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Community and Q&A

Sizing a Minisplit Heating System

bc_new_england | Posted in Mechanicals on

Hi All,

Long time reader, first time poster. I’m grateful for this website, I’ve learned a lot and appreciate all the resources.

I’m hoping to get some advice around sizing a system to heat a 1987, 2,200 sq ft colonial in Dover, NH 03820. We have an oil boiler on it’s last legs and I would like to decarbonize our home by going to mini-splits (plus the added benefit of AC + air filtration), and a hybrid HWH .

We are going through the NH Saves insulation program which to improve the envelop of the home. I’m not sure what details would be helpful in this regard, but the sealing work should get the house from 2950CFM to 2000CFM50, and the major insulation pieces will be having rigid foam insulation/spray on rim joist for the basement and blown in cellulose for the attic.

We also paid to have an energy study done by the auditor & insulation contractor to help size a heating system for the home. He is BPI certified and, if relevant, uses TREAT software . I have in hand (attached) the heat load needs for our home (after insulation/sealing work).

The auditor’s position is that although great, mini-splits are not cost effective as an option to heat our home since “To do it right, you’d need to spend between $25-28K”. Over time, my relationship with the auditor has soured and I’m looking for advice/second opinions. I’ve found installers who can do minisplit systems that can serve the heat load on paper, but when I spoke of them to our auditor – he said the price is too low/a red flag because he knows what these systems cost.

I know it is an oversimplification, but I’m stuck on having found a system/installer/price that serves the ~49K BTU needs of the house (Fujitsus, operating down to -15 deg), versus his adamant position it’d be insufficient/unsuccessful based on the price (no system details discussed).

He will not assist further in system design unless I’m willing to spend the amount he projects and/or he GCs the project. To me, this is a red flag, but I also acknowledge he’s an industry professional with many years of experience in the area, so he could be right.

I’ve attached a very crude layout of our home (excluding basement — the greatroom/garage is on a slab fyi). I’m researching folks in my area who might be able to provide a second opinion and thought I might try and seek some advice here as well.

Any advice on what to consider, how to approach would be greatly appreciated.



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  1. Jon_R | | #1

    > no system details discussed

    Success very much depends on the system details. On a room-by-room basis. Find someone who will provide you with full Manual J, S & D information.

    Consider a VRF system (like Airstage J-IIS) instead of a multi-split. 4K btu/hr indoor units are available. Also consider single head ducted units. But don't expect anyone to be able to quantify the comfort advantages (and disadvantages) of either.

    1. bc_new_england | | #6

      Hi Jon,

      Thank you for the notes here. I updated below with more information.

      Forgive my ignorance, but regarding the Manual J, S, & D - how does a D play in to this? I'm self educating on what this all means. I haven't asked explicitly, but I believe the "Design Heating and Cooling Loads" is a Manual J & S rolled in to one.... I say that because the energy consultant ran this and another one for an oil-fired boiler that had different numbers. I could be way off here - not sure how to tell.

      I will look in to a VRF system - my initial research shows they're mostly commercial applications for AC. A question around this is the affordability aspect if it's intended for commercial application?

      1. Jon_R | | #10

        Manual D is for duct design. Which may or may not apply in your case. Should have listed "T" also.

        Airstage J-IIS is the right size for residential like yours - it would be interesting to know about price and "how much better than multi-split? "

        1. bc_new_england | | #12

          I think I am missing some things about the Airstage - is it simply a larger heat pump than standard with minisplits? Looking on FujitsuGeneral it shows the outdoor compressors(?) but I'm not sure how the heat/cooling makes it indoors.

          Forgive my ignorance - does it just connect to heads/cassettes etc like a minisplit system?

          Looking at the AOU48RLAVS - it looks like its heating operating range is -4 (Heating -4 to 70 °FDB (-20 to 21 °CDB)), I'm not sure if that covers my needs - to be honest I'm still a bit confused about what our design temp should be.... the closest weather station on this list: is Pease Airport at 8 degrees Fahrenheit if I'm reading it right, but we're about 9 miles inland of there.

