You say you will use these for cooling as you have the radiators, but I think you will find that you end up running your boiler only when it get really cold. Heating performance on these mshp's is outstanding. I heat and cool my home with Fujitsu's and Rinnai Energysavers. I pulled the entire central system, not because of poor equipment but because of very poor ductwork. I was the rep for Fujitsu for 12 yrs prior to selling my business and moving west. They make great equipment. It is difficult to advise without a floor plan, but as I see it, you have a few choices. You could go with a multi 2, 3, 4 or up to 8 evaporators on one outdoor condensing unit. Those tonnages can go 1.5, 2, 3, or 4 and the interior unit combinations and evaporator styles can mix and match. One thing I will say about the cassettes. Make sure you have service access. I concur with Ionize on the mini-ducted unit. they can work out very well in the transoms over closet doors and in attic areas. I hands down prefer the wall unit and find that they, just like everything else in the house blend in over time. The longer you live with them the better looking they become. You choice, but I think they are the best. I actually did a daily extensive, but informal, study on this. many times if I was involved at the beginning of the job and was told, "none of those ugly high wall units", when, for one rason or another they were installed I would go back after a while to see how they were. The comments were universally positive. Grudgingly so, at times, but people were happy with them. Mini splits are tremendous for dehumidification. I installed a dual system upstairs. I went with singles downstairs. These represent the ultimate in zone control, so we rarely run the upstairs units a lot. Downstairs I run a 25 seer 12RLS and I think it is a 21 seer 15RLS. That to me is the right combination. Very highest efficiency units in the primary living area and combis in the less used areas. Do not have a unit blowing right down on top of the bed. The cassettes can be 2, 3 or 4 way blows so that is adjustable. i'd suggest that you look at fujitsugeneral.com. Go to the Contractors tab and there you can download the entire catalog and they are very good about publishing specs. As to the hi-velocity units. They work. They are installation sensitive in the ducting and they are much less efficient. When the 13 seer mandate went into effect the hi-v folks used an exclusion (confined space) to not have to meet it. In my experience they cannot compare to mini-splits for either comfort or efficiency. As with so many things mechanical, hire the best guy you can find and pay him what he is asking. Having made a lot of site visits on problem installs I can tell you that there is no substitute for "do it right the first time" on these.

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.

That is a very good question. Thanks for asking. I should have thought of that. No, I am not using a good meter at all. I am using a really cheap clamp-on meter. I repeat, really cheap. I checked its performance relative to a multimeter ammeter using a resistive load (toaster). It seemed OK. It is hard to justify an expensive meter for a hobby. Maybe I can borrow a better one. At the disconnect, It would be relatively easy to introduce a conventional ammeter. Would that work better or worse with distortion? Some tables listing the electrical characteristics for these units have a column heading "P/F (%)". I assume that is power factor. I know what that is supposed to mean in practical terms, but I've thought little how the rating is determined. Is it for steady-state operation or does it include start-up, for example? Fsq4cw, would the "distortion and hash" be higher during start-up or at speed? The "P/F" of these units is 95 with one indoor unit running and 97 with 2 or 3. Those numbers would indicate very little distortion wouldn't they?

No one is going to decide for you, sorry. Here is yet another way of looking at it. Your calculations for the hot part of the home is 6400 BTU for 500 sq so that leaves (9000-6400) 2600 BTU for the cooler part. That is 12.8/sq BTU for the part with more heat generators and perhaps the sunny side. If you take the 2600 BTU that is "left over" that is 7.9 BTU/sq for the part of the home that has less heat-generating stuff. 9000 seems like enough. Roadking is right, though, only 9% difference in the minima between the two so it probably won't make that much difference in humidity control at low load. You'll probably be happy either way.