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.

Smaller units will require a smaller backup genset to run one at a time if you want to do that. (Inverter units are already very good with gensets, btw.) As others have pointed out, efficiency may be higher with the 1:1 systems. Some mini splits can cool and heat simultaneously transferring heat from one room to another, but these are rarely seen in residential situations because they are more expensive. It may only be available in systems with more indoor units. There is one downside to heat/cool units if you only need cooling. Complexity is increased meaning more things to break. I don't know if that is significant. I had a reversal valve fail under warranty which cost the installer, but not me. Note that if you have oil heat, the heat pumps may be a less expensive way to heat. Two 1:1 systems means another compressor unit sitting outside. If I had all 1:1 systems, I'd have seven compressors sitting on pads :-( You should know that the better mini splits have inverter-controlled compressors and variable speed blowers. This combination increases efficiency and comfort by stabilizing temperature and improving humidity control. Now consider this. A single unit that can throttle back to 30% of 6000 BTU can run at a 2000 BTU minimum. If you have an outdoor unit that can run 12,000 BTU your minimum becomes 4000 BTU. I believe that the smallest multi Mitsubishi compressor unit is 20,000 BTU so in this example, its minimum might be 6,000 BTU so you gain little from variable speed unless you are running both units at the same time. This would be enough to make me go with 1:1 in your situation despite "slight" increased cost and another unit to maintain. In addition to those arguments, if the price is similar, but you are running one compressor at a time most of the time, you get more longevity for the compressors.

"What size system would you recommend?" That all depends. Full capacity heating on your Manual J seems to indicate 3-ton. Existing duct system seems to indicate a little over 2-ton. Installing a 3-ton system could be noisy and possibly result in the heat exchanger in the fan coil unit freezing up if sufficient air cannot be moved over it. 3-ton may also result in poor dehumidification and short cycling. Duct modification may be required to handle the additional cfm of a 3-ton system. Sensible cooling seems to indicate 2-ton. You could install a 2-ton geothermal unit and be fine based on the sensible cooling load. This would mean that you would be OK for about 95% of the time in heating mode during winter. The other 5% to be made up by supplemental resistance backup heat. Remember, geothermal heat pumps have no defrost cycles ever. Supplemental or aux heat comes on in addition to the HP not instead of the HP so that the backup is on for relatively short periods of time even during that remaining 5% of the winter. This means that the smaller geothermal system will be OK in cooling, with better dehumidification, provide heat almost all of the winter, be a better match to your duct work and cost a lot less to install in that you will require one less borehole and smaller equipment. On the other hand you could install a 2-1/2 ton or 3-ton 2-speed compressor geothermal HP and be assured of full capacity heating 100% of the time. You should install a variable speed fan coil unit with stage resistance backup on any system. SR

For example, these are the questions that would be asked for the exterior walls of a house. Two identical houses that face in different directions would have two different losses, because of windows/doors facing different directions. Walls: (Each exterior wall, on each floor, must be done individually) Any windows and doors in a wall must be included with that particular wall. A rough drawing of each wall can be supplied, if easier, showing windows and doors. All measurements, in feet and inches for walls, windows, and doors, must be included for the length, width, or height. What direction does the wall face? O North O Northeast O Northwest O East O East Northeast O East Southeast O West O West Northwest O West Southwest O South O Southeast O Southwest O A: Wood frame, with sheathing, siding or brick Wall cavity O None O R-11 3 ½" O R-13 4" O R-19 5 ½" O R-27 7 ½" O R-30 8 ½" O R-33 9-10" Select exterior insulation O None O R-1.3 ½" Asphalt board O R-1.8 ½" Beadboard O R-2.5 ½" Extruded poly board O R-2.7 ¾" Beadboard O R-3.6 1 in. Beadboard O R-3.8 ¾" Extruded poly board O R-5.0 1" Extruded poly board O R-8 Sheathing O B: Masonry, above grade Select insulation value O R-5 1" O R-11 3 ½" O R-19 5 ½" Wall construction  no added insulation O 8" or 12" block O 8" block and 4" brick O C: ICF Insulated concrete form, above grade Select ASTM R-value O R-12 to R-14 O R-14 to R-16 O R-16 to R-18 O R-18 to R-20 O R-20 to R-22 O R-22 to R-26 O R-26 to R-30 O R-30 to R-36 O D: SIP Structural Insulated Panels O 4 ½" panel O 6 ½" panel Exterior finish O Stuco or siding O Brick O Split logs on both sides O E: Logs Circle the log thickness: 6" 7" 8" 9" 10" 11" 12" O F: Block or brick, extends to 5 below grade Select insulation value O R-5 1" O R-11 3 ½" O R-19 5 ½" Wall construction  no added insulation O 8 or 12 inch block O G: Block or brick, extends over 5 below grade Select insulation value O R-5 1" O R-11 3 ½" O R-19 5 ½" Wall construction  no added insulation O 8 or 12 inch block O H: ICF, extends to 5 below grade Select ASTM R-value O R-12 to R-14 O R-14 to R-16 O R-16 to R-18 O R-18 to R-20 O R-20 to R-22 O R-22 to R-26 O R-26 to R-30 O R-30 to R-36 O I: ICF, extends over 5 below grade Select ASTM R-value O R-12 to R-14 O R-14 to R-16 O R-16 to R-18 O R-18 to R-20 O R-20 to R-22 O R-22 to R-26 O R-26 to R-30 O R-30 to R-36