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Advice on a Mitsubishi minisplit installation for cooling only

rubycon | Posted in GBA Pro Help on

Hi, I have some questions for you heating and cooling experts.

My situation:
– Seattle, WA 3-storey house (climate zone 4), 3500 sqft, no basement, on concrete slab.
– The house is adequately heated with hydronic heat, so we only need minisplits for cooling.
– There is a single open stairwell which spans all 3 floors.
– The house is also internally insulated (inside walls/floors have insulation).
– Our master bedroom is on the 3rd floor and is south and west facing with huge south facing windows. It’s easily the hottest room in the house, can be 84F when the other 3rd floor rooms are 78F. Hotter in summer.
– We have a MIL unit above our garage which isn’t used too often. It’s split into a bedroom and a kitchen/living room, around 1000 sqft total.

We’re planning to install the following Mitsubishi system:

Compressor 1 – 42k – MXZ-5C42NA2
Heads
12k – basement – MSZ-FH12NA
15k – main floor – MSZ-FH15NA
15k – master – MSZ-FH15NA
(42k total)

Compressor 2 – 24k – MXZ-3C24NA2
Heads
6k – small bedroom – MSZ-FH06NA
6k – small bedroom – MSZ-FH06NA
6k – MIL small living room (above garage) – MSZ-FH06NA
6k – MIL small bedroom – (above garage) – MSZ-FH06NA

My questions
– Does this system seem oversized, undersized or goldilocks? I haven’t done a Manual J but will be sure to ask for this from the installer.
– I’ve been told by one installer that a compressor can comfortably service 20% over its stated capacity, e.g. a 36K compressor would be able to service 43.2k total from the heads. I specifically confirmed with this installer that that applies to all Mitsubishi compressors. However, another installer told me that only HyperHeat compressors can service 20% over, and non-HyperHeat compressors should only service their stated capacity in heads. Who’s right?
– If we attach the 42k compressor to the outside wall on the 2nd floor (the master is on the 3rd floor just above it), how noticeable/annoying is the vibration from the compressor?
– In the spring and fall it’s common to have the first floor at 65F and the third floor at 80, meaning I want to redistribute heat to get to 72F. Can I create a single duct in the stairwell, separate from the minisplit system, running from the 3rd floor to the 1st and push the hot air down it with a squirrel cage fan? Or would this be a bad idea?
– The MIL isn’t used very often, mostly for guests. Would it be better to have a super large 5-head compressor (e.g. 50k+) to do the main part of the house (basement, main floor, master, + 2 kids rooms), and a single or 2-zone compressor just for the MIL? Or does the above split (42k/24k capacity) make more sense?

Thanks for your advice! I’ve been super impressed with the dialogue on this site.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ruby,
    At first glance, the system certainly seems oversized -- but it's impossible to know without performing the required Manual J calculations.

    Why would anyone trust the advice of a contractor who suggests equipment before performing the Manual J calculations?

    For more information, see Who Can Perform My Load Calculations?

  2. ranson | | #2

    The money you save upfront by right-sizing your equipment will very likely offset the entire cost of the manual J. I had Energy Vanguard perform a manual J for me. The owner is a Green Building Advisor author/contributor. They did a good job. You'll have to draw up a rough set of house plans for them to do the job. You'll also need to know the way your house is insulated and the U-values of your windows and doors. If this seems like too much work, then you'll probably want to hire someone locally, not your HVAC contractor, to walk through your house and do a room-by-room manual J.

    --John

  3. Expert Member
    Dana Dorsett | | #3

    Goldilocks would run away from this screaming (and so would all three bears)!

    This is the classic "grotesque oversizing by micro-zoning" problem. The minimum compressor output of the MXZ-5C42NA2 or in cooling mode is 6000 BTU/hr, which is about 4x the minimum output of an FH06 head if it were on it's own dedicated compressor. This is a recipe for short-cycling the compressors into truly abyssmal SEER efficiency, no matter what the nameplate tested SEER happens to be.

    Even in much hotter more humid locations than Seattle the whole-house cooling loads of a 3500' slab on grade usually comes in under 3 tons, often under 2 tons. You're proposing 5.5 tons, for what's likely going to be less than 2 tons of cooling load. You have micro-loads on the micro-zoned rooms, but very macro minimum modulation levels on the proposed compressors! The minimum cooling output of the MXZ-3C24NA2 is even worse, at 12,600 BTU/hr, which means it will be cycling the compressor on/off on a continuous call from just one FH06 head, since the max cooling output of the FH06 is less than than 12,600 BTU/hr, even when the outdoor temp is colder than the indoor temp.

    http://meus1.mylinkdrive.com/files/MXZ-5C42NA2_Submittal.pdf

    http://meus1.mylinkdrive.com/files/MXZ-3C24NA2_Submittal.pdf

    https://nonul.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

    Get the room by room cooling load numbers using aggressive assumptions (per the Manual-J). The 1% outside design temps anywhere in King County are between 78-83F- not too brutal, and the latent loads are negative.

    Some simple sizing rules of thumb for the Mitsubishi FH-series: Doored off rooms with design cooling loads less than 3000 BTU/hr should not get it's own zone. Any room/zone with a design load between 3000- 12000 BTU can use a dedicated FH09 on it's own compressor, since with no latent load and a 1% outside design temp in the ~80F range it really can deliver 12,000 BTU/hr of cooling. I suspect the load for even the MBR will come in under 12,000 BTU/hr.

    https://nonul.mylinkdrive.com/files/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf

    Room with loads less than 3000 BTU/hr can split the output of a dedicated KD09, assuming they are reasonably close to one another and it's possible to route the ductwork:

    http://meus1.mylinkdrive.com/files/SEZ-KD09NA_SUZ-KA09NA_Submittal.pdf

    The minimum cooling output is 3800 BTU/hr, but it can deliver fully 10,900 BTU/hr at your 1% outside design temp and negative latent load. Split between 3-5 rooms with cooling loads between 1500-3000 BTU/hr it will offer reasonable modulation, efficiency and comfort.

    I'm not quite sure how to match up the statements:

    "...no basement, on concrete slab"

    ...and...

    "...12k - basement - MSZ-FH12NA..."

    There is simply no way that the cooling load of a basement (especially a basement that doesn't really even exist) is anywhere near the 13,600 BTU/hr max cooling output of an FH12.

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