best way to start is to do a heat load for your house. If that has not been done you are being ripped off. The question of vents and dust and moulding can easily be answered with a unit from http://www.unicosystem.com/ These people and a couple of similar companies units are made for this condition and will answer all your questions. Delay the work and call the company directly. Simply put they made for historic restoration jobs, the move less air so smaller vents or different types usually hidden, less air means less dust blown around. Yes they are just as quiet or more when installed correctly and you will be happier! Cost is comparable and if you have not been given this option you need to question if the contractor is the right one for you, he should have offered this to address your particular concerns.

Last sentence: 1) Minisplits control humidity well when the temperatures are not high, but high enough to want a little cooling and dehumidification. A lot of AC systems make a lot of cool, humid air when cycling on and off. 2) What kind of cooling did you have up there before and how well did it work? I can't see anything in your picture. Re. your last post: Cooling works better if placed high on the wall. Heating works better placed low. In a mixed heating and cooling environment any were is a compromise, I am not an HVAC pro so I won't answer your last question authoritatively. The answer will probably be that no one can tell without making detailed observations and calculations. In addition to that, you will have to have some way of adjusting heating and cooling seasonally for a single system to serve a loft like that. With just one set of conditions, it will either be too hot in the summer or too hot in the winter. Check out the possible mini split arrangements on the Toshiba, Mitsubishi and Sanyo web sites.

I doubt this is an issue for fresh air. There is a limit to how much fresh air you can bring in and it is much lower than the output of the air handler (most likely). So you have to use some recirculated air. Second, if there is something in your basement air that you don't want to breathe, then you have a problem independent of the setup. I am guessing the basement is unfinished? Air intakes are at the air handler all the time - I have a new house that is set up that way. So don't worry about that aspect. When I finished my basement, I found that I can meet all of my HVAC needs by just having a return there. It circulates the air enough to regulate humidity. I suspect the basement will get a little cold in the winter and I can increase the airflow but in the summer, closing the vents made the area perfectly comfortable. Hey - if it makes you feel better, I have a newer house with an expensive zoned system and they had to install an extra return in my master because of door slamming. When I have one zone calling for stage 2, there is more airflow than is comfortable. I have been building work arounds for the last 3 years. What I can say is that my bills are extremely low and after the work arounds, it is near perfect.

Can't really improve upon tigerdunes minimun specs above, efficiency performance dehumidification (if sized correctly..)staging electric strips. R-8 should be for all ductwork supplies and returns. All connections should be mastic..not foil or duct tape but paint on mastic and/or mastic tape. load calc to show how much air is needed and how to distribute air.(duct sizing & design) I'd invest in the room by room load calc and see how they plan to size the ducts and lay them out. you don't mention where ahu & heaters will be located. one in crawl space the other in attic? both in conditioned space with ductwork in attic and crawl? one thing to know going in is that sheet metal duct is best under houses. animals love flex duct, while you can usually deal with mice in attics..coons, possums can wreak flex duct. also sheet metal duct ( I call it hard pipe) leaks air at all connections. in addition to screwing duct together all joints and seams should be mastic sealed...before duct is wrapped. duct wrap should be stapled, and taped with duct tape (fsk tape) 80 years ago we built houses for ventilation. tall ceilings, balloon framed walls. to add central air to these homes is to change the dynamic of the house. in my area we usually foam floors with closed cell as it is the most cost effective way to both air seal and insulate the floors. here is a link to LaHouse (LSU Baton Rouge, La) about insulating floors: http://www.lsuagcenter.com/NR/rdonlyres/D33F711D-DC4B-4E4C-9ED6-A97DCE9DB026/79806/pub3187insulatingraisedfloorsHIGHRES.pdf you'll have to cut and paste for it to work.. if ductwork is in attic, another option is to foam the roofline and keep ductwork in a conditoned space. this will minimize problem areas like recessed lights that are not air tight, which allow attic air and insulation particles to enter the house. we use open cell installed in rafter bays and covering the face of the rafters. in my experience with older homes you have to fix the house in terms of air leakage and insulation for the central installs to provide comfort, dehumidification and savings. otherwise you just throw more tons of a/c at a leaky structure. insulating floors and attic would be to your benefit. air sealing the house is a good investment. there are lots of energy raters in Texas. you should talk to a few and get more information before spending so much for a/c. unlike a lot of audits an energy rating unbiased information that provides a blue print for efficiency, recommendations specific to your home and performance & install verification. best of luck.