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Ductless minisplit – completed Manual J

JeremyArch | Posted in Mechanicals on


We purchased a 2 story passive solar designed house in southern Ontario that was referred to as a 1980s eco house design similar to this ( Slab on grade, southern exposure, minimal northern windows. Wall design 2×6 framed, vapour barrier, 2×2 horizontal strapping for all electrical runs and then drywall. Minimal holes in the polyethelyne (a couple holes to run exterior wires, kichen exhaust, no bathroom exhaust). No holes in second floor ceiling, all lights are on walls and attic access is via outside gable hatch. Currently has electric baseboard but we only used the woodstove to heat the house this year going through about 2.5 cords of wood (which means we didn’t leave the house for extended periods).

We have lived in it for about a year to see how the house functions and plan a few renovations, mostly update a few things for our comfort level. As much as I like the wood stove and the heat is provides we are planning on installing ductless heat pumps. This will free up some of my time for my young family and allow us to use a more efficient electrical when we cant feed the stove.

So first thing first, i contracted out a manual J by a third party company as recommended by the fine folk here.

-8F outside design temp, 72 inside design temp
82F outside design temp, 75 inside design temp

Our load after adding attic insulation and new windows which is planned work
32000btu/hr heat, 13900 latent cooling 11000 sensible
Main floor – 18500 heat, 5500 cool, 2nd floor 13700 heat, 8400 cool.

This numbers may be adjusted, we are having a blower door performed next week which we can pass on results to our hvac designer (currently assumed 3.57 ACH).

I want to plan on 2 single zone heat pumps, one main floor and one upper, please see attachment for locations. I understand that there may be temperature variations in the rooms by a couple of degrees when doors are shut. The models I am looking at are

Fujitsu XLTH 12RLS3HY
Advertised to provide 12000 btu cooling (3100-13600) and 16000 btu heating (3100-22100)

I am looking for opinions on if this will provide enough heat or if i should move one of the units up one level to slightly oversize to 15RLS3Y and 18000 btu. I will be asking this question to my hvac designer but i am always open to a 2nd (and 3rd, 4th 5th) opinion.

Thanks for any insight

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, I hope you can update your user profile so that your real name is displayed. Here is a link to an article that explains how to do that: How the GBA Site Displays Readers’ Names.

    Second, I think that your plan will work.

  2. Expert Member
    Dana Dorsett | | #2

    The 16K nominal heating capacity for the 12RLS3HY only has to be met a +17F outdoors/+70F indoors, a requirement of the HSPF test protocol to be allowed to test efficiency at that modulation level. Capacity falls with higher indoor temps &/or lower outdoor temps. Your load was calculated a both a higher indoor temp and a lower outdoor temp, so it's important to consult the capacity tables.

    You should (just barely) be able to get 13,700 BTU/hr (the top floor's heat load) out of the 12RLS3HY at -8F outdoors, 72F indoors, but it would take the 15RLS3HY to (just barely) cover the main floor's 18,500 BTU/hr @ -8F. See the heating capacity tables on p16 & p17 of this document:

    It would not be insane to bump both up to the 15RLS3HY size.

    There is almost no way that design-day heat distribution to the master bedroom and the bedroom diagonally opposite to it with a head mounted in the location drawn, but with the doors open it may do OK with the bedrooms directly opposite the stairwell.

    On the main floor throwing heat the full length of the house from the family room down the hall is also unlikely to result in cool-weather comfort in the dining room & living room.

    It would be useful to have the room by room load numbers in those floor plan drawings.

    It probably makes more sense to have a 3/4 ton head in the family room and another 3/4 tonner in the living/dining area. Upstairs a 9RLFCD (or 12RLFCD) slim-duct cassette in the closet could serve both the master bedroom and the adjacent bedroom, with another serving the other two rooms (which would need some building out of the walls to accommodate.)

    The Fujitsu multi-zone compressors don't have a guaranteed capacity at your outside design temp, so you would be looking at a pair of 9RLS3HY for the downstairs (2 compressors, one per head). The 9RLFCD mini-duct cassettes have a specified output at outdoor temps of -5F, which is close enough to your -8F design temp that it's not too risky. The capacity at 72F/-8F should be pretty close to the capacity at 75F/-5F design temperatures (both an 80F temperature difference.)