  2. charlie_sullivan | | #2

    This is an unusual situation. Usually it's the energy consultant who figures out that a simple, inexpensive minisplit option will work well and the HVAC contractor who is skeptical. I would either simply go ahead with the system design you have or find another consultant if you have doubts. Or post more details of that system design here--not sure how much you have.

    Another option to consider is to do some of the heating by putting hot water from an air-to-water heat pump through the old radiators. Then add either some minisplits or some fan coils to do the remaining heating load and the cooling.

    On the insulation work, given your goal to decarbonize, make sure you are using foam that doesn't have high global warming impact--conventional spray foam and XPS foam boards have extremely high climate impact, thousands of times higher than CO2. There are lots of foam board options that avoid the high GWP of XPS:

    EPS has lower R-value per inch, but higher R-value per dollar.

    Polyiso has very high R-value per inch, but is expensive.

    GPS, aka Neopor, is graphite infused EPS, and gets up to the same R/inch as EPS.

    NGX is Owens Cornings new type of XPS, which can be hard to get your hands on, but is much better than normal XPS for climate impact.

    For spray foam, if it's two-part foam applied by a insulation contractor, you can specify "HFO" foam for much lower climate impact. If it's just for sealing cracks, the "one part" foam in small cans doesn't have the same problem and is fine to use.

    1. bc_new_england | | #3

      Thank you both for the recommendations. I will look more into the proposed insulation materials - I wasn't aware there could be such a difference in materials/carbon intensity, but it makes sense.

      My experience so far is that for the pricing of the system to fall in budget, I need to be more prescriptive of the system. I think the installer is simply an installer, and not a system design whiz.

      As of right now, the system quoted is:

      Total - 39K cooling / 42K heating

      2nd floor + great room off garage
      Fujitsu - AOU36RLXFZH
      SEER = 20
      EER = 13
      HSPF = 10.3

      Indoor evaporators:
      7k ASU7RLF1 x 2 (two smaller bedrooms upstairs)
      9k ASU9RLF1 x 1 (master bedroom upstairs)
      15k ASU15RLF1 x 1 (great room)
      1st floor
      Outdoor: AOUG15LZAH1
      indoor: ASU15RLF1 x 1 (off of sunroom, pointed towards dining room - largest open area in home)
      SEER = 25.3
      EER = 13.9
      HSPF = 13.3


      This is what was quoted to me. It seems that both the master bedroom and great room heads are oversized. I believe the overall 36K BTU system can modulate down to 12K. I'm not sure if/how this should factor in to the design.

      My thought as of now is to go back to the installer, and have him quote doing the 36K unit to heat the upstairs of the house, plus put a head in the basement (7-9K BTU?) to warm the first floor / off-set the hybrid HWH. Then, we would need a 9K in the great room, which we could do a 1-to-1 for greater efficiency.

      Please let me know what other specifics I can provide. I'm quite new to wrapping my head around home heating systems. I've always just replaced whatever was there before, but I'm hoping to get off fossil fuels in what is likely a long-term home for us now.


  3. bc_new_england | | #4

    I updated the floor plan with (hopefully) some helpful info. Attached here:

  4. Expert Member
    Akos | | #5

    You mention oil boiler, so I'm assuming the place has rads. The only proper way to get a comfortable setup in that case is to add in ducts and go for ducted mini splits, or at least a ducted mini split for the bedrooms upstairs.

    A bunch of wall mounts will never quite be comfortable in older house, you'll have a hard time getting heat to where it is needed (outside walls and windows).

    Without going to ducts, your best bang for your buck is a central wall mount in the main area, another wall mount in the hallway upstairs and replace the oil boiler with a smaller electric resistance unit. You can also always replace the electric boiler with an air to water heat pump down the road (if / when they become more common and affordable).

