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Mini Split sizing feedback

evantful | Posted in Mechanicals on

Howdy all wanted to gauge your thoughts on Mini Split sizing

House facts:
– Built in 1955
– 1008sqft feet
– Single level ranch
– Poured concrete foundation with full basement, unfinished.
– First generation Corning/Owen Fiberglass batt insulation in walls, probably r-11
– r-30 batt insulation in ceiling installed within the last decade..
– White Cedar siding, laid over some kind of particle like panel material coated in black as sheathing.
– Multiple single pane windows with older retrofitted storm windows, two newer double paned windows.
– Ellenville, NY

I used Coolcalc, room by room to try and complete a Manual J as accurately as possible. Heating load was listed at 27740btu and cooling at 18k BTU. This was with Outdoors temps of 88F and 5F, Indoors Temps at 75F and 70F.

Now especially on the winter temp side, I may only hit 5F for a week or so during a cold snap, generally average lows at worse are in the upper teens.

Do those load number seem alittle to high?

My HVAC installer recommended Mitsubushi MUZ-FH18NA2, single head (at my request due to cost) located in the living room. This would be in addition to a forced air oil furnace, installed in 1991. I have attached the drawing of my house layout. The Bedroom end of the house (24ft shorter side) faces North East, the Bathroom/Dining Side South East, Living room side North West, and the SouthWest wall with the Kitchen has a one car attached garage on the other side so it is not directly exposed.

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  1. evantful | | #1

    Sorry, the image attached did not have the partition walls in between the kitchen modified, please refer to the image below

  2. evantful | | #2

    Just as an update, I realized I think my heating load was being effected heavily by the door type I had inserted. I modified it to steel door with polyurethane insulation and that dropped that heat loaded to 22,900btu.

    I only have one entry door that leads directly outside, the front 32x80in door. There is another door in the kitchen that leads to the Garage

  3. GBA Editor
    Martin Holladay | | #3

    It's hard to second-guess your heating load calculations without knowing all the details of your house (especially the air leakage rate).

    However, you have a lot of leeway in your minisplit sizing in light of the fact that the house is also equipped with an oil-fired forced-air furnace (equipment that presumably can help heat your house if necessary when the outdoor temperatures drop to the single digits). The Mitsubishi minisplit suggested by your installer is rated at 18,000 BTU/h, and should serve your needs.

  4. Yupster | | #4

    Entry doors usually contribute less than 4% of the total load to my heat loss calculations. I don't know what you had specified before you changed it to steel with polyurethane but an 18% drop in heat loss suggests something very odd with your calcs.

  5. Expert Member
    Dana Dorsett | | #5

    A heat load of 27,740 BTU/hr @ +5F/70F for a 1008' house (27.5 BTU/hr per square foot) or 22,900 BTU/hr (22.7 BTU/hr per square foot) with a very efficient basically rectangle footprint seems high by quite a bit. I would have expected it to be in the ~15,000 BTU/hr give or take a few 1000, and definitely less than 20K. A cooling load of 1.5 tons for a 1000' house (a ton per 667') is also on the high side. Most houses that size come in at a ton per 1200-1500'.

    Slab on grade, crawlspace or full basement? Is the foundation insulated? If it's a leaky uninsulated basement the load COULD be over 20,000 BTU/hr, but if it's tight and insulated probably not.

    If you have a heating history on the place you don't have to guess. Run a fuel-use based heat load calculation- wintertime fill-ups only, or it will be higher than reality. Are the ducts in the attic, above the insulation? That too would cause overestimation of load. See:

    The FH18NA2 has a capacity of 20,300 BTU/hr, and should cover your whole house load, but if the heat load is where I think it is you'll probably be better off with something smaller- the FH15NA (18,000 BTU/hr @ +5F) or even the FH12NA (13,600 BTU/hr @ +5F).

    The down-side to oversizing has to do the minimum modulated output. To hit the efficiency numbers the mini-split needs to be in it's modulating mode rather cycling on/off. If it's cycling on/off it's taking an energy hit every time it spins up, and has a standby loss to boot.

