He is using one 36,000 Condensor for all three units. I am not sure if he has done a manual j heat load calculation. Can I do that on my own?
By all means you MUST do a (room by room) Manual-J prior to installing any equipment! If you oversize a ductless head for the space it will cause more comfort problems than undersizing it (!), and getting all three zones reasonably well balanced with their design loads is essential for multi-zone ductless systems. Undersizing can create it's own issues too, but with multi-zone units undersizing one zone and oversizing others can create significant comfort issues. Getting to the "right" cooling or heating solution calls for a room by room, zone by zone load calculation.
My concerns in this instance is that 18K might be right-sized or somewhat undersized for the Great Room, and a 9K head is almost always OVERSIZED for a bedroom, even a master bedroom. (Indeed even 6K units are usually oversized unless it's a large room with a large amount of window area.) If the 18K head is right sized or slightly undersized it'll still be comfortable there, but whenever ANY zone is calling for cooling there is refrigerant running in all other heads, even those that are nominally "OFF". If the head is way oversized for the actual zones in that room it will collect condensation on all surfaces inside the housing of the head, and when it turns on it will spit water for several seconds on most cycles. In the extreme cases just the refrigerant running through the head (no blower running) can overcool the room.
Neither of these are perfect tools, but try either
LoadCalc or
CoolCalc freebie online Manual-J-ish calculators for determining the room loads. The output numbers of those tools will almost always be wrong in absolute terms (usually overshooting reality) but it's the
proportion of load size to ductless capacity size is what matters. If the tool says the 18K head can only delivers 75% of the calculated load to the Great Room that's fine, as long as the 9K head only delivers 75-100% of the load calculated for the Master Bedroom. What is more likely a 9K head will be capable of delivering 200% or more of the calculated load, which means it will cycle on/off a lot making it prone to the water spitting problem, but if it's 300% or more there is a strong chance that it will overcool the room even without running the blower.
If it's a big wide open kitchen off an archway to dining area it might be able to handle a 9K head on a multi-zone compressor, but it's a <200 square foot kitchen doored off from the rest of the floor, maybe not. Kitchens have high peak loads from internal heat sources, making them harder to get right.
Multi-zone ductless systems have high minimum-output levels on the compressor unit, and unless two or more heads are serving a significant load it's going to do a lot of on/off cycling which eats into both efficiency and comfort. It's often better to use multiple single-zone units, which generally have larger turn down ratios between maximum & minimum. For bedrooms a single zone 9K head that can throttle down to under 2000 BTU/hr would still be fine, whereas the minimum output of a multi-split compressor is usually well north of 6000 BTU/hr, and some won't go under 12,000 BTU/hr, 3-4x (or more) the 1% design cooling load of a typical bedroom.
Bottom line- the "ductless head for every room" microzoning approach is great for marketing (and for the contractor's ability to make their boat payments), but usually bad for actual comfort and efficiency. Combining the loads of 2-4 small-ish bedroom/den/home-office loads and right-sizing it with a "compact duct" cassette with a few short runs of duct is usually more efficient AND more comfortable.
[edited to add]
Both Mitsubishi & LG have some pretty good cold-climate single zone 9K units that efficiently modulate down to the 1000-1600 BTU/hr range in cooling mode, with max-output in the 11-12,000 BTU/hr range (higher max capacity than the "rated" capacity at which they were tested for efficiency.)
A good place to compare ductless capacity and modulating range at a few different temperatures (notably 82F- relevant for NH style 1% design temps, but also 95F, AHRI test temps for SEER efficiency) as well as heating performance at +5F is to use the
NEEP product search page. If you open up the "details" in a new tab it'll save some time re-selecting the filters, but the typical output page looks like this one, the single zone 9K
Mitsubishi FS09. Note the very high COP 6+ performance at minimum 1882 BTU/hr @ 82F and nearly 5 @ 95F, which makes it a real winner for bedroom loads.
The same FS09 head married to an MXZ multi-zone compressor won't go that low though unless there is at least 5000-6000 BTU/hr of load on the other zones.
