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Community and Q&A

Review mini-split setup

RMaglad | Posted in Mechanicals on

good afternoon folks,
I’m building a 2000 sqft bungalow with conditioned, semi finished basement. 9′ ceilings on both levelss, to a little below “pretty good house” standards. R70 attic, R30walls, R7 triple pane windows, R5 doors, R30 basement walls, and R16 subslab, aiming for excellent air tightness. Fully ducted ERV, ventless dryer, recirculating range hood, and the achillis heel, a wood burning fireplace, “sealed” combustion with outdoor air supply kit.

Heat load calcs came in at 22,000btu/hr for the whole house incl basement (-25*C outside, 22*C inside). Cooling load, incl latent came in at 26,500btu/hr (30*C outside, 24*C inside). The calcs are further broken down based on my “zoning”.

Kitchen/Living Dining: Heat 6800btu/hr, cool 12500Btu/hr : Going with a 12000 mitsu single head to come close cooling load (glazing kills me in this room in the summer, site is an open field right now, but intend on planting strategic trees for summertime shading, as i lost two 100′ tall giant ash trees to the asian beetle), I’ve got 3′ overhangs all the way around, but south-west is killer.
Master (incl ensuite and 2 closets): Heat 2900 btu/hr, Cool 4800Btu/hr: Going with a 6000 mitsu single head
Bedrooms + basement bath and basement bed: Heat:5800btu/hr, Cool 6100btu//hr: going with a 9000btu/hr ducted mini (all short runs, unit will hand from floor in basement)
Remainder of Basement: Heat: 6400btu/hr Cool 3300 btu/hr: going with a 6000btu/hr mitsu single head (large mechanical storage area can just get spilled from adjacent rooms heat)

Total capacity: 33,000btu/hr for both heat and cool, so i am about 1.6 times oversized for heating and 1.25 times oversized for coolingm, when i look at it from a whole house perspective.

The only place i think i could potentially reduce capacity would be in the living room, and go with a 9K unit, with the understanding that on the 10 super hot days of the year, when the sun is beaming in through the unshaded south-west facing windows, that the room will get to be warmer than the set point, and the unit will be running full tilt, there will also be spare capacity from the other units to help balance out the load. This doesn’t really bother me that much, as during the summer, i really only like it cool for sleeping, and will have a separate unit for the bedrooms.
Thanks for the review and advice.

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  1. lance_p | | #1

    Hey neighbor, I'm in Ottawa as well.

    Hopefully you'll get someone more experienced than I to chime in on your load distributions.

    What I can offer, though, is that with so many units in the house, being concerned about oversizing is probably not necessary. If you'd like your units to modulate to lower outputs to keep efficiency high you just need to use fewer of them during weather that's not too cold. Once the real cold sets in you can bring them all on line.

  2. Expert Member
    Dana Dorsett | | #2

    The modulation limitations of multi-splits (or even separate mini-splits when ridiculously oversized for the room loads) are always an issue that needs attention.

  3. Jon_R | | #3

    Note that a 40% oversized heat pump that modulates only 2:1 will still perform much better than a 0% oversized heat pump that doesn't modulate. And the latter used to be common and acceptable.

    Open interior doors will resolve high humidity in a single room caused by too much capacity in the single room.

  4. Reid Baldwin | | #4

    Are you proposing four separate outdoor units or a multi-split?

  5. user-2310254 | | #5


    Did you see Martin's post summarizing Bruce Harley's mini split tips?

  6. Expert Member
    Dana Dorsett | | #6

    Bear in mind that the cooling specs in the short-sheets are at an 8C indoor to outdoor temperature difference, and don't include the maximum capacity, only the output level at which it was tested for efficiency. Your cooling indoor to outdoor temperature delta is only 6C, so it will have more capacity and likely higher efficiency than the "rated" or "nominal" cooling capacity.

    Your kitchen/living/dining area has a cooling load of 12,500 BTU/hr, heating load of 6800 BTU/hr, which can probably be better served with the FH09 than the FH12, since it has a much lower modulation range on the low end (less than half that of the FH12), and will still cover your load at -25C. It's max cooling is 12,000 BTU/hr, which isn't a whole lot less than the 13,600 BTU/hr max of the FH12, and at your 6C cooling delta-T it will be able to deliver more than the AHRI tested 9000 BTU/hr. Your design heating load at -25C out/22C in is 6800 BTU/hr, which is about 145 BTU/hr for every degree below 22C. The minimum output of the FH12 is 3700 BTU/hr @ 47F (8C), but your load is only ~2000 BTU/hr at that temperature.
    That means the FH12 will be forced into cycling whenever it's above roughly 0C. But with the FH09's minimum of 1600 BTY/hr @ 8C it won't be forced into cycling until roughly 11C and higher. There are a substantial number of hours per heating season that fall between 0C (when the FH12 stops modulating) and 11C (when the FH09 stops modulating), and that will add up to a difference in both comfort and efficiency.

    If the bedrooms MBR & ensuite are all on the same floor you might be better off with a 1-ton mini-ducted unit serving all. But without a floor plan it's hard to make better tuned recommendations.

  7. RMaglad | | #7

    floor plans attached, however we wanted the small 6K unit in our bedroom for full independent control.

    My HVAC contractor is suggesting a 2 zone multisplit mxz-2c20, with 6K FH09NA and 9K KD09 heads, instead of the 2 individual units for each "zone". Reason being that the 9K slim duct unit has a minimum rated temp of -15*C, and my design temp is -25*C. Any thoughts on this? Seems when only 1 zone calls for heat, the outdoor unit will be grossly oversized.