    The idea is to get the bulk of the heat from the mini splits and only use the existing rads as a bit of supplemental heat to even out the temperature in rooms.

    This would also let you size the mini splits very close to your heat load as the baseboards can always supplement during those polar vortex days.

    Generally best in most cases to go for one to one setup, that is one outdoor unit for each indoor unit. One-to-one setups have much better modulation range and tend to be more efficient than multi splits. Plus the material cost of a two zone multi split is about the same as a pair of single zone units.

    The 2nd floor mini split will be doing mostly cooling and only a bit of heat, you can save a bit of money by getting a non-hyper heat unit (ie Mitsubishi MUZ-GL instead of MUZ-FS ) for upstairs.

    1. bc_new_england | | #7

      Hi Akos - yes the home has baseboard radiators. I added an updated (crude) map above that includes their rough placement if relevant.

      RE: Ducts - that is originally where I started looking for the upstairs since we have an open attic. I was told by multiple contractors that it'd be more expensive than multiple wall mount heads.

      Forgive me, having lived in Massachusetts for some time - by my standards a 1987 house is basically brand new (the last building I lived in was believed to be built ~1795). Is there a true concern here because of insulation code being different in '87 when the house was built? Or something else?

      RE: electric boiler --- I've been considering this as well, I'm glad you brought it up. Most plumbers I've spoken with are against them due to high operating costs in New England, but I understand if it was intended to be minimally used + backup for polar vortex conditions - operating costs would be minimized. I agree that it might be a good option (based on my initial research) against the high costs of an air-to-water heat pump today.

      A few questions:
      -Are they a simple swap-out from an oil boiler?
      -If my oil boiler heating size is ~59K BTU (# from energy consultant), but I'm replacing lets say 36K BTU with minisplits (call it two 1:1 - 18K BTU systems, one for each floor) --- am I looking simply for the difference between those, 23K BTU elec boiler, or is there a lot more to it?
      - Do electric boilers modulate?
      - I have a 200 amp service, any considerations I should have?

      1. Robert Opaluch | | #8

        Instead of an electric boiler, you might consider electric resistance or electric radiant units. Smaller ones can be plugged into existing electrical outlets. Best to rely on the minisplit(s) for the bulk of the heating, as they are cheaper to run than plain old electric resistance heating units. Individual electric heating units are relatively cheap to buy, can be easy to install, and can each be run independently on their own thermostat (so are only used when needed by the room occupant). But otherwise cost twice as much to heat a space vs. a minisplit. You might just wait and buy/install after you see how comfortable you are with the minisplit(s) alone.

        Good idea to be adding insulation and doing more air sealing to reduce your heating and cooling load. In MA and RI, they evaluate then do the work for less than I can buy the materials.

      2. Expert Member
        Akos | | #9

        Usually when GBA refers to a well sealed and insulated house, it is the neighborhood of R30 walls/ R60 ceiling and sub 1ACH50.

        Assuming your ceilings are 8' your 2900CFM blower test works out to 10ACH50, assuming 2x4 construction so R12 walls, unfortunately your place is still quite "old and leaky". Probably a big improvement over your previous place built in 1795, but nowhere near sealed and insulated enough to be heated with point source equipment.

        The good news is that with a house that leaky, there are a lot of low hanging fruit for air sealing. Currently I would guess around 25% of your heat is lost to air leaks, best efficiency improvement you can do is fix this. Note this is not the same as insulating, you want to find where the leaks are and plug them with caulk/spray foam/rigid insulation panels.

        This means you need to keep the baseboards for supplement heat or install a fully ducted setup.

        The best installed attic system (air handler in insulated/sealed doghouse, ducts and registers encapsulated in SPF), is still less efficient than having all your ducting and air handler inside your conditioned envelope. It is also about the same amount of work, so, it only makes sense if there is absolutely no way to run ducting inside the house (I have yet to see one of these).

