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Community and Q&A

Sizing a Minisplit

TNREM | Posted in General Questions on

Hi folks,
We are totally off-grid in the rural Southeast US with a solar system as follows:
6.7 KW of panels
8800 Watts of inverter (2 – 4400 units slaved together)
24.5 KWh LiFePo battery bank (Simpliphi)
12 KW Kohler generator on auto-start for back up

We moved to the house full-time in March and this was our first full summer there. At the start of the summer our battery bank was 17.5 KWh but I recently expanded it to 24.5 KWh.

We decided we needed AC for the summers but couldn’t make a firm decision so I put in a 110 VAC window unit rated at 14,000 BTUs as a temporary measure. It was in one bedroom and we blew it all over the house via some pretty inefficient box fans. FYI the house is a 1450 SF manufactured home with 6″ walls and upgraded insulation throughout. Surprisingly (to me anyway) is the house is pretty tight and the little window unit was able to keep the house quite comfortable even on the hottest days. Also, the batteries were floating most days before noon, even with the AC going full blast. However, and I suppose obviously, all bets were off when the sun went down if we continued to run the AC and box fans. The genny would crank up around 2 or 3 AM. I set it to auto start at 20% SOC and kick off at 50%, just so as not to burn as much propane to get us to full sun on the panels. After I upgraded the battery bank, some nights we could make it without the genny kicking on but most times it still cranked up just before sunrise and I’d let it run a bit and shut it off.

Now I want to retire the window AC and go with a multizone mini-split unit. I’m leaning towards Mitsubishi as we lived overseas for a while and that’s what we had (and really liked) there. Also, we heat (for the time being) with propane as well, so I’m figuring on cold days with the sun shining, we can save a few bucks with the mini-split heat pump.

The 4 zone Mitsubishi system I’m looking at is rated at 3940 watts, but obviously that’s with all four inside units running full bore. Given that we’ll be doing ‘zone cooling’ (and heating) with minimal load in the unoccupied rooms, and that we could ‘almost’ make it through the night this past summer with our newly expanded battery capacity and our (extremely inefficient) window unit/box fan combo, I’m thinking the ‘bigger’ split unit should work just fine without starting the genny in the middle of the night.

All that said, I’m betting there are some folks here with deep experience with mini-splits off-grid, so any thoughts or comments are appreciated.

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Replies

  1. nickdefabrizio | | #1

    I do not have off grid experience but I did see somewhere a mini split that worked off of DC power, so yo could actually install it direct to solar panels without using an AC inverter. I don't know if that system is more efficient, but it does seem interesting to consider.

    1. TNREM | | #5

      For sure a DC unit would be more efficient but that's basically off the table due to my current installation without actually adding panels and a battery bank specifically for the AC. In the interests of brevity, I'll forego a long explanation but it basically has to to with cable runs, voltage drops, and the rather 'spread out' nature of my existing installation.

  2. user-2310254 | | #2

    You should have an independent engineer complete a Manual J on your home. Search for “who can conduct my Manual J” for more information. I suspect it will reveal that you need a fairly small system. For comfort, you want a unit that runs longer at fairly low speed to maintain the set point.

    1. TNREM | | #8

      Concur I need a relatively small system, but the problem is I want a 4 zone system which seems to be limiting the choices in regard to the sizes of the compressor/outside unit. I also leaned toward a Mitsubishi product which limits me to one authorized dealer in this area (southern Middle TN) who is about 45 mile away. I've only spoken to the dealer by phone and given him room sizes etc., so I'm just in the beginning stages of this. However, he came back with a 'budgetary' price of almost $12K installed, which seems a bit high. My biggest problem however is that I'm concerned about operation and power consumption. We have a 1450 SF, 3 BR home with a central living area (Living Area and Kitchen Combined). Typically we'd want to run the AC in the living area during the day (about 625 SF) and in the Master BR (about 180 SF) and possibly a second BR (about 144 SF) at night. The unit he's proposing has a 'rated capacity of about 35,000 btu/hr, and a 'capacity range of 9,500 to about 35,000 btu/hr. Frankly almost 3 tons seems like WAY overkill, given that we've been cooling the house (the WHOLE house) with a 14,000 btu 110 VAC window unit (with 4 box fans blowing the air around) on even the hottest days of summer. And I take your point about the Manual J calc, but I also figure that until I understand what my available options are in terms of 4 zone cooling, that the calc is kind of irrelevant until I have a better feel for the capacities of the 'off the shelf' units. I'm also somewhat limited by our location which is WAY out in the country. Frankly, there just aren't that many choices unless I want to go the DIY route. I've done that on the solar, building the barn, and a ton of other projects but I'd really like to offload the AC side of things if possible. I recognize the result might not be 'optimal' but I'm just looking for 'relatively comfortable' with a reasonable power consumption.

  3. Jon_R | | #3

    This isn't for everyone, but I would install a hydronic chiller system with a large cold water tank and fan coils. Done right, it would minimally use the batteries (small amount of fan and pump power) and no generator at night (if the day had sun). No issues with small zone loads or poor part-load dehumidification ratios.

    1. TNREM | | #9

      Thanks Jon, but that's way more effort than I want to expend. With multiple other projects underway, what I'm really looking for is the best available 'off the shelf' solution I can find.