  3. JeremyArch | | #3

    Thank you both for your replies.

    Dana, i have attached the same layout drawings with the loads highlighted as requested. I was really hoping to minimize equipment in the house but perhaps that might not be possible based on the layout. When heating with the woodstove the bedrooms were able to stay at a pretty comfortable temperature at the current design load of 42000 btu so i was hoping that the 2 head set up would be able to cover it at the expected load. Once again, i appreciate the feedback.

  4. Expert Member
    Dana Dorsett | | #4

    On the main floor you have less than KBTU/hr of heat load in the family/entry hall side of the house, and over 10,000 BTU/hr of heat load on the kitchen/dining/living room side of the house. Assuming you'll still be using the wood stove, if you're going for a 1-head solution, for that floor, put it on the other end of the house, the area that has nearly 2x the heat load.

    Is there space for a 15K head on the wall between the living room window & dining room window, so that it can blow toward the hall & kitchen? That would probably still heat the family room under shoulder season conditions, and you can still cozy it up big time with the occasional fire in the wood stove.

    Upstairs is still tough on the heat distribution bit though. With wood stoves you have the benefit of column of 150-250F air driving convection to mix it up, but with mini-splits you're looking at only 100-110F air at design condition. The distribution won't be quite the same. A single 18RLFCD would have sufficient capacity for that zone but it's not clear where the ducts would run.

  5. JeremyArch | | #5

    Thanks Dana, seems like a pretty good option.

    Could i possibly put a ducted unit in the utility room to service the family room area and the master bedroom above running off one outdoor unit. A second multizone unit with a single head (on the living room wall (without the window) and a ducted unit in the closet beside the stairs. This could have a supply to one bedroom and a second supply into the hall directed at the other two bedrooms? I know that this wont provide the load to those two bedrooms if the doors are closed, but the kids are young with doors open most of the time and we always have the backup heat if needed.

  6. Expert Member
    Dana Dorsett | | #6

    Running an upstairs room on the same zone as a downstairs room rarely (if ever) works out very well.

    Fujitsu multi-zone units don't have specified output near your outside design temp. If going with multiple heads/cassettes on a single compressor you may have to use Mitsubishi MXZ series, assuming there is reasonable installer & distributor support in your area.

  7. JeremyArch | | #7

    Thanks for the information. I will definitely look into the Mitsubishi and plan on distributing the heads. My orginal plan would have been most convenient but if it wont hit the bedrooms with much heat/air the its time to change it up.

  8. JeremyArch | | #8

    Hi Dana,

    Ive been looking at the mitsubishi units and was wondering your thoughts on this layout.
    2 outdoor units 1 on each side of the house.
    MXZ-2C20NAHZ - 1 msz-fh09 in master bedroom, 1 msz-fh09 in family room over patio door
    MXZ-2C20NAHZ - 1 sez-kd09 in closet split between 2 bedrooms, 1 msz-fh12 in dining room over patio door.

    That third bedroom is in a pretty awkward spot, its a guest room and rarely occupied so i may have to leave it out of the equation. It looks like there is a certified installer in my area as well.

  9. Expert Member
    Dana Dorsett | | #9

    That seems like it would work, but have the contractor verify the capacities in those combinations against your load calculations.

    It's probably worth checking to see if it isn't cheaper to go with the MXZ-5C42NAHZ, which would give you the option of an FH06 heads in the bedrooms, a pair of which may have a lower minimum modulation than KD09, and no ducts.

    Also, the minimum modulation of a 5C42 compressor is 7200 BTU/hr @ +47F, which is less than the 7400 BTU/hr @ +47 F of just ONE 2C20, so as a system you'll have more modulation range with the single 3.5 tonner.

  10. JeremyArch | | #10

    Thanks Dana, I will look into that option as well.

    Out of curiosity, would there be a level I should get my current house so my original plan may work. If its going to cost me several thousand more in equipment perhaps investing that money in other places could lower the amount of equipment used.