        Adding this in as retrofit and not part of a larger reno gets to be pretty expensive thus my suggestion of the in-between with some wall mounts and a bit of boost from an electric boiler.

        The electric boiler is a pretty simple swap from your old unit, it does mean running a larger power feed to it. Since you would need to be running power for the new mini splits, it shouldn't be too much extra.

        Your 200A service should be plenty for a reasonable electric boiler+mini split setup. Your electrician can run the demand calculation to double check.

        If you have old oil fill data, you can run through the link bellow and figure out your heat load and verify if the 48K is correct. Feels a bit on the high side, but could be the case with a leaky house:

        Assuming the 48k is correct you would need about 33k for the first floor and 15K for the 2nd. So probably looking at a 20k+ of heat (AOU18RLXFWH
        with ASU18RLF, the 24k unit does only an extra 3600BTU of heat at 5F, so not sure if worth the extra cost) and a 12k wall mount upstairs (say AOUG12LZAH1 and ASUG12LZAS).

        The 18k unit moves a lot of air, so make sure to place it in a location where it will not be blowing directly at people.

        The boiler would need to be sized to make up for the difference (48-21-15 = 12000BTU) so a 5kW unit would probably do. On a 200A service, you probably have plenty of spare power, so you can always bump that up to 10kW for not much extra cost as a bit of buffer. Besides the a larger power feed, there is no efficiency difference for a larger unit, few places where a bit of upsizing doesn't matter much.

        Some electric boilers also come with outdoor reset, which would work well in this application, you just dial in the reset curve to match the extra heat output you need from the baseboards instead of having to deal with two separate thermostats.

        If your baseboards have adjustable louvers, you can further tweak the amount of heat they are putting into each area to get a better balance.

        1. bc_new_england | | #11

          This is helpful - thank you for all your advice on this. In case it's helpful, I've attached 3 docs that detail more our current R values and house characteristics, identified improvements to bring CFM50 down to 2000, and the overall sealing/insulation scope of work. I plan to have all the recommendations done as part of the NH Saves program. I don't understand all the acronyms and naming conventions - but I figured better to attach here in case it's all industry-standard speak others can discern.

          You are correct, a ducting retrofit is out of scope for this project in budget and desire. Thank you for the article shared, I'll try and work through that, although my primary intent of hiring the energy auditor was to do help me size appropriately :D

          For what it's worth, the BEFORE heating load requirement with oil boiler is 77,368 BTU, and AFTER the sealing/insulation he had it projected to be 58,655 BTU. I'm not sure why the AFTER BTU need projections are different based on if we do an oil boiler (58,655) vs. mini-splits (48,796).
          Is that a red flag? Or perhaps there's more to it?

          I'm trying to play out how doing a few distributed (2-3) wall mounts, an electric boiler w/ ODR, and a hybrid HWH would be configured physically/interact. Should I be concerned about a very cold basement with a HHWH? Or would an electric boiler throw off enough heat to offset that? I know it's likely dependent on how often each are running. And one goal in this scenario would be to minimize how often the elec. boiler runs (my elec cost is about $0.224/kWh). One of the initial recommendations from the energy auditor was to add a wall mount in the basement to offset the HHWH and help heat the 1st floor.

          Initial thoughts here, but perhaps a 15LZASH1 off the sunroom (serving the sunroom/kitchen/dining area) I believe this would do 18K BTU heat with a higher HSPF of 13.3 vs the AOU18RLXFWH at 10.3 HSPF. Then perhaps a AOU36RLXFZH with a 9K in basement, 12k in great room and 12k in upstairs hallway/master bedroom(pointed to hallway)?

