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3-zone minisplit, 2 outdoor units

Matthew_M | Posted in General Questions on

New construction, my interior HVAC is going in this week, and a comment from Dana in another discussion prompted me to double check things. I realized my 1 outdoor unit would be oversized at the 47F design temp load. Now I’m planning to use 2 outdoor units. Can someone with more knowledge check my numbers and plan here and see if I’m making mistakes?

I have 3 zones, 3 x 12k slim-duct units. 1 for each floor. I’ve rethought plans from using 1 outdoor AOU36RLXFZH instead to 1 AOU24RLXFZH and 1 AOU12RLFC. These units minimum heating capacities are 12,000 / 6,800 / 3,100 respectively.

My modeling software TREAT uses TMY3 data for my zip code ( 12205 ) and is conservative vs. manual J, and states it uses the peak load in a year for the load calc. Frankly, I’m unsure what that temp is, so I’ve assumed it’s 0F and then applied that to 47F for the below load calcs.

Sq.ft.: ~3,700 ( bsmnt 1,400 / 1st 1,400 / 2nd 900 )

0F deg Total heat load: 26,048 btu/hr

Basement (finished, lived in): 6,056 btu/hr

1st Floor : 10,664 btu/hr

2nd Floor : 9,328 btu/hr

47F deg Total load : ~8,510 btu/hr

House is 2×6 16″oc w/ cellulose dense packed, + Zip R-9. Ceilings are half R-60 other half sloped roof with R~37.

Windows : 267 sq.ft. of U.19

I trust this software to tell me the capacity I need for my climate here, and figure it’s calculating to -15F or maybe lower. But a 2nd opinion would be nice. Is only 1 24k outdoor unit enough? Or is it safer I do 2 outdoor units, a 24k & 12k, with the 12K zone 2nd floor or basement?

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  1. Expert Member
    Dana Dorsett | | #1

    The 99% outside design temp in ZIP 12205 is +6F:

    A 26K load at 0F becomes 24K, which is (barely) within range of a MXZ-3C24NAHZ, but to have some margin on Polar Vortex disturbance cold snaps an MXZ-3C30NAHZ wouldn't be terrible. With a MXZ-4C36NAHZ there would be gobs of capacity margin (45K @ +5F, 7.2K min @ 47F), but the MXZ-2C30NAHZ would probably cover it.

    The KD09 is good for 10,900 BTU/hr when there is enough compressor behind it, and would be more appropriate than the KD12's 13,600 BTU/hr, at least for the second floor and basement, if the basement MUST be ducted. The basement would do just fine with an FH06 head if it's not all cut up into doored off spaces.

    That can all be done on one compressor, but the KD cassettes may need to be swapped for MVZ-A12 or SVA- KP12 to push enough air through longer duct runs.

    Looking at Fujitsu's line up:

    The 6800 BTU/hr minimum modulation of the AOU24RLXFZH isn't dramatically less than the 7200 BTU/hr minimum of an MXZ compressor, but as you discovered, the AOU36RLXFZH won't modulate below 12K @ 47F. The xxRLF miniduct cassettes can push higher static pressures, and can be mounted vertically, which makes it a bit more flexible, and the 8100 BTU/hr heating output of the ARU7RLF cassette is also more appropriately sized for the basement loads.

    The AOU24RLXFZH with a ARU12RLF for the first floor (good for 13.5K heating) and a ARU7RLF for the basement , with a separate AOU/ARU 9RLFCD for the upper floor is probably as good as it gets with a ducted multi-split + mini-split solution. The combined minimum modulation of the two compressors is 9,900 BTU/hr (compared to 12K for the AOU24RLXFZH)

    If the basement can be done with a single head, a Fujitsu AOU24RLXFZH split between the two upper floors and Mitsubishi FH06 for the basement would have a combined minimum modulation of 8.4 KBTU/hr @ 47F.

    1. Matthew_M | | #2

      Wonderful, thank you Dana.
      There's some added complexity to my house here not factored into above loads. The 2nd floor unit will condition 2 corners of the 1st floor furthest from the 1st floor unit. An office, and master bath. About 220 sq.ft. Call it 2,200 btu/hr. The 1st floor unit is a slim-duct but will be essentially ductless as I'm building it into what would have been a fireplace.

      The 2nd floor load calc includes bonus room/FROG load of 3,444 btu/hr. An area not essential, say during a polar vortex. The duct design is such so that the plenum is accessible to close the damper off.

      The basement is completely separated in half, 2 ductwork runs, ~25' each

      Accounting for all this, loads should look like:
      1st floor unit: 8,464 btu/hr
      2nd floor unit : 8,084 btu/hr with bonus closed off. 11,528 btu/hr with it open.
      Basement : 6,056 btu/hr

      So if I want to design for the 99%, one AOU24 split 9/9/7 works.
      If I want to design for some safety margin, use Mistubishis 30K unit, but I think Mitsubishi is ruled out from the vertical mounting necessity for my install, so, split the system to 2 outdoor units as you've said.

      Tweaking your suggestion then, it'd be an AOU/ARU 12RLFCD for 2nd floor, and now just an AOU18 with an ARU7 in the basement, and either an ARU9 or 12 1st floor.
      This sounds all around like the right direction, with some redundancy in case an outdoor unit goes down.

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