    The manual J used the following data
    R50 attic
    R19 walls
    windows triple pane, double low e, argon - vinyl
    casement - whole u 0.18, SHG 0.22
    fixed - whole u 0.16, SHG 0.27
    Patio doors triple pane, double low e, argon - vinyl
    whole u 0.18, SHG 0.29
    Unfortunately i dont know what insulation is in the slab, but during the manual J i assumed there wasnt any.
    ACH assumed at 3.57

  11. JeremyArch | | #11

    Hi Dana,
    I just wanted to follow up on my previous question. By bumping the the attic up to R60, adding 1.5" iko enerair to the exterior and getting my ACH down to 2 or lower, could a 2 head solution work?


  12. Expert Member
    Dana Dorsett | | #12

    If the bedroom loads can be brought down to ~1500 BTU/hr or less at design condition it'll be just fine (with the doors open), but that's likely to require boosting the window performance (or smaller windows?), not just modest tweaks to the wall & ceiling U-factors. Run the numbers, see where they come out.

    There's at least a shot at getting there on the corner bedrooms, but probably not on the master bedroom. The middle bedroom that has only one exterior wall and possibly the corner bedroom next to it are likely to already OK due to the location of the doors relative to where the single-head solution would be blowing.

    Downstairs I think you're still looking at 2 heads if you want the family room to be cozy at -8F outdoors WITHOUT a fire in the wood stove. If you're willing to let the family room run cool when it's -8F outside, a single head in the living/dining/kitchen area should work.

  13. JeremyArch | | #13

    So your saying there is a chance ....

    Lol, i guess its going to be pretty unlikely that i can make it all function on a couple of heads at design. I will work these numbers a bit more and see what kind of changes i can make to the envelope, but given the budget i likely wont get it to where it needs to be for 2 heads!

  14. Expert Member
    Dana Dorsett | | #14

    Most people could manage to stay comfortable with just two ductless heads, but it may mean adding some resistance heaters in the doored off or remote rooms down the hall to meet code for heating system requirements.

    After the wall foam over & attic insulation upgrades the master bedroom and family room might need ~1000 watts each of electric baseboard or something to satisfy codes (3.412 BTU/hr per watt), the corner bedrooms ~600 watts, and the center bedroom ~400 watts to guarantee that even with the doors closed they can be automatically heated to a code-min 68F indoors @ -8F outdoors. Some inspectors would insist on that room by room heating, but others wouldn't care, as long as the capacity of the 2 mini-splits covers the whole house load.

  15. Robert Opaluch | | #15

    Do you notice that its the very overcast days, the coldest days, or the upstairs rooms that are coldest or require the most heating when you live there? That might give a clue about slab insulation level. I suspect you must have some insulation, at least around the slab edge. Could you see plans from the builder, or from the building authority that may have the plans used for a building permit? Is there any foam board insulation on the exterior of the stud walls? Or what is your assumption?

    Certainly upgrading your attic insulation could reduce your heat loss significantly at fairly low cost, if you have space available in your attic. Just R-10 seems insufficient. Could you run the numbers again assuming you add R-20 or R-30? Or what percent of your total heat loss (and gain) is from the attic?

    You mentioned that you assumed no insulation at your slab. You have no insulation on the outside perimeter of the slab?? That’s something you could modify that would make a real difference if there is no insulation at the perimeter.

    Dana has mentioned many times in other Q&A's, about how important it is to avoid underestimating insulation or overestimating air leakage, as it leads to oversizing heating systems unnecessarily.

    Finally, you do have a wood stove and you have electric baseboard heating. Either could be used to provide heat during the most overcast and the coldest days mid-winter. Meanwhile, most of the winter, a reduced output of two mini-splits might work okay, or might supply heat to the majority of your rooms. The other rooms (or most brutally cold or overcast days) could use the baseboard and stove to make up the difference. Or if you underestimated insulation levels, you might rarely need those backup heating systems.