          If I understand correctly, multi-head units have a higher floor for modulation (and therefore are less efficient), the AOU36RLXFZH for example has min heating of 12K BTU, vs. the 15LZASH1 of 3,100K BTU. I imagine, based off the distributed locations of the 3 heads in my above scenario - that across them I would likely need 12K at any given moment and therefore that setup would work as intended? If that's the case, I imagine I lose efficiency in the shoulder seasons possibly but being new to this I'm not on sure footing that I understand how these pieces all work together.

          What I really hope to avoid is getting backed into a corner with our boiler on its last legs as we head into the coming winter. The simplest solution is to swap the boiler (I've got a quote for an 87% efficient unit with an outdoor reset + piped in outdoor air for combustion) - but I want to avoid locking myself into an oil future.

          1. Expert Member
            Akos | | #13

            I would work through that link first, I highly doubt your place is anywhere near 85000BTU as is, you would never be able to afford to heat it with oil if that was the case.

            Unfortunately lot of designers are not careful with inputs into design software, common enough to see heat load calculations being 2x to 3x of actual building load. Since you have fillup data, it gives you an ACTUAL number for the building as is, you can work form that to size things based on improvements.

            There are some very good items in the list, insulating the basement and spraying the rim joist is a big improvement. I didn't see enough about sealing knee walls, these are notorious air leaks and should also be addressed.

            To get good efficiency out of a multi split, it needs to be sized exactly for the load, this includes all heads.

            For example, your post insulation upgrades basement will probably have about less than 1/2 the heat output of a 9K head, chances are it will cycle and efficiency won't be great. If you are worried about the HPWH cooling the basement, get one that can be ducted and duct it to the main floor.

            You also want to avoid putting small heads on a big multi split, chances are the unit will need to bypass refrigerant to run the small heads which is terrible for efficiency. You generally want to make sure that the load for each head is close to their rated output and the min modulation on the outdoor unit is bellow the smallest head size. Multi splits don't modulate all that well, the indoor heads are usually either on at rated capacity or off. This is why it is important to get sizing just right.

            Since your hydro is pretty pricy, you do need to get the heat pump setup right otherwise it will cost more than oil. You probably need a seasonal efficiency of around COP of 3 to be cost effective, you can definitely hit this with one-to-one setup but you won't make it with an incorrectly sized multi split.

  5. bc_new_england | | #14

    Thank you for this. If I understand correctly, it's a lot easier to make a successful (efficient/lower operating cost) mini-split system using 1-to-1 configurations because they can modulate across a wider range to fit the needs of the day? Whereas a multi-split unit really needs to be sized perfectly by room to work as intended?

    In conjunction with decarbonizing, I'm definitely trying to minimize our annual bills and create some ROI for this system - so I really appreciate the advice on designing a system to avoid operating cost spikes. I'd never forgive myself if I spent all the money on minisplits to spend more running them per year than with oil!

    I took a stab at calculating our home's heat needs. I'd attach my work here for validation - but I'm not sure an excel sheet is downloadable from this forum?

    Here's what I came up with:
    From 12/31/20 - 2/11/21 we used 205.7 gallons of oil. Our boiler is rated at 86.4% efficiency by the spec sheet, with 103K heating capacity MBH, 90K net IBR rating water MBH(? - assuming this is net output). It is an old, tired boiler - so I took a tiered approach to degraded efficiency 86.4%, down to 82% and 80%.

    Using oil BTU/gal of 139,350 * 80% efficiency = 111,480 BTU * 205.7 gallons = 22,931,436 MMBTU during that period. I'm guessing my 'balance point' is the 65 deg Fahrenheit based off guidance in the article. I THINK my 99% design temp is 8 deg F. The HDD sum at 65, is 1614.5.

    22,931,436 MMBTU / 1614.5 / 24 hours = 591.81 BTU/hr

    65 deg - 8 deg (design temp?) = 57 deg

    591.81 BTU/hr * 57 = 33733.1437

    33,733.1437 * 1.4 = 48,226 BTU ----- if I understand correctly, this is my heating requirement for the home currently and from here, the heating load would only go down with improved sealing + insulation. *EDIT* Just realizing, my hot water is off the boiler, so an amount of the oil used was for hot water - therefore my load might be even lower?

