I am in the very early stages of planning a building (about 3,600SF) for employee housing. 6BR with private baths plus common kitchen/living space. We are exploring a very efficient prefab system with near Passiv House stats (R35 slab, R50 walls, R80 Roof and 0.6ACH50) This is located in zone 5 but more like a Marine Zone 4. Standard equipment in these houses is resistance base but we have electric costs of $0.25/kWh. With our climate some summer humidity control would be welcome too, though sensible heat is usually not bad. If I installed an unzoned system with six unrelated occupants and no benevolent thermostat overload I fear constant discomfort. Is my problem solved with 2 zoned mini splits (one 4 zone for 4 BR's and one 3 zone for 2 BR's plus common space). Would this be grossly oversized and fail to control humidity without over cooling? Would it be grossly oversized for heating too? Thoughts?

Without the actual load numbers, design temperatures and model numbers for the multi-splits there is no way to even guess if this approach will be good enough.

As I said I am in the very early planning stages and so trying to gather preliminary info to make some basic design choices. I'm working from the very general towards the specific, so specifics are scant. The design temp I have used in this location is +5F. During the heating season our dry bulb temps are moderated by ocean temps, however our average wind speed is about 15 - 16 mph so infiltration loads can be huge if air sealing is shoddy or neglected. Given a 3600 SF occupied space with the high R envelope noted I'm guessing our peak design heat loss will be on the order of 25 to 30 Mbh. A guess is all I have at the moment. For cooling, peak design temps would be approximately 95F but with an RH in the 85% to 95% range so high latent heat load. (Due to the high electric cost and moderate temps and constant ocean breeze most homes here have no cooling at all - just "double hung thermostats")
Looking at Mitsubishis the smallest four zone unit I find is a MXZ5C42NA2 4-SLZKA09NA4 which is a 36,000 Btu unit with four 9,000 Btu ceiling cassettes (again all new construction so cassettes seem more appropriate than wall mount) That could serve four of the six BR's. The remaining 2 BR's and common space could be served with a MXZ3C30NA2 3-SLZKA09NA4 which has 3 9,000 heads for a total of 27,000. These were the smallest Mitsubishis I could find (not married to Mitsubishis but have not had good luck with LG's). With a total capacity of 63,000 Btu for a load of less than half that am I dangerously oversized?
Because the BR's need individual control (way more critical for heating than cooling) another strategy might be to try to maintain a "baseline" space temp (say 65 for example) then provide small resistance base in each room to allow the last few degrees to be adjusted by the occupant. Resistance is cheap to install so perhaps this would allow the whole building to come off one "right sized" ASHP.
Dana, your input, insights and generosity on this site are huge and greatly appreciated - many thanks. I'm hoping this is an intriguing conceptual design exercise!

Where to begin?

First, specifying the HVAC solution before even the preliminary envelope design is shaping up and preliminary load numbers known is folly. That said...

The minimum modulated output of a multi-split is usually the max output of a half-ton head. In a high-R house (or even a code-min house) the design heat loads of an individual room is well below the capacity of a half ton head. So the "head per room" approach is going to short-cycle both the compressor and heads into net-LOUSY efficiency, well below the nameplate HSPF/SEER of the system. Most multi-splits are NOT full VRF (variable refrigerant volume) systems, with either limited (or no) modulation of the heads themselves. The minimum modulated output of the MXZ5C42NA2 is 7200 BTU/hr @ +47F which could easily be more than the whole-house load of a near-PassiveHouse 3600' house @ +47F. The MXZ3C30NA2 min-output at +47F is even worse, at 11,400 BTU/hr. Together that's probably going to be where your whole house load numbers come in at +5F, which means these compressors will NEVER modulate, and only cycle! Cycling the heads is bad enough for efficiency, but you're looking at cycling the compressors too. Whatever the nameplate HSPF numbers are, if you're short cycling the whole mess you won't get an as-used HSPF of more than about 7, 7.5.

Most single zone mini-splits modulate and track load at high efficiency, but even those have limitations of how low they can go. The Mitsubishi FH06NA half ton mini-split is one of the best in class, but can only throttle back to 1600 BTU/hr @ +47F. If the design heat load of the bedroom at +5F is 1000 BTU/hr (it'll probably half that at PassiveHouse levels unless the windows are big), the load at +30F will already be below the modulation range, which means it will be cycling on/off most of the season at a very low efficiency-eating duty cycle rather than modulating at very high efficiency.

BTW: If this is Block Island the 99% outside design temperature is +16F. The 99.6% design temp might be as low as +5F (in Providence the 99.6% design temp +7.2F in the most recent ASHRAE 25 year database) but irrelevant for a high-R house. See:

https://higherlogicdownload.s3.amazonaws.com/ACCA/8e4cf5b4-e984-4971-bb79-7889082c7cf2/UploadedImages/Outdoor-Design-Conditions-1.pdf

https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf

In a high-R house room to room temperature differences are small. How critical is individual room temperature control, really? Heating the whole house to 70F with a system that's modulating most of the season will use dramatically less electricity than short-cycling a multi-split into low efficiency keeping it at 65F.

