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Community and Q&A

Single minisplit for AC?

jeffesonm | Posted in Mechanicals on

Looking for feedback on adding AC to a ~1600 sq ft ranch in central NJ. Current setup is a window unit in the bedroom and a 1980s 220v beast of a wall unit in the dining room. Together they do all right in those worst weeks but for the most part we’ve just been toughing it out. I’m on a well shaded lot and the house stays reasonably cool during the summer, but also a bit damp. So hoping to dehumidify with the AC and of course cool the place down during the hottest weeks of the year.

1960s construction, plaster walls with foil-faced fiberglass batts of indeterminate insulation value. Similar on the ceiling, plus R-30 insulation unfaced batts I added on top, running perpendicular to joists. Original single pane double hung aluminum frame windows, with storm windows. Solid doors. Garage and bedroom ends of ranch are gable ends with no overhang, front and back of house have 18″ or so roof overhang. Front of house (kitchen in front) faces roughly NE.

Heat is primarily from a Jotul F55 wood stove, supplemented by hot water baseboard and Burnham MPO-IQ65 oil boiler when it gets under 20 or we’re not home to feed the wood stove. Not really looking to heat with the mini split but might be a nice option to have during shoulder seasons.

I did a room by room heat loss calc using and came up with 53k btu heating and 21.5k btu cooling. Floor plan, square footage and cooling btu numbers are attached. I did the one bath together with the adjacent hall and the other together with adjacent den just to keep things simple. Also of note is the high living room btus due to 80% of that wall being glass.

That turquoise box on the drawing is where a 2’x2′ whole-house fan is located. I was thinking a simple solution would be to replace it with a ceiling cassette mini spit in the neighborhood of 18k btu/hr. Seems like it would possibly be a little undersized, but only on the worst of days. Given how well we’re getting by now, maybe close the doors to the unused bedrooms and it’d be fine. My better half and I occupy the home and I expect it to remain that way for the foreseeable future. I understand this would require open bedroom doors for air circulation but that’s how we do it with the wood stove and it works fine for us.

I have a full basement so plenty of room down there to run ducts, but I’d really rather not. Don’t want to cut a bunch of holes in my beautiful wood floors or lovely plaster walls, and I hate banging my head on basement ductwork. 4:12 roof so not much room up there. I did consider a ducted mini split centrally located in the attic, with short runs to the bedrooms. Not sure it’s worth the hassle of ductwork for marginally better air circulation, and then I’d have to figure out returns, ducts in uninsulated space, etc. Again I’m designing for the two of us and it’s predominantly the kitchen, living room and bedroom 1.

Thoughts about this plan, or other options I should consider? Thanks in advance.

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

    I don't see a downside. Try it and see if it works.

    You can always add another minisplit in the dining room if you decide you need to.

  2. User avatar
    Dana Dorsett | | #2

    As a general rule coolcalc overestimates loads, often by 35%. A cooling load of 21.5K for a 1600' house is about a ton per 1000', which isn't unheard of, but somewhat bigger than average. A heating load of 53K for a 1600' is 33 BTU/hr per square foot, which is about twice what I would expect for a reasonably tight 2x4/R11 house with single-panes + clear glass storms. When using that tool be aggressive on all assumptions, particularly on air-tightness.

    Since humidity is a primary issue, be sure to look at the minimum modulated cooling output as well as the max capacity- the lower the better, since that will give you the longest cycle times.

    eg, The SLZ-KA15NA & SUZ-KA15NA combination can throttle back to 3800 BTU/hr in cooling mode whereas the Fujitsu AUU18RLF + AOU18RLFC can back off down to 3100 BTU/hr, about 18% lower. That may not seem like much, but it makes a difference.

  3. User avatar
    Dana Dorsett | | #3

    With a 1.25-1.5 ton ceiling cassette and through-wall unit in the dining room you probably have more than sufficient total cooling capacity.

    The beastly wall unit in the dining room is probably ridiculously or even ludicrously oversized for the combined cooling load of the dining room + den/bath area, which leads to lousier latent load control. (What are it's nameplate BTU/hr and EER specs?) Since it's 30 years old it probably no longer hits even it's original EER numbers. The load numbers for the dining/den area don't really add up to a mini-split worthy load, since the minimum modulation of most mini-splits will be above the cooling load of the dining room, so it won't really modulate with load (much).

    But replacing the past-it's-prime cooling-only wall unit with a half-ton point terminal heat pump (PTHP) or more appropriately sized half ton through-wall air conditioner might be "worth it". (A half-ton PTHP runs about $200 more than a half-ton AC-only unit. $700-800 vs $500-600) If you get one that has a scroll or rotary compressor it will be a lot quieter than what's there, and by being closer to right-sized for the loads the duty cycle will be higher, with better humidity control. Something like the Amana PBH073G35CB (high wall mount) or PTH073G25AXXX (better as low-mount) would do it, and would probably more than cover the heat load for that zone in heat pump mode too. There are others.

  4. jeffesonm | | #4

    Thanks for the feedback. I was eyeing up that Mitsu but will check out the Fujitsu as well. I will have to dig around for the manual for the beastly wall unit, I don't see the BTUs anywhere visible. Probably give the mini split a go and replace the wall unit down the line, if it's still needed.

  5. User avatar
    Dana Dorsett | | #5

    The BTUs, EER and model numbers are usually on a plate inside the front grille where you can peek at it with a flashlight, or by the filter access door/slot, sometimes on the side of the unit (making it impossible to read when it's installed.)

    All else being equal, Fujitsu AUU18RLF / AOU18RLFC ceiling cassette is also about 20% more efficient than the Mitsubishi SLZ-KA15NA / SUZ-KA15NA . (SEER 20 vs. SEER 16, HSPF 11.5 vs. HSPF 9.6). That's enough to make measurable difference in the power bills. It's AHRI rated capacity at +17F is literally twice that of the Mitsubishi too- 21,600 BTU/hr vs. 10,500 BTU/hr. Even if it's a bit more expensive up front, it's probably going to be worth the up-charge to go with the 1.5 ton Fujitsu than the 1.25 ton Mitsubishi for the extra capacity alone.

  6. Vanessa | | #6

    Dana, thanks for the great info comparing ceiling cassettes between Fujitsu and Mitsubishi, its just what I was looking for. Where did you find the capacity at +17F for the Fujitsu AUU18RLF / AOU18RLFC ceiling cassette, please? Only the 47F min-max capacities are listed on the submittal sheet I found.

    Do you have any info comparing the Daikin ceiling cassettes to these two brands?

  7. User avatar
    Dana Dorsett | | #7

    The ARHI "Nominal heating" capacity of 21,600 BTU/hr is the modulation level at which it was tested at +47F for efficiency, not necessarily it's maximum. But to run the test it has to be able to deliver at LEAST that much at +17F.

    If it's consistent with some other Fujitsu cassettes compatible with the AOU18RLFC compressor it can probably still deliver the full 21,600 BTU/hr even at +5F, and something close to 23,000 BTU/hr @ +17F when running at it's maximum speed.

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