First off, 4 stories sounds like a big vertical head for residential mini-split equipment just from my casual notice of those requirements. I would check that out first. I am testing a whole-house mini-split installation right now (Mitsubishi, 7 in, 3 out). I will let you know in a year or so if I like it ;-) On paper they are more efficient due to inherent efficiency and the inherent zoned nature. We have been having pretty hot weather and with my wife out of town this week, I have been pretty much been cooling the bedroom and the kitchen with some intermittent cooling in the utility room where the exercise machine lives. Note that I am not much of a sit and watch TV type. You are really a leg-up over lots of others on this decision as you have experienced mini-splits. One thing to think about is that, IIRC, most of them don't have back-up heat so you will not have any heat while they are defrosting. (Maybe none of them have electric resistance back-up, but I think I ran across at least one brand that does.) You also have to pay close attention to cold weather performance for the mid-Atlantic region. I know that with Mitsubishi's current residential equipment, the 1:1 systems have better low temp performance than the multi-splits. They are more efficient too. Even if they do have back-up heat, you can kill the efficiency pretty fast if you need back-up very often. Keep in mind that there are lots of options to the wall-hung indoor units. They cost more, but there are units that can be dropped into ceilings and others that sit on the floor. There are also short, ducted units that would solve that big room problem by splitting the supply and return. (You could also install more than one indoor unit/room.) Condensate needs to be dealt with using pumps. I am not an HVAC or building pro, so take my observations with a grain of salt. After replacing a ducted system with mini-splits, and talking to my install guys a lot, here are some observations and conclusions. Installing a mini-split through outside walls, as is it usually promoted, is pretty quick and straightforward (read fast and cheap) compared to installing a new duct system in an existing house with no central AC. My install was done through interior walls running the lines/cables under the raised house and out to the side which was much more complicated. If I were doing this over and I had a good duct system within a house envelope, I would stick with it. The trouble is, what is a good duct system? More often than not, they are outside of the living space. In addition, they are considered good when they leak 5% of the air and they only get worse as they age. My system was in the attic with parts of the duct system in the attic and partly in a furred-down space over the hallway. The worst part was one duct that started at the AH in the attic, dropped down under the house in a chase and ran for about 30 feet before it reached the first floor register in an addition with a flat roof (thus no attic). It then reduced in size and ran another 25 feet under the house. That part might have been mini-splitted even if the rest of the house retained a ducted system. By your description, your duct system was not installed very well, but if the design is sound, it could be revamped less expensively than installing minis. In general, I think that ducted systems are trickier to design and install than mini-splits so there is more tolerance for less than careful and knowledgeable installers with mini-splits. Due to their ability to handle variable loads, choosing the right size is not as critical either. (I had 4 tons cooling plus a gas furnace before, now I have about 106% of the cooling and 60% of the heating. The heating was way oversized.) My system will handle cooling fine with the house the way it is, but I should not have trouble after I make improvements to limit infiltration and add insulation. My installer is knowledgeable, smart, very workmanlike, and experienced (he has taught the trade for 30 years). He would have installed new ducts and a two-speed system for a slightly lower price than the chosen mini-split equipment. I doubt that he would do it for the same price if doing it on a similar home going through interior walls. He did not have much mini-split experience and brought in a collaborator that does, on this project because he was looking forward to a learning experience. I don't think that he lost his shirt, but I don't think he made as much as he usually does.

You have it. "Evaporator" refers to the inside heat exchanger in a dedicated cooling device commonly called "air conditioning" or "air conditioner". The pressurized liquid evaporates inside it cooling it. Outside is commonly called the condenser because the hot, compressed gas coolant is cooled turning it into a hot liquid then a cool liquid. If we are talking a heat pump, the positions are reversed in heating mode so there is sometimes confusion. I would not worry about an outside unit with respect to dust. You might need to put a regular cleaning routine in place that the average homeowner does not, especially if it is under cover where rain does not reach it. I don't really have the experience to say one way or another, but the cleaning should be relatively easy. Given my personal experience, I would not put a mini split indoor unit in a dusty environment unless the installer would say that it is not a problem and refund your money or install something else if you find it clogging up. I am sure that the YouTube mini split cleaning vids have proliferated since I took up the problem and I have not reviewed the situation for a year or three. They should be worth a look, but beware that if some look much easier than others, they may not be doing a complete job. My feeling is that it is best to keep up with the buildup and it may prevent the need to do more complete dismantling, but I don't know for sure.

weedmeister - Thanks you so much for the help. Just to follow up on that's going on in case you're interested" I also contacted Ecobee support a few days before I posted here. I was worried I wasn't going to hear back, which is why I posted here. But it just took a while. Now I've been going back and forth with somebody helpful at Ecobee. The Ecobee guy sent back the same wiring you did, except that he said I can't double-wire the transformer into the Rc and C contacts as you suggested. I did try wiring it up both the way you and he suggested (although without the Transformer), and confirmed the Mini Split would not provide power to the Ecobee. He then suggested it might provide power it I re-wired it using the "O" wire, but it still did not. At first he said that to use the transformer for power, it would need to be done through an Isolation Relay https://support.ecobee.com/hc/en-us/articles/227874527-ecobee-thermostat-installation-with-an-isolation-relay But he later updated that to say I can't use that method because then the Ecobee would send power out to the Min-Split. I actually don't think this is a problem - my reading of the Dry Contact manual is that that's why there's a "volt" switch in it - it's to set the Dry Contact to be OK with power coming into the Dry Contact from the thermostat. I'm hoping I'll hear back from him confirming that, and I can buy a relay and wire the transformer into the Ecobee that way for power. If it turns out that I really can't do that, then I do have an option of running new thermostat wire up from the oil furnace that so I can get another, powered "C" wire that way. Thanks again. Oh, and MiniSplit Heat, thanks for trying to help, but I don't understand how your comment answers anything I asked in my post.

@fsq4cw - how about a ballpark price just for the air-to-water heat pump? Just out of my own curiosity. Note that due to @mtvhike having a fairly large solar array and the specifics of his tariff scheme, I'm not sure that any significant capital cost to reduce his electricity usage will pay off well. This shows one of the oddities about specifics of solar tariffs. As I understand the way it works is this: his base tariff for electricity is 9.5 cents/kwh. Any solar he generates offsets the electricity he consumes (calculated annually?). BUT: any solar generated in excess of consumption is only paid at the wholesale rate, which is dramatically lower than the retail rate (call it 1 cent / kwh). Right now - without full year data - he expects to consume somewhat more than his panels generate. The upshot is that his savings from reduced consumption are limited (You could analyse this differently but I find it easier to just take what he already has as the base case, I think the conclusions are the same). Let's say, for example, he consumes 4,000 kwh annually more than he generates, for a bit less than $400 (netted out annually). If he saves more energy than that, every 'extra' kwh saved only gets him 1c / kwh. At least unless or until he adds some significant extra load, like buying an EV or putting an aluminium smelter in his garage or something. So: I'd say it makes sense to consider a minisplit primarily for the air conditioning comfort and if it also offsets some of his straight resistance heating, great, there's some modest savings there (within a 'limit' of however much his consumption currently exceeds his solar generation). Spending much additional for the 1c/kwh savings just won't make much sense financially. This is pretty simplistic as right now he has historical data for consumption and the solar array is relatively new so annual generation only estimated, and of course from year to year both may vary. And at any rate, a good argument to not rush into anything with high upfront expenses or to prioritise other projects for the time being.