A better approach (once you have the room by room load) is probably to use a pair of mini-ducted mini-splits. The Fujitsu -18RLFCD can modulated down to 3100 BTU/hr @ +47F, but can deliver 21,600 BTU/hr @ +17F. A near-PassiveHouse 3600' building would probably have a load of less than 20,000 BTU/hr @ 0F for the whole building so a pair of them (or the 1-ton versions) would likely cover the whole load at temps well below the 99th percentile temperature bin. And a pair of them would have a minimum modulation less than just one Mitsubishi multi-split.

http://portal.fujitsugeneral.com/files/catalog/files/18RLFCD1.pdf

Fujitsu's mini-duct units are easier to design with than most of the competition, due to the more powerful blower motors, but also because they can be mounted vertically in a very shallow easy-to-service mechanical closet/room. A short plenum to soffited ducts (or ducts in joist bays, where appropriate, but NOT panned joists) isn't that hard to pull off. Take a look at this retrofit installation heating a whole (not superinsulated) house in northern CA:

https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg?w=800&h=1097

https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg?w=800&h=1097

You'll still need to keep the duct runs short, and use reducers at the register-boots and long-ish throw register grilles. A Manual-D duct design and balancing vanes can be used to keep the room-to-room temperature difference reasonably well bounded, and individual room flows can be tweaked to taste by the occupants for individual room temp control.

Note: Most mini-duct cassettes have a wired remote that is often mistaken for a thermostat. The Fujitsu (and other) mini-duct cassette default mode is to control based on the incoming return air temperature, but can be programmed to use a sensor in the wired remote, making it behave like a thermostat.

But even this might be overkill for the actual building. You really need some sort of load calculation for reference to know where to start, even if you known the actual load numbers are going to move around a bit as the details of the building get nailed down. Right now it's really nothing more than a WAG.

The sub-20K WAG I threw out for the whole house heat load is actually ~2x the design limit for a true PassivHaus (Darmstadt, not PHIUS). A 3600' PassivHaus is allowed a maximum peak load of 10 W per square meter, which works out to ~3.2 BTU per square foot, which would make that 11,520 BTU/hr for the whole house. They measure the square footage differently than the US real-estate convention of including the wall thicknesses and all interior space, but the real estate measurement is usually about 15-20% bigger than PassivHaus methods. That would make the max PassivHaus-legal load about (0.85 x 11,520=) ~9800 BTU/hr, so call it 10K. PHIUS PassiveHouse allows different peak loads depending on location. On Block Island they would allow 4.2 BTU/hr per square foot (yes they have a spec for that location- zoom in: http://www.phius.org/phius-2015-new-passive-building-standard-summary ). So you'd really be allowed about 13,000 BTU/hr under PHIUS rules. If you don't really design to PHIUS levels everywhere it'll creep up some- under 20K is probably still realistic, but if you don't run some calculations you'll never know.

In houses of that performance levels individual bedroom loads are so low they're essentially heated by the occupants if the occupants are awake and standing (but maybe not fully heated by a sleeping occupant, at the 99% outside design temp.) So if you're really building something of comparable performance, one or two mini-splits serving somewhat centralize common areas might be the right approach. A 3/4 ton Mitsubishi FH09NA can deliver 10,900 BTU/hr @ +5F and modulates down to 1600 BTU/hr @ +47F. A pair of them would deliver 21,800 BTU/hr (way more than the 13,000 BTU/hr PHIUS limit) and would modulate down to 3200 BTU/hr out and cost a heluva lot less than a couple of multi-split Medusas or a pair of mini-duct Fujitsu units.

And THAT is why trying to select HVAC equipment without load numbers a waste of time (and usually money too!)

BTW: As luck should have it, today's GBA blog touched on the issues of multi-split inefficiency, and how even the smallest mini-split heads are oversized for individual room loads (even in code-min houses):

http://www.greenbuildingadvisor.com/articles/dept/musings/bruce-harley-s-minisplit-tips (you'll probably have to sign up for a trial membership to get by the paywall issue)

Dana, As always thanks for your response and sharing your spot on insights in such detail. I am trying to fast track this project and therefore I'm exploring stuff on a lot of parallel tracks, hence the consideration of heating systems without a full data set on the building envelope. Not ideal but, I need to start somewhere. Without being too facetious, the way to accommodate individual room temp preferences might involve a trip to Yankee Candle. Thanks!

There are a couple of aspects to this. Codes in the US usually require that bedrooms be capable of being heated to at least 68F at the 99% outside design temperature in an automatic way.

That COULD be a mini-split keeping the common areas on the other side of the partition walls 73-75F or so, and if the numbers imply that that approach to compliance works, that's the way to go. A room with an R50 wall and 10 square feet of U0.20 window with about the same square feet of R4 partition wall as exterior wall, when it's +5F outside and 75F in the common areas a small bedroom will stay above 68F when occupied by one sleeping human emitting 200-250 BTU/hr. (That worked well for a DER I was involved with in Worcester MA a handful of years ago.)

For those who would truly suffer if the room temp dropped to 68F on the coldest hours of the coldest nights of a year (say, a visitor who has only lived in tropical countries), making a small finned radiator-type space heater available would more than cover the room load. Electric blankets could help too. Those for whom 68F is too sweaty for sleep can always crack a window.

That might preclude trying to get a little STC improvement with rock wool in the interior walls..... Privacy and sound kind of eliminates a transfer grill
Perhaps a tiny resistance wall mounted heater on timer would work to add the last few degrees.
Each room will have a bath, which will have an exhaust fan (also on a timer) so some make up air provision will likely be needed too. (my own house is very tight and any one of the dryer, central vac, range hood or bath fans will pull air down the chimney and through the wood stove)
I'm sure this can be finagled since we're looking at tiny numbers.....

Did you ever consider panelised walls for youre project

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