    Are my assumptions correct here?

    1. Expert Member
      Dana Dorsett | | #15

      Without re-running the arithmetic myself the overall approach looks correct.

      The 99% outside design temp of +8F is correct for the nearby Pease AFB/Portsmouth weather station.

      The unaccounted for hot water use load is offset by the unaccounted for solar gains (yes, the sun does shine at least a few hours in January :-) ). The duty cycle is also high enough in January to overcome the worst effects of the efficiency hits from oversizing, so the total error is really "in the noise".

      De-rating the boiler to 80% is probably a bit excessive, but it isn't going to change the whole-house block load numbers by very much.

      If you were keeping the place 65-72F during that period (no weekends away the thermostat set to 50F?) the ~34KBTU/hr @ +8F design load is about right. As a sanity check that's about 15.5 BTU/hr per square foot of conditioned space, slightly higher than the average tight 1987 vintage 2x6/R18 with clear-glass double panes type construction, but neither crazy high nor suspiciously low.

      Do NOT upsize a modulating air source heat pump system by 1.4 per the ASHRAE recommendation, since that usually cuts severely into modulation comfort & efficiency. It will usually use LESS electricity seasonally to size it between 0.9-1.2x the design load and burn some heat strip power (resistance heaters) to cover the few hours per year of shortfall that might actually impact comfort. ASHRAE's 1.4x oversizing factor makes sense for a gas fired furnace or non-modulating boiler, but isn't doing the owners or the planet any favors with modulating equipment (or even single speed heat pumps). With a 1-2 stage gas furnace the efficiency doesn't change at any oversize factor, but you want the duty cycle to be high enough to provide actual comfort, not just heat the space. With modulating equipment sized at ~0.9-1.2x the duty cycle is close to 100% even at temps several degrees above design temp, and ideally it would run similarly high duty cycles even during the shoulder seasons, which it CAN. But at 1.4x oversizing most modulating heat pumps would be cycling rather than modulating most of the heating season in Dover NH.

      If you want to take a stab at a Manual-J-ish load calculation, the BetterBuiltNW design tool ( ) uses suitably aggressive default U-factors and air tightness assumptions for sizing modulating heat pumps. It also has a duct design tool that you may or may not want to use. This free tool was targeted specifically at HVAC contractors installing air source heat pumps to get them to STOP SUBOPTIMALLY OVERSIZING the equipment. I've only used it on one project, but it's numbers are within spittin' distance of other methods, and much simpler to use (with fewer places to screw up) than professional Manual-J tools.

      1. bc_new_england | | #17

        Thank you Dana for the article, guidance and commentary here. Very much appreciated. I will tinker with the BetterBuiltNW design tool - it looks pretty neat!

        It sounds like I can feel reasonably assured that our need for a heating system should add up to around 38-44K net output (1.2X - 1.4X ASHRAE - using 83% boiler efficiency). As of now, I'm leaning towards trying to quote a system comprised of three separate 1-to-1 mini-split systems: 18-24K off the sunroom, 12-15K off the greatroom, and another 12K upstairs (hallway or master bedroom pointed towards other bedrooms?). If I understand correctly, the modulation allowed for in this setup will ensure efficient operation and de-risk me of the room-by-room balancing act of a multi-split.

        Does this all sound right?

        Since we have an operable but dying boiler in place, I would keep it there until it quits and am also considering an electric boiler in the 5-15kW range with an outdoor reset, as advised by Akos.

        But under this setup, I could rest assured that the vast majority of my heating needs would be served by the minisplits, with comfort supplemented by a boiler / electric radiators (another potential option).

        Does this sound right?