  16. JeremyArch | | #16

    Ok. Well perhaps we will still consider the two head system. Each room already has the required electric baseboard and im okay with using them or the woodstove on the handfull of days when ill need to

  17. JeremyArch | | #17

    Thanks for your input and i will definitely check with the planning department to see whats on file for the house. The attic currently is a meger r20 so ill be looking to add another 40. The good thing is i still have time to fine tune some of these details before i get going on the retrofit and purchase the mini split

  18. Expert Member
    Dana Dorsett | | #18

    If you're keeping the pre-existing fully functional baseboard heating (have you tested it in every room?) the 2-head solution with judicious use of the resistance heat has the best lifecycle economics. Spending the difference in rooftop PV would probably be more beneficial than the cost of additional system complexity, and more than make up the difference in electricity use. It requires some vigilance on the baseboard thermostat settings though. Feel free to set the mini-split setpoints quite a bit higher than the baseboard thermostats, which allows the heat pumps to carry a larger fraction of the load.

    You're probably looking at a pair of 15RLS3s, or FH15NAs, or FH18NAs. The 15RLS3s have a lower minimum modulation which should deliver a greater modulation range than the FH series. Dropping to 1-ton units would work too, since you have the full baseboard backup, but you sometimes get higher air flow by upsizing (look up the specs), which usually translates in to further throw, better mixing.

  19. JeremyArch | | #19

    I have tested the baseboards and they are all good to go and have installed programmable thermostats in the bedrooms so i should be able to set it up to minimize the baseboard usage. I most certainly plan on putting a rooftop PV on the house in the near future,. Our current daily base load is 24kwh/ day. I am not sure how this will change after converting over to the heat pumps. The roof is going to need replacing in a few years and I am hoping the tesla solar roof is a success and prices are reasonable when its time to tackle it

  20. JeremyArch | | #20

    I meant to ask, would the two head unit be able to cool the place or would i expect similar struggles hitting those 2 bedrooms. The models appear to have substantially more then enough capacity to cover the cooling load

  21. JeremyArch | | #21


    I have a question regarding my manual J output and i was wondering of you could provide some insight.

    I had a manual J completed on my house a few weeks ago, they used the wrightsoft program and used and in filtration rate based on how the home was built and its age with and ACH of 3.57, it provided an output of 32147 btu per hour heating.

    I requested it to be recalculate after my blower door test of 2.2ACH and increased my attic insulation by an r10 (from 50 to 60) from the previous one and got an output of 32607 btu per hour.

    Does this make sense? shouldn't this number drop if everything else stays the same? or am i missing something here? Your thoughts are appreciated,

  22. GBA Editor
    Martin Holladay | | #22

    You're right. The design heat load should have dropped, assuming that the only two changes were a lower rate of air leakage and thicker attic insulation.

  23. JeremyArch | | #23

    Ok. Thanks Martin! He must have changed something else as well then i guess.

  24. JeremyArch | | #24


    No real questions, just a little update though fell free to comment.

    The designer determined the issue. There was a factor that he assumed the program completed but needed to be changed manually. I guess he usually uses program defaults as most people don`t conduct a blower door test as part of the process around here.

    My planned work will include R60 attic, new windows/doors and adding R12 to most the exterior walls getting the ACH to 2.2 (or better!!).

    25558 btu/hr heat, 13290 latent cooling 10223 sensible
    Main floor - 15343 heat, 5324 cool, 2nd floor 10215 heat, 4896 cool.

    I am planning on the two head solution one Fujitsu XLTH 12RLS3HY in the landing and one Fujitsu XLTH 15RLS3HY between the dinning and living room wall as recommended. Electric baseboard backup if needed

    Thanks for all your feedback, Unfortunately the archive department here didn't have any building plans on file so i could look further into the under slab insulation. I am planning on adding 2" to the exterior slab down the road as well about 2 feet down just in case which should further help my efficiency.

  25. JeremyArch | | #25

    Oh and here are the room by room numbers if your interested.

  26. Jon_R | | #26

    Keep in mind that the feasibility of using few heating/cooling sources is highly dependent on how much design day temperature drop/rise (source room to remote, open door room) you can tolerate. 2F, 5F, 10F? 10F isn't so bad where you can over-heat the source room by 5F.

    Want that master bath warmer than other rooms? - it will be more than 100% electrically heated.

