Getting the most out of Fujitsu multi-splits
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
2016 BOILER ONLY
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.