    The minimum output of the FH18 or FH is over 5000 BTU/hr @ 47F, which is probably more than your whole house load at that temp. The minimum output of the FH12 is 3700 BTU/hr @ 47F, which is probably still less than the load at that temp, which means it will be modulating a lot more during the shoulder seasons, and deliver higher as-used efficiency.

  6. evantful | | #6

    Thanks everyone,

    Dana after working adjusting my doors, The heat load is being registered at 22,800btu, LoadCalc is giving me a similar number.

    Along the lines of what you said, my basement is uninsulated and It's seems to be penalizing my heat load heavily in CoolCalc, insulating the basement to r-11 drops my heat load down to 17,400btu BTU.

    I'm trying to export the data from CoolCalc into a PDF for everyone to see, it's possible they are using a very conservative calculation.

    In regards to a fuel usage calculator, we have only been in the house since february 8th, I do have fuel bills from every 20 days with fill ups to full each time, unfortunately theres one data point that will skew the usage... about 650sqft of the basement is heated by the furnace, at one point it was a "finished" basement, done at some point in the early 60's but completely uninsulated and in poor shape.

    I could still provide that, do set backs at night effect a fuel calculation?

  7. evantful | | #7

    Dana, also the ducts are all located in the basement

  8. Expert Member
    Dana Dorsett | | #8

    The overnight setbacks aren't usually a big factor unless it's actually hitting the setback temperature EVERY night. It's typically no more than a 5% error. If your full-heat thermostat settings are above or below the 68-70F range we can make adjustments.

    The error factor of the sometimes-heated basement will skew the load to the high side of the as-used reality. With the ducts in the basement duct leakage or conducted losses aren't completely lost, but there is some amount of air-handler driven air infiltration if they're leaking into an unsealed basement.

    So, run the fuel use numbers, see if there's any correlation at all to the loadcalc results.

    With the load tool's 22,900 BTU/hr with the basement un-insulated and 17,400 BTU/hr with an 2x4/R11 insulation tells you something important. That's a 25% reduction in total heat load, and total heating energy use, and a very realistic fraction for typical houses. Installing the IRC code-minimum continuous R15 for DOE climate zone 6 (Ulster County NY) is going to be worth it no matter how you're heating the place. With the mini-split carrying a large chunk of the load the duty cycle of the oil-burner will be low, and the temperatures in the basement will be lower than previously. With an insulated foundation the average wintertime temperatures down there may even rise from where they are now.

  9. evantful | | #9

    Dana, what equation would you use for a fuel use based heat loss calculation?

  10. Expert Member
    Dana Dorsett | | #10

    Details of fuel-use heat load calculations are spelled out in detail here:

  11. evantful | | #11

    I seem to be getting a number around 27,000btu.

    With the basement being apart of the heating system (as in it has it's own ducts to service it), completely uninsulated, so about 650sqft of it, does that seem to make sense on top of the CoolCalc number of 22900?

  12. evantful | | #12

    Thanks again to everyone. I think my plan is going to be installing a FH-12NA in the living room, see how it does and worst case I could install a FH06 in the master bedroom, lined up and facing out the door way in the hall to help carry the load on the side of the house during the winter months.

    I already have a 1250 watt baseboard w/ remotely located honeywell thermostat I installed this past winter in the master bedroom to create a zone at night (the furnace when it cycles on is pretty noisy and sits right below our room) which could help carry some of the heating load and I could close the door to the rear bedroom.

    I certainly want to insulate the basement, but I need to address some other issues down there before I can get to that

  13. Expert Member
    Dana Dorsett | | #13

    If the fuel use number is dramatically higher than the loadcalc estimate it's likely that your ducts are unbalanced & or leaking copiously, which drive air infiltration rates very high whenever the air handler is running. You're probably looking at pretty serious parasitic load - a parasitic load that goes away with a ductless solution.

    Loadcalc usually comes in more than 20% higher than reality. A typical ducted system has about a 10-15% parasitic load, a fully commissioned Energy Star ducted system will have less than 5% parasitic losses. Your fuel use load numbers are more than 10% higher than the usually high loadcalc result, which means either the fuel use calculation wasn't done correctly, the combustion efficiency is far worse than advertised, or the ducts are seriously out of whack.