  27. JeremyArch | | #27

    You raise a very valid point Jon. We are definitely going to see some temperature variations, most notably in the laundry/master bath/master bedroom areas of the house house.
    To me the issue is installing more equipment just isnt worth it right now. If I have to bump the heat up with the baseboards in the master bathroom a few degrees for 30 minutes in the morning and evening in the middle of winter it really isnt going to cost me much. I dont know if i can justify the cost of installing extra equipment like another ductless head, or figuring away to run a ducted head and all the costs associated with finishing the work.. Who knows, later on if i am uncomfortable and using to much electricity, i can look at adding another unit, that the beauty of the ductless i guess.
    But yes, you are right and i will likely have cooler rooms throughout the house with out using the baseboard zoning

  28. JeremyArch | | #28

    Hello again,

    We have narrowed down our contractor for this job who is reasonably priced. It looks like we are going to commit to a third head now instead of later and put it in the master bedroom The contractor will give us a discount and there is a recently released rebate program. So now i have a decision to make regarding the other upstairs head and i thought I would ask for some feedback.

    Option A - 9LFCD in the closet between the bedrooms, very small duct run to both bedrooms with potential a third short run out to the hallway
    Option B - 9LFCD in different closet, output into the bedroom and into the hall
    Option C - 9L3HY in stairwell high on wall

    Also, thoughts on keeping each on a single outdoor unit each, or creating a 2 head multi split (1500 unit main floor and 9000 unit 2nd floor).

    Thanks for your comments

  29. JeremyArch | | #29


  30. Expert Member
    Dana Dorsett | | #30

    It's easier to keep refrigerant lines well within specified maximum lengths with two compressors rather than one. Having two also gives you redundancy- if one fails the other can still cover a large fraction of the load while awaiting repair.

    Option A, with a feed to the hall as well, would give you the most optimal distribution, since you can then tweak the flows to both bedrooms for temperature balance. Think about where the condensate drain has to run and access to air-filters, etc. That may limit the location somewhat.

  31. JeremyArch | | #31

    Thanks for your feed back Dana. That will likely be the option I go with then if I can fit the unit in the closet and still leave some room to hang up some kids clothes! I will look at having the 15k downstairs as a 1-1 and the 2 9k units as a 2-1.

    Thanks again

  32. JeremyArch | | #32

    Hello again,

    I just wanted to follow up an say thanks for the advice I received here. I had a busy summer of upgrades and is seems to be paying off so far this winter. I thought i would provide a little summary of what was completed and how things are working out so far.

    I put the attic insulation up to R60
    Replaced all windows and patio doors with triple panes
    Removed exterior siding on 3/4 of house, taped plywood seems, added WRB, 2 inches of polyiso and new vinyl siding.
    Original blower door 2.2 final blower door 1.9 (when i get around to doing the other part of the house i should be able to get this even lower)
    Installed 1 Fujitsu 15RLS3HY on the main floor and one 12RLS3HY upstairs. So far, slight temperature variation of about 1-1.5 degree C from where the ductless heads are to the opposite side of house.

    As an all electric house, I was using an average of 22 kwh a day from April - Oct and am assuming it as my base load with no heating or cooling. Heating started at the end of Oct, Oct-Nov averaged 33Kwh a day and Nov- Dec so far 39Kwh day

    The energy audit provided my with an expected annual energy use of 52 GJ while the typical new house in my area would use 89 GJ annually. 15% of that space heating, 34% of that water heating, 6% space cooling, 20% lights/appliances and 25% other electrical.

    My next step is to put in an HRV, and add some insulation on the exterior of the slab foundation in the spring, hopefully some solar panels in the not to distant future. It wasnt cheap but luckily there were plenty of rebate programs i could take advantage of and covered close to 20% of the work but seeing as i dont plan on moving for a long time, its well worth the investment.

    Thanks again for all the ideas, recommendations and information provided on this site! Have a great holiday season and an energy efficient new year!

  33. GBA Editor
    Martin Holladay | | #33

    Here at GBA, we enjoy reports like yours -- reports about how a house is performing after retrofit measures have been completed. Congratulations on your achievements -- and thanks for letting us know about them.

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