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Getting the most out of Fujitsu multi-splits

kjmass1 | Posted in General Questions on

I had some mini splits installed in Fall ’17, had issues over that winter so this is the first winter they are all fully functional in heating mode. Worked great in the shoulder season so far. Fujitsu tech checked operating temps a couple weeks ago and said all was good.

I have 4 heads attached to TWO 24K non-hyper heat multi outdoor units. A 12K serves about 850sf on the first floor open living area, 12k top of open stairway, 7K in the master. A 4th unit is in the attic playroom but only used occasionally.

I turned off the boiler backup this past weekend and the house dropped from 71 to 65 and held there, temps were in the 20s and 30s. I pulled some data from a couple years ago to compare the boiler only and with minisplits the last couple days which were similar temps.

Boston area, 1800sf on floors 1&2



Average 7.6 therms per day heating

34HDD average base 65, keep house at 71 (+7HDD)

7.6 therms * 99999 BTU/therm *.80 (old steam boiler) /24 hours = 25,333 BTU/hr, ~617 BTUs/degree hour at 41 HDD

Past Two Days with Mini Splits

3.6 hrs/day runtime (192,000 boiler input)

33.5HDD average base 65, keep house at 71 (+7HDD)

3.6 hrs * 192,000 BTU boiler * .80 efficiency / 24 = 23,000 BTU/hr, ~568 BTUs/degree hour at 40.5 HDD

It’s like I’m not getting any heat from these things yet they are running nonstop (double $$$?). The first floor unit, attached to a wired thermostat (set a couple degrees higher than room sensed temp, high fan, set to regular insulation mode) is blowing hot air all the time. It’s not blasting like max high speed AC, but steadily blowing heat 24/7. Temp gun was close to 100 degrees on the plastic.

2nd floor units seems to be using low speed fan, and will kick up fan speed as needed. ODU will occasionally cycle off but it’s running probably 90% of the time. ODU’s have been defrosting if needed.

I was under the impression that the first floor head should output up to 13k assuming the compressor is keeping up…the AOU24RLXFZ is rated at 14,100 @ 17 degrees, and a max of 19,800 BTU at 17 degrees, and max 16,300 at 5 degrees. Surely that is plenty for temps in the 20s and low 30s, with just one active head attached?

Am I missing something? These worked great with temps above 40, but doesn’t seem to be having any meaningful impact on my gas usage. House is old but good insulation in attic, updated windows and blown in on first floor walls. Rough coolcalc put load around 50-55k at 9 degrees.



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  1. GBA Editor
    Martin Holladay | | #1

    Your description is confusing, as is your math.

    During the recent two-day experiment -- described as "this past weekend" and "the past two days" -- was your boiler on or off? I think you told us it was off ("I turned off the boiler backup this past weekend.") So why does your math for "Past two days with minisplits" include a calculation for the runtime of your 192,000 BTU/h boiler?

    If your complaint is simply that your indoor air temperature is 65 degrees with the minisplits instead of 71 degrees (and you want your indoor air temperature to be 71 degrees), try increasing the thermostat setting on your minisplits until the indoor temperature hits 71 degrees.

  2. kjmass1 | | #2

    Sorry Martin- I didn’t mean for it to be confusing. Last weekend I turned off the boiler to see how well the splits would operate on their own (thought they would have been able to handle those temps, they couldn’t keep up).

    I then monitored a couple days earlier this week with similar temps with the boiler as a backup to see how much it was running. I was surprised that it doesn’t seem like I’m getting much BTU output from the splits (compared with 2016 similar month, no splits). They are almost identical. The splits are definitely pumping out heat it just doesn’t seem to be having an effect on my boiler usage, at least at these temps.

  3. GBA Editor
    Martin Holladay | | #3

    Try turning off your boiler, and increasing the thermostat setting on your minisplits. (Set them to 78 degrees if you have to -- the resulting indoor temperature may be 71 degrees.)

    I doubt if your conclusion that "they couldn't keep up" is accurate.

    1. kjmass1 | | #4

      I will give that a try this week when we are gone for thanksgiving.

      As far as actual performance / heat output, at roughly 30 degrees, should I expect the air to be blowing as hard as say 47 degrees with a nonhyper heat model, even if the outdoor unit should have the capacity? It’s definitly not blowing as hard as in milder days.

  4. Jon_R | | #5

    If "boiler off, high thermostat" doesn't work, then I'd guess a refrigerant charge problem. This is probably consistent with a lower than expected airflow rate.

  5. DavidLeff | | #6

    To test if there is a refrigerant problem...try the "Powerful" button (assuming remotes are similar to mine). My 15K single head unit puts out about 105-110 degree air down to about 35 degree outside temperature.

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