I know a few of you are following my addition build and most of the work being done by me. I have done a load calc and at around 5 deg f I have gotten the 1200 sft space down to around 11000 btu heat loss. I have decided on the 30.5 seer mitusubishi .75 ton unit for the 900 sft open room on the second floor. The room on the first floor will simply be attached to my existing first floor rheem 2 ton unit. Looking at these units I want to come up with a plan to phase out my traditional heat pumps before their service life is up. I looked at the mitsubishi multi splits but their seer ratings top out at about 18 and they do not have hyper heat. So I have options. 1) slowly place individual hyper heat units in individual rooms in my house. 8 in total would completely replace my traditional units. 8 different out door units would be excessive but more efficient and easier to manage replacing individual zones in the life of the home. Also with h2i i would never need split heat. 2) do 3 or 4 30 seer units for main rooms and do a multisplit to finish it up. I still wouldnt need strip heat since even the 6k wall units would be oversized for the smaller bedrooms. 3) This is where I am a little foggy. I am seeing these city multi units with h2i. The smallest is 6 tons but it is a dc inverter just like the other ones. I could run all the indoor units off one but I can not find them for sale and most are 3 phase power. 4) I saw on Matt Ricengers video blog a 4 ton residential multisplit with h2i. This would be great but I also can not find these anywhere or even the specs. Opinions?

Do you really need units for every room? We're running our whole house (1400 sq/ft) on a single 12K mini-split, and as long as you keep the internal doors open, the whole house is very comfortable, and stays within 1 degree F. Granted, this is a new ICF home, well detailed, and R-50 in the attic. We have two units, and found the 9K in the MBR works OK alone, but can't seem to manage the humidity as well as the more centrally-located 12K.

I figure I will need one for the master bedroom and maybe one per each bedroom someone will be staying in. I can probably get away with maybe one in the kitchen and one in the living room. One in the second floor of the addition and one in the shop. So, maybe 6 at minimum. The existing house is 2x4 construction with R11 in the walls and 1/2 inch foil faced polyiso on the exterior instead of plywood. Until I replace the R11 batts I assume I have an absolute zero chance of not needing a head in each room that can be closed. I could probably do a couple of hidden short ducted units to combine zones but I think the efficiency hit would not be worth it. Now, if I can finish foaming the existing attic when I do the addition and redo some walls then it is a different story.

Been doing a bit of research tonight and also came across a small problem with the plumbing in my kitchen which will give me the excuse to replace the drywall when we eventually remodel as well. I am looking at maybe 3 of the stand alone single zone units for most of my heating and cooling. I could then get maybe a 4 zone multisplit for the remainder of the house. I would keep the temp on the heads a couple of deg above the higher efficiency units and they wouldn't kick in until they load had been reached on the higher efficiency units. I still wonder what the practical difference would be between a VRF 18 seer multi split and VRF 30.5 seer single split. I am thinking the numbers might lie in Virginia weather since both are only rated at 100%. I would be interested to see what the actual efficiency difference is in a VRF DC inverter that is over-sized by a ton or two and running at only a fraction of their capacity 90% of the time. Then on those random 105 deg day or 9 deg night once or twice a year reaching 100% efficiency.

One ductless head per room is extreme overkill- to the point where they won't work in an efficient range. If the design load (heating or cooling) is less than about 2/3 the rated output of the ductless head you won't hit nameplate efficiency on the unit. The smallest ductless heads out there are about 7000 BTU /hr (cooling or heating) so it has to be a fairly large or fairly lossy space to really work well.

A 3/4 tonner for a 900' space will probably have it running in an efficient range, but not for a 250' bedroom.

Mini-duct cassettes can be set up to split the output between 2-3 rooms, and can be more reasonably sized for the cumulative loads of those room, and will run MORE EFFICIENTLY than 2-3 smallest-of-the-line 3/4 ton H2i units.

Awesome info as always Dana. You might have saved me quite a bit with this. My fear is the efficiency loss with a small ducted unit. I read where they top out at 16 seer. At the same time I also read where they are easy to integrate with an HRV, the efficiency loss is less if they are in a conditioned space, and SEER is really meaningless with VRF. I know the wattage rating alone on one of my 2 ton outdoor units is greater than a mitubishi 4 ton at 100%

I guess I should really get to it and finish the insulation upgrades first before considering replacing my entire heating and cooling system. For now I will probably shoot for one 30 seer VRF in the addition, since there is no hvac in there now, one in the living room, and maybe one in the master. Three largest rooms in the house and would probably at least eliminate the need for strip heat in the winter and cool most of the house in the summer. After that I can probably reevaluate when it is time to replace my current units. A three or four zone mitsubishi hooked up to a couple of ducted units might be all that is needed.