        I aim to make the right decision, and ideally I would pay the HVAC/consultant/installer to help me make these decisions - but out of the 10-ish quotes I've received (most from very positively reviewed companies in the area - google/yelp reviews etc) none have discussed this set of options and achieving a balance of heating systems. I've been met with skepticism of using only minisplits to heat the home. I now realize that I'm likely targeting SOME sort of backup system regardless - but planning to use it very rarely. It seems most folks I've spoken with think in terms of one system, not a balance of two or more. I would be very happy with a system that is 95% served by minisplits and 5% served by another option that falls into budget.

        My goal coming out of this is to be confident in a system design I can prescribe to an installer that will keep my family warm, hopefully with a reasonable level of comfort.

        Acknowledging that this is an open forum and no-one has been to our house, all this advice is very appreciated. The energy auditor I was working with was very explicit about how these systems are complex and all variables need to be balanced. It sounds as if he is right, but I might be getting wrapped around the axle on details, when what I really need is 38-44K of heat output down to +8F.

        1. Expert Member
          Dana Dorsett | | #18

          >"It sounds like I can feel reasonably assured that our need for a heating system should add up to around 38-44K net output (1.2X - 1.4X ASHRAE - using 83% boiler efficiency)."

          NO! You're looking for only 30-38K of output, not more. That would be a 0.9-1.2x sizing factor. You are keeping the boiler (or electric boiler) as the "Hail Mary" backup for covering the shortfall during cold snaps.

          Even at 0.9x (undersized by 10%) for the 99% heat load in most zone 5 locations that is still going to be able to fully heat the place for more than 97% of the time without needing any auxiliary heat. (This could be verified against hourly binned temperature data, which I don't have free access to for your location.) Except during Polar Vortex disturbance cold snaps slight undersizing isn't going to result in comfort issues, since the coldest hours of the day occur in the pre-dawn hours when everyone is in bed.

          >" I'm leaning towards trying to quote a system comprised of three separate 1-to-1 mini-split systems: 18-24K off the sunroom, 12-15K off the greatroom, and another 12K upstairs (hallway or master bedroom pointed towards other bedrooms?)."

          Don't assume those sizes are anywhere near correct before doing the Manual-J (or BuiltBetterNW simplified Manual-J) for the zones those heads would cover. Once you have the zone load numbers, calculate 0.9x to 1.2x , and narrow the min/max band on the "Heating Capacity (Max Btu/hr @5℉)" slider on the NEEP search page to find suitable models. You can also narrow that down by vendor/manufacturer if you have specific vendors that are better supported in your area than others. See:


          When comparing models, take a look at the minimum output @ +47F as well as the efficiency at that minimum. Some will have nice low modulation numbers (great for comfort) but LOUSY COP effiency at low modulation. Others have quite respectable minimum-modulation COPs & +47F. eg:


          ... vs.


          1. Expert Member
            Dana Dorsett | | #19

            >" I'm leaning towards trying to quote a system comprised of three separate 1-to-1 mini-split systems: 18-24K off the sunroom, 12-15K off the greatroom, and another 12K upstairs (hallway or master bedroom pointed towards other bedrooms?)."

            Note that maximum cooling capacity of a head is usually higher than the nominal "rated" AHRI cooling rating.

            Similarly, with cold climate minisplits in heating mode a nominally 9K (rated cooling) head delivers 11-13,000 BTU/hr @ +5F":



            Typical better class 1-ton (12K) cold climate heat pumps deliver 15-17,000 BTU/hr @ +5F, a typical 15K ( 1.25 tonners) delivers >18-20K, etc.

            In raw BTU/hr terms it's possible cover your entire ~34K heat load with three 9K heads, and a cold climate 9K head is usually considerably oversized for a bedroom, even a master bedroom.

            It's worth considering a few ducted cassettes for better heat/coolth distribution into doored off spaces such as bedrooms, with the cassette right-sized for the combined room loads. Both Fujitsu & Midea ducted cassettes can be installed in an upflow orientation, which makes installing them in the back or side of a large closet relatively easy (or make a 10 square foot "utility cabinet/closet" at the end of a hallway, etc.) See:


            Take a look at some of the pictures in response #13 from Larry Waters.

