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Community and Q&A

Making sense of the first month of minisplit heating data

kjmass1 | Posted in Mechanicals on

I have 4 Fujitsu mini splits (non xlth) hooked up to (2) 24k condensers. They were primarily for AC but I’m trying to reduce usage of my aging 199k/152k steam boiler while the temps are relatively mild (Boston). Right now I’m using a 12K head per condenser, per floor, so 2 total. Dana had some good advice as the min modulation range was about 6200 BTU/hr per unit, and to run both units, so this weather seems to be the sweet spot for efficiency and load.

11/6-12/5 I used 1107 kwh of electricity. Subtracting out .85kw/hr of household electrical usage, I used .68kw/hr for heating in a 26 HDD period. Both units have been modulating nicely, running almost nonstop. One day i watched the condenser and it ran 13/16 minutes or 80%.

I do feel either the first floor unit can’t keep up with the ~800sq, or just isn’t measuring the temp accurate enough to keep it at a reasonable set point. It’s positioned in the best spot for the layout, however logistically could only be 2″ from the ceiling above a window. The second floor unit does a good job keeping the floor consistent, but heat also rises so it’s load could be less.

I keep the remote at 80 for it keep a setpoint around 69. With the colder nights I’ve kept my boiler on as a backup at 68, and it ran for 40 hours in the same period adding in 8500 BTU/hr @ 82%, not including pickup losses.

Do these numbers seem reasonable for the HDD period for a roughly 2000sq ft home that is well insulated? I thought the splits would be able to shoulder more of the load, or maybe I’m just picky and can’t handle a swing of more than a degree or two. Even at a COP of 4 at temps of 40, that’s only 9200 BTU/hr split over the 2 units? It can’t even go that low, can it?

I’m going to try bumping the thermostat up a couple more degrees to see if I can get my boiler usage down even more. Last resort would be to put in a wired thermostat, but I’ve heard those are pretty pricey installs. The ecobee is averaging a couple of sensors on the first floor within the split zone area, so I know it’s accurate to how we feel.

I’d appreciate any advice or insight.


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Your question is a little confusing to me. It seems to boil down to this: You want your minisplits to shoulder more of the load, and you want your oil boiler to come on less often.

    The solution seems simple to me: Raise the thermostat setting on the minisplits, and lower the thermostat setting on the boiler.

  2. User avatar
    Stephen Sheehy | | #2

    Lots of people report problems when the minisplit is located too close to the ceiling, as yours is. Either move it, push up the temperature setting or get a remote thermostat. I believe Fujitsu says the unit should be no closer than six inches from the ceiling. In Boston, you should never need the boiler.

  3. Keith Gustafson | | #3

    Mini splits do tend to allow more deviation than your standard HVAC system, but 11 degrees...

    Are you letting them run full auto?

    I found playing with the settings can be helpful to get around control quirks

    Maybe try forcing the downstairs fan to run on medium or high with the vanes pointed straight down and see if it changes its ability to see the temp of the room

  4. kjmass1 | | #4

    I have been running them in heat mode, high fan, set to 80+ degrees. I will admit the closeness to the ceiling is probably causing the issue, but unfortunately that was the best location for the floor in a 1940's home. It seems like a wired thermostat would give it a better sense of the actual room temp.

    I did play with having a lower boiler setpoint but the overnight temp drops were just a bit too much for us with a baby.

  5. Keith Gustafson | | #5

    Throw a thermometer in the output of the unit and see what the actual output temps are, with that setpoint differential it ought to be running flat out, pretty hot air, if not there might be something else going on.

    How much area is this unit trying to cover and how obstructed by walls?

    2 inches, yeah, way too close. These units get most of their intake air from above so you have really constricted the airflow, basically blocking the intake

    A silly thought occurs to me, if it will run with the front cover open, try that, it might increase airflow[or it might drive the thing crazy]

    Long term dust is an issue but might be worth checking out

    I think I would overhang my window and trim rather than have it that close to the ceiling

    I have broken the rules before, but I don't think that much

  6. kjmass1 | | #6

    Here is a floor plan showing their locations- no real other locations given windows, built in cabinets, etc.