    If you're going to continue to use the furnace it's worth sleuthing out and fixing any duct balance/leakage and house air leakage issues. With a hand held 2-port manometer that has 0.01 water-inches resolution you can usually find the worst offending duct balance problems. An Energy Star duct system will have less than 3 pascals (0.012" ) pressure difference between adjacent rooms under all operating conditions (doors open, closed, etc.). With an inexpensive hand held manometer you'll have to measure in both directions if 0.01" is the bottom of the scale, but any measurement over 0.02" is telling you something, and a differential measurement over 0.05" is an energy use disaster, forcing at least part of the return path to be "the great outdoors" unless the house itself is exceptionally tight.

  14. evantful | | #14

    The duct system, unsealed, is original to the house from 1955, and was modified (poorly) at some point when they replaced the furnace (also done poorly), it's very unbalanced and a good portion of it's load is being blown out the basement vents.

    It's a 156K BTU furnace.

    Ultimately my goal is to have the Mini Split or splits completely replace the oil furnace, and once I feel confident after a few winters, remove the entire system and ducts and wire in some electric baseboards to serve as backup. This would allow me reclaim quiet a bit of space in the basement

  15. evantful | | #15

    Just wanted to follow up with the final choice's made:

    I ended up working with my HVAC installer and I had a FH12 installed in the living room per the original plan and a FH09 installed in the master bedroom, in line and facing out the bed room door. Originally I asked for a FH06 but the HVAC installer offered me a FH09 for the same price because he had one on hand from a cancelled job. See that the low end modulation is the same I went with it.

    - I have attached a diagram below showing the layout of the units.

    The installer was extremely nice to work with and did excellent work. Both units were installed at 6ft (I have 8ft 4in walls), I had to balance how low I could get my wife to sign off on.....
    Total price for both units installed was $4,800 after rebates (which amounted to $2400)

    As far as performance, today it was 79 as the high, very humid. I have had the FH09 turned off and the FH12 running on full auto, it's modulating perfectly and the out door unit hasn't turn off once. Plenty of Condensate coming out the line and humidity levels feel really nice in the house. During the evenings I will run the FH09 with the door open and the FH12 off depending on loads.

    My next purchase will be a Honeywell D6 Pro Thermostat, they just released it and designed specially for Mini Splits. It uses an IR blaster so no gateway or wiring is needed. It has it's own built in temperature and humidity sensor which will serve as a remote sensor overriding the head units. It integrates with the Lyric App.

    I just wanted to say thanks to everyone who contributed, it was a large purchase for us and in the long run I'm glad we didnt go with the FH18, I think in a cooling situation it wouldn't be able to modulate low enough given out load.

    1. seae | | #18

      Question for you did you end up purchasing the D6? I have the same system you have and I have been having trouble with the D6. Honeywell claims it is a faulty D6 but I'm wondering if it just not compatible. The problem is with the scheduling , when i choose to have the unit scheduled to be off it actually turns on what seems like every 5 minute on full blast. Any insight would be great!

  16. evantful | | #16


  17. Expert Member
    Dana Dorsett | | #17

    The internet price difference between an FH06 and FH09 is only a couple hundred US tops, probably less at installer wholesale type pricing. They both have the same minimum modulating at 47F so there isn't an efficiency penalty for the FH09, and it's nice that he could make that deal for you, and you probably saved him re-stocking charges.

    So the total installed cost was $7200 all-in before subsidy, or about $4.1K/ton, which is in the right ball park compared to quotes on recent installations I've seen near me, a bit cheaper than some.

    Between the two you have over 24,000 BTU/hr of capacity at +5F outdoors, 70F indoors (or +3F outdoors, 68F indoors). Given your likely very-high duct losses on the old system you won't be running out of capacity until it's well into negative digits, even if the other bedroom might start to run cool while it's still in the teens (TBD.) I suspect you'll be pleased overall with the heating season performance despite having a room that runs cool when it's cold outside. The oil delivery guy is going to hate you though. :-)

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