          2. bc_new_england | | #20

            Dana - thank you for the advice and direction here. I ultimately went ahead and attempted the simple Manual J via I attached my current results, but have some follow up questions/confirmations that might impact the final number. Currently, the executive summary: whole home = 31,820 BTU heating / 19,608 BTU cooling.

            I made some assumptions/over-rides based on what I understand the end state of the insulation/sealing work I'm having done to be, plus the current state the auditor reported on:

            -attic insulation to achieve R60
            -current wall insulation is R19 (I selected "R13+R5 board", assuming getting R18 in the software was the closest/best number to use)
            -basement wall insulation R11
            -our greatroom has a ~16 foot wall shared with an unconditioned garage, so I assume that should be added to the "exterior length" room calc?
            -window area is measured on the inside for glass only (no inclusion of wood/vinyl framing)?

            I am a bit turned around on the "Unconditioned above ceiling" + "Unconditioned below floor" percentages. My basement is unconditioned, but will be insulated to be part of the envelope .

            For example:

            -should my 2nd floor master bedroom be 0% / 0% respectively?
            -should 1st floor be 0% / 100% respectively? (Sunroom, converted 3 season porch on stilts, and the greatroom, on slab, are exceptions)


            I've started to dig in to products, but before I present my initial design thoughts on that I'm hoping to get my calculations more tuned in via the questions above.

            After doing the initial rough heat load calc based on my oil boiler, running through the BetterBuiltNW tool, reading the suggested articles, and yours and other's advice - I'm feeling much better about having a close idea of what my heating needs look like, and how to better consider sizing some units.

            Due to some limited availability of program funding, contractor scheduling, and a low-interest HVAC financing option tied to the insulation program; I am eager to move forward, but hoping to be very confident in my chosen system components. Even if I can get the downstairs done this year and address the more challenging space (upstairs bedrooms/bathrooms) next year - I'd be very content - since it'd allow me to move forward on some portion.

    2. Jon_R | | #16

      Some additional fuel use (vs Manual J) errors to consider when sizing a system:

      a) outdoor temps go well below design temp - this occasionally happens for extended periods
      b) load calculations using average wind won't be accurate for days with higher winds

      1. bc_new_england | | #21

        Jon - that makes perfect sense. Thanks for noting that. How should I adjust for that? Assuming my self-directed simple Manual J is correct at 31,820BTU, would Dana's suggested sizing a mini-split system method of 0.9-1.2X that load cover me on those rare occasions?


        I also intend to get the -15F degree heating operation units, although system design temp for my area is +8F, unless there's a clear reason not to. Hopefully this will accommodate those rare, sustained low temps.

  6. bc_new_england | | #22

    I've been exploring some of the!/product_list/ Fujitsu products to size one-to-one configurations of mini-splits to serve the first floor primarily.

    Using an average of the simple Manual J results, and the heat fuel results for my heating needs it looks like my range of 0.9-1.2 is 28,385 - 37,846 BTU

    From the Simple Manual J, my first floor accounts for ~14,750 BTU, the great room is ~7,200. I think since the upstairs presents a trickier circumstance to do effectively and efficiently - I'm going to pause that for now and try to get just the first floor and great room accounted for.

    As such, I'm looking at doing:

    - one AOUG12LZAH1 (!/product/51109) off of the sunroom, as it does 11,800 AT -15F, and 16,000 at +5F
    - one AOUG09LZAH1 (!/product/51108) in the great room

    They both modulate down to respectable loads from what I can tell. COP at full capacity, -15F is 1.55 and 1.46 respectively which seem less than ideal, but I imagine them running at full capacity during a -15F period is going to be a small portion of the time.

    Any thoughts on what a layman like myself might be missing in my considerations?

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