  7. kjmass1 | | #7

    They were primarily installed for AC and luckily the top of the stairway unit is 6" down from ceiling so that one works really well. Unfortunately this was the only space for the first floor unit for it to cover the majority of the first floor ~800sq ft. We have high window trim and built up crown so it was tight.

    I guess in theory, with the 2nd floor unit more of an AC unit, the first floor unit could have gone below the window as I've seen some do here. I can't imagine it working well in the summer but certainly would do better in the winter. Again, we approached this from an AC point of view so that was my fault not truly understanding how well these would work in the winter. Attached is a pic of the unit on the first floor. Probably could have cut in to the trim a little bit and gotten a couple more inches out of it.

    It certainly has no trouble cranking out air at high speed, so with it being in a little alcove, I think it is tricked in to thinking the surrounding air is warmer than it thinks. A wired thermostat should help with that, no?

  8. Jonathan Lawrence CZ 4A New Jersey | | #8

    So you are the one who has been hoarding all of the $2 bills :)

    Did the installer ever suggest going with a floor mount unit?

  9. User avatar GBA Editor
    Martin Holladay | | #9

    I'm pretty sure that the manufacturer's installation instructions note the minimum distance between this unit and the ceiling. (Once again, an argument supporting the reading of instructions.)

    In retrospect, it would have made more sense to cut into your window trim than to cut into your crown molding.


  10. Keith Gustafson | | #10

    And to make up for putting it up against the ceiling you pushed the trim right up on either side.....

    This image is going to be saved under 'this will never work'

    sorry couldn't resist

    If you centered it on the window and basically removed the top window wait......leave the trim, build the wall out so that it is flush with the trim, rehang the unit centered on the window with the vanes so when open are in front of the window shade. Basically fill that entire corner with trim, extend the top horizontal window trim to the corners of the alcove, the entire space above the window is trim, two little squares of green either side of the unit. The horizontal trim will hide the umbilical going sideways to meet the existing exterior location. Days work plus paint, probably no new parts needed[other than trim]

    And it will look better than that off center business you have going

  11. kjmass1 | | #11

    I thought it would look better centered between the wall and the kitchen cabinet...unfortunately the window is no longer centered so it is a little off putting.

    The crown was there first and was notched out to make room for the split.

    I think this is what you were describing? Not sure I like all the white "hanging off" the window like that. I would probably just take the 3/4" panels straight up and follow the windows width to the crown. Ideally I'd rather box in that whole area above the window with some horizontal bars to give it more of a built in look but not sure if that can be done either without restricting airflow. I've seen a couple examples out there.

    Thanks for the suggestions. Ironically it worked pretty well in AC mode.

  12. User avatar GBA Editor
    Martin Holladay | | #12

    It's possible to shim behind the unit (with a shim that is the same thickness as the window trim) without making the shim visible. If the shim is the same size as the minisplit unit, you'll still see green-painted drywall on either side (instead of the huge white-painted panel of plywood, shown in your mockup).

  13. User avatar
    Dana Dorsett | | #13

    According to the manual, the top of the backing plate for the ductless head can be as close as 2" from the ceiling for xxRLS3 heads. See the diagram on p11 (PDF pagination):

    That just means it will be able to deliver the requisite air flow, but doesn't address the temperature error issue.

    The heads have internal temperature sensor offset adjustments which can help, but the "right" solution is a remote thermostat. When you go that route you lose some of the functions (such as the "MIN HEAT" function) that you have with the hand held remote, but in general it's a net win.

  14. User avatar GBA Editor
    Martin Holladay | | #14

    I stand corrected, and I apologize. Evidently you read the instructions and I didn't. (I must say, however, that I am surprised that such a clearance is allowed.)

    Thanks also to Dana for keeping me on my toes. Just to be clear: The top of the mounting plate has to be at least 4 inches from the ceiling to ensure that the top of the unit is at least 2 inches from the ceiling.