    The 9K mini duct has a HSPF of 10.0, SEER 15.0 heat cap: 10,900 cooling cap: 8100
    The 6K mini has a HSPF of 13.5, SEER 33.5 heat cap: 8700 cooling cap: 6000

    The 6K+9K multizone has an HSPF of 9.8 and SEER 17.0, heat cap 18400 and cooling cap 15000.
    This unit is more costly than the individual units listed above, and i worry about cycling for most of the winter.

    The whole home heat load of 22500 is *almost* entirely met with the 6K in master, 9K in living, and 6K in basement. Surely, the 9K mini duct will continue to produce enough even below its rated capacity at -15*C to make up the theoretical missing 1500btu from the whole home load...

  8. Expert Member
    Dana Dorsett | | #8

    Your worries are well founded- a multi-split solution would be a BAD idea here!

    Multi-splits won't modulate down anywhere near the minimum output capacities of the FH06 or a KD09 . The minimum output of a the MXZ-2C20NAHZ compressor @ +47F is 7400 BTU/hr, which is more than half the max output of the FH06 @ 47F, more than 4x the minimum modulated output of an FH06 on it's own dedicated compressor. This means both heads will simply be cycling on /off (at fairly rapid cycles) nearly all of the time.

    Also note, you won't get more than 22,000 BTU/hr out of the MXZ-2C20 compressor at even +5F/-15C, and substantially LESS than that at -25C. (Probably about 17,000 BTU/hr max @ -25C)

    Individual mini-splits on their own separate compressors will be able to modulate, and at their minimum modulation won't be grotesquely oversized for the room loads. Even four FH09s have a combined minimum modulated output of 6400 BTU/hr @ +47F, which is less than that of the 2C20, and you have the option of turning individual mini-splits completely off during the shoulder seasons if they're cycling too much during the shoulder season when the remaining mini-splits can carry the load, which lowers the minimum modulation range further.

    Fujitsu's mini duct units have rated output at -20C. Their minimum modulation +47F is 3100 BTU/hr (roughly the same as a pair of FH09s.) A pair of 12RLFCD or 18RLFCD would have sufficient capacity for the whole-house load. Having one of them dedicated to the rooms/zones that will have little to no load during the shoulder seasons (and can be turned off when it starts cycling too much) might be a better solution, even though the capacity at -25C is unspecified.

  9. RMaglad | | #9

    Dana, thank you.
    I think you are confirming my suspicions, that regardless of the rated capacity of the 9K ducted, whether its mitsubishi or fujitsu, the dedicated compressor for the slim duct unit + a dedicated compressor for the 6K would be more efficient on a day-to-day basis, while at design conditions i shouldn't suffer miserably, nothing that an extra blanket or a wall panel heater couldn't make up for.

    I guess the appropriate question would be, whether the KD09 or the 9RLFD would still deliver some heat below its rated capacity. Again, at design temps with just the individual 6, 6, 9 i am very close to the whole home design load, and also have a wood fireplace for ambience/backup.

  10. Expert Member
    Dana Dorsett | | #10

    Yes, they will all continue to deliver heat at well below the specified minimum operating temperature, as long as the compressor is still working. The capacity isn't specified at temperatures colder than that, but it doesn't suddenly stop.

    Some Mitsubishi H2i compressors are designed to actually stop when it drops below -28C, and automatically re-start when it warms up to -26C or so, which may be an important consideration if your design temp is -25C. Read the specs carefully. In the fine print on the FH-series minisplit submittlal sheets it reads:

    " ** System cuts out at -18º F (-28º C) to avoid thermistor error and automatically restarts at -14º F (-26º C). "

    Fujitsu units don't have that "feature", and keep on going no matter how cold it is outside.

  11. Expert Member
    MALCOLM TAYLOR | | #11


    Ha! I see you got burned on the size of your closet.
    Out of interest, did you do the working drawings yourself?

  12. RMaglad | | #12

    Malcolm, i did not prepare the working set myself. I prepared the overall concept myself, and i had a designer take it to the next level and reconfirm code requirements (not much changed from my final drawings with regards to layout). To date its cost me 1750 in drafting (23 revisions!! Ocd of an engineer i guess) 400 liability and 800 structural engineer review..all canadian bucks.
    I used sketchup for first quick plans, then a chief architect to make it a bit more precise and do some renderings...then passed it off to designer.

    Closet..hah! Yes she gets the bigger closet and bathroom access...i had a detached garage..but we cut it out due to budget constraints.

  13. Expert Member
    MALCOLM TAYLOR | | #13


    it looks like a nice house that I bet you will enjoy living in. I brought up the drawings because the dimensioning seems a bit eccentric. If I was framing from the plans I'd find it difficult. Some are missing (like the width of the pantry and powder room); a number are illegible, falling on dimension lines (like the dining room dimension of 25'-?"); and some dimensions for walls also include a window in their length (like the wall with the fireplace), requiring the framer to do math on site while laying out plates - which is something you probably don't want. The convention is to move inward from big to small, but the window dimension line is often closer to the outside than some longer wall dimensions. I'm just bring this up in the hope that it prevents problems further down the road. Good luck with your build!

  14. RMaglad | | #14

    Noted, thanks..ill get it cleaned up.

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