Woa, Tippi, you have a serious and expensive moisture problem. It has to be leaks or infiltration of moist air. Is it very humid where you live? Do you have dry indoor air in the winter? My experience with mini splits indicates that potential and existing customers are very confused by the manufacturers' language ca. dehumidification. Yes, there is a dehumidify setting, but there is only one coil in the indoor unit. They can only be biased to water removal over cooling by running the fan at the lowest speed and keeping the coil at the lowest practical temperature. This is a very nice feature and is very effective in humidity control when only a little cooling is needed or a little can be tolerated. It is not as effective as a dedicated dehumidifier when one of those is desirable. The commercial units might have two coils in the indoor unit. I have not seen that, but I am not an HVAC pro. They'd also need two sets of lines, one for each coil. What I know they can do is cool and heat adjacent units in the same room, or in adjacent rooms at the same time with the same outdoor unit. This would be an effective dehumidification device in lieu of a dedicated dehumidifier. BTW, this link was posted in a current thread. I thought you might find it of use: Here is a link that might be useful: Roof in FLW-like house

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 are right, but I had no choice really. Fortunately our climate is fairly mild and the house is very energy efficient. I spent the day doing looking at options in the structure for running ducts to be able to pull return air from down low on the 1st level. I converted everything to sq.in. and started from the supply ducts back. My math shows that I need a 18" return trunk going vertical into the attic for about 20'. Then it Y's into a 14" and a 12". The 14" runs about 8' and y's into two 10"'s. One of those runs 4' into the cieling return for the 2nd floor (centrally located), the other runs 6' into a wall cavity down to the 1st story floor. The other 10" off the Y runs nearly 35' and drops into another wall cavity to the 1st story mst.BR floor. This area was previously landlocked fram any return action by a wall and short hallway. The 1st story has a return ducted into the wall the furnace backs up-to under the stairs. It will be a custom plenum that basically yields a 12" x 12" volume and utilizes three stud bays for another low return. Mathematically I end up with returns on the first story being oversized by about 15%. On the second story I am purposely undersized by about 20%. The only return (10")on the second story si in the cieling. All of the returns on the first story are just up off the floor in stud bay's. The 2nd story has 6-6" supplies and the 1st story has 9-6" supplies. The furnace is a 3 ton Carrier infinity. Am I on track or crazy? TIA

Thanks all, it did seem pretty outrageous to us too but had no baseline to go from so weren’t sure. We are in central CT, so not a low COL but not so high that this doesn’t make us blink. We are definitely getting other estimates. We have been using a portable unit from our old house to keep us cool, it just seems to defeat the purpose of having a central system. We moved in in the winter and the heat seemed to be adequate up there (and it is), the cooling power just isnt there on hot summer nights. The reason the cost of improving or changing the duct work is so high is because a lot of it is asbestos wrapped - so the high environmental/hazardous material disposal fees factor in and drive up the cost. The bedroom is at the end of the run on the opposite side of the house on the second floor, so the recurring theme has been that it’s probably not worth it. We did think about possibly DIY with the Mitsubishi but figured we would get some prices and go from there, thinking that if we had a pro install we had some kind of protection/warranty in case something failed. We do have someone else coming out and deoending on their estimate, will probably reconsider DIY or just deal with the window or portable unit for awhile longer - or forever. Thanks again for the feedback.