  15. kjmass1 | | #15

    Interesting that Fuji is ok with it going that high. One drawback to lowering it over the window trim is that on the outside that cover would be in an awkward place overlapping the exterior window trim. I could probably lower it 3" before the outside cover would start to interfere with the trim outside.

    So maybe a wired thermostat is the way to go for now. I think the unit has to be taken apart and a communications board installed so maybe that would be the time to at least drop it down a little bit, although I might have to punch a new hole for the drain.

  16. kjmass1 | | #16

    No problem Martin. I felt like I was reading the directions more than the installers at times, unfortunately...although I won't take credit for knowing the clearances as all of the units were pretty much locked in by either width or height.

  17. User avatar
    Dana Dorsett | | #17

    For the record, I've seen 7RLS heads mounted less than 2" from the ceiling that still somehow worked OK, partly due to the fact that they were located in low-load walk out basement bedrooms, and never really ran at max speed (except when they were the only head calling for heat/coolth from the 2-ton multi-split compressor.)

    In the same house on the same compressor a 12RLS head mounted ~9' up, above a window on the gable of a vaulted ceiling living/dining area had to be tweaked internally to deal with the temperature offset, even though it was still a good 2-3' below the peak, and nowhere near the sloped ceilings.

  18. Keith Gustafson | | #18

    More or less, yes that is what I was saying. Balance the trim by extending it on both sides, maybe pull the crown back to allow more airflow.

    I was proposing leaving the exterior trim right where it is, moving the lines as they pass through the wall to get there, hidden by the new interior trim.

    I was actually thinking about it and I did mount a no name minisplit on a 6x12 beam in a hallway, which could not have been very much further from the ceiling


    Hindsight being 20/20 I would use a ducted head unit there with maybe some false soffits to get air pointed in the right direction, but that is not real helpful right now.

    Try opening the cover. If it maintains temps better with the cover open[assuming it will run] then it is worth moving it down

    If not, then it is a function of the large disconnected space it is trying to heat. AC is easier, because it is first about dehumidifying and as long as a space is dry, it will feel comfortable. If you had 8 people in your den it would get stuffy and hot fast, but just wandering through the house it is going to be fine. Heat you are trying today to be 50 degrees over outside, and that den is going to be cold, and the dining room not that much better. The living room is straight ahead, so it would be ok, but really that head unit is trying to cover a lot of territory. I cannot immediately think of a way to get more air moved around that space

  19. kjmass1 | | #19

    The little alcove would have been a great spot for those ducted heads that are long and slim. Would have looked real sharp if it was all finished off and trimmed out...but a little out of the budget.
    That back den is spot heated when the boiler isn’t running.

    It does add meaningful heat among kitchen, living room and dining room. At a minimum it’s cutting my boiler usage in half and getting me about 30% savings overall (before this cold snap now at least).

    The boiler room is under the living room so the residual heat when the boiler does run acts as a nice radiant floor.

    Hindsight I might’ve been able to get away without the master unit, but it was nice to keep humidity down in the summer though. Haven’t used it for heat yet we just keep the door open.

    All said, I’d say there is maybe a 2 degree difference between dining, kitchen and living. I’d have to check the ecobee logs.

  20. Julie Mccormick | | #20

    I have 3 mini splits in a small 5 room house, zone 7. They are all 2-4 “ from the ceiling. A 9000btu unit in a120sf room was the most problematic with uneven output and a lot of creaking/cracking noise. All issues were resolved with a remote thermostat. Another unit in a larger area encompassing 2 adjoining rooms was always quieter, however, it required regular thermostat adjustments in the winter. If outdoor temps are >40F the thermostat is fairly accurate in maintaining an equivalent indoor temp. As outside temps drop, the thermostat needs to be adjusted incrementally, up to 8-10 degrees higher to keep the same inside temp. Ceiling fans are in use. I should probably use remote thermostats for all of the units but it’s kind of fun trying to guess what the settings should be. The handheld controllers have many functions most of which I’ve not found to make any difference.

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