  4. Expert Member
    BILL WICHERS | | #4

    I can't claim lots of off-grid experience, but I do have a lot of experience with power systems design since that's a big chunk of my work.

    That window A/C is probably just doing on/off cycling. There are some losses running that way. Mini-splits tend to modulate, as far as I know ALL of them do that. This means they adjust how much they're doing in terms of BTU output based on the needs of the space, so they're not just cycling on/off. This is a good fit for your application. I think you'll see much better efficiency this way.

    If you can find a DC-only minisplit, you'd need to match your system bus voltage which limits your options. With an AC (alternating current this time, not air conditioning :-) system, you have inverter losses, but those are pretty low these days since there has been a lot of design effort put into maximizing power conversion efficiency with those systems.

    I agree with Steve also -- you want to put in an optimally sized unit to maximize your overall system efficiency. There are some limits to how much these systems can modulate, and you want to keep them in their respective sweet spots as much as possible. An oversized unit won't be able to do this well.

    Bill

    1. TNREM | | #12

      Well, first of all, I expect we're probably 'abusing' the window unit and I don't really expect it to last. It was very much a 'stop gap' measure to get us through this summer. It's a GE unit with 'smart technology' and the compressor is 'inverter driven' so it's SUPPOSED to ramp up or down with cooling demand, similar (as I understand it) to the mini-splits. However, I expect we've been operating pretty far outside it's design parameters. Basically we just got the biggest 110VAC window unit available at Lowes (14,000 BTUs) and have been running it at it's lowest setting in the end BR (64 degrees) and then used box fans to circulate the cool air. No one sleeps in that end BR and you could probably hang meat in there. And the compressor definitely does cycle on and off , because there is voltage drop when it cycles on along with an attendant load increase on the instrumentation in our utility room. I'm sure it would be fairly efficient cooling the 700 SF it's rated for, but we're not really operating it that way. Honestly, I'm totally surprised that it's cooled the house as well as it has.

      Secondly, I'm not looking at DC units because of the current arrangement of our solar. The panels are on our barn (250' away from the house) and the 'DC side' of things (charge controllers, inverters, battery bank, etc.) live in our power shed (which is a re-purposed 20' shipping container) near the barn. The only power coming to the house is AC from either the inverter or our generator (located adjacent to the power shed). We'd have some pretty significant voltage drop issues to run DC to the house, and besides, I'm really looking for the most effective 'off the shelf' AC solution I can find.

      What I hearing (at least what I think I'm hearing) both you and Steve say is that I need to be looking at a harder working smaller unit vs a bigger unit that will mostly be loafing. That pretty much conforms to what I was already thinking but my problem now is how to accomplish that with an off the shelf system that will accommodate 4 zones. Any suggestions as to brand, unit, etc.?

      1. Expert Member
        BILL WICHERS | | #17

        >"What I hearing (at least what I think I'm hearing) both you and Steve say is that I need to be looking at a harder working smaller unit vs a bigger unit that will mostly be loafing."

        Yes, exactly. The ideal situation for efficiency is a unit that is running flat out 100% and just barely maintaining the setpoint temperature. Since the real world outdoor temperature varies throughout the day and with the seasons, your unit needs to be sized for a scenario close to worst-case conditions but without going overboard (which is why we have the concept of "degree days" for sizing HVAC equipment). The modulating units then modulate down when they're full capacity is not needed.

        BTW, if you put protectors on all your cabling in and out of that steel shipping container and bring all those cables through a common entrance panel to keep all the grounds together, you'll be nearly lightning-proof with your system inside the container.

        Bill

        1. Jon_R | | #19

          > The ideal situation for efficiency is a unit that is running flat out 100%

          Let's check this. Say with the Gree Sapphire. Hmm, COP = 5.5 at min modulation and 3.41 at max/100%.

  5. bfw577 | | #6

    I have electrical monitors on both my mini split's and its eye opening how little power they use. My Gree Sapphire at its lowest cooling modulation only draws 80-100watts and puts out 2900 btus. I heat and cool my entire house with 2 12k splits running of my net metered solar panels.

    They both blow away window acs that usually cycle on and off drawing 500-1000 watts.

    https://ashp.neep.org/#!/product/25406

    1. Expert Member
      BILL WICHERS | | #7

      For the sake of completeness, remember that "watts" are a rate, while "watt hours" are a unit of energy, a volume. Watts are like "gallons per minute", kilowatt-hours are like the number of 5 gallon buckets you fill in an hour.

      This basically means that if you have a unit using 100 watts all the time for an entire hour, that is 0.1 kw/h. If you run a 1000 watt air conditioner for 6 minutes (one tenth of an hour), that's also 0.1 kw/h.

      Minisplits try to modulate down until they're putting in (for heat), or taking out (for air conditioning) the same amount of thermal energy that your home is either losing or gaining, respectively. This tends to result in a very comfy, constant temperature. Often times the modulated minisplit will end up using less overall energy in terms of kw/h as a conventional cycling air conditioner to maintain the same comfort level. That's one of the places your energy savings comes from.

      Bill

      1. TNREM | | #14

        Yep, got that. :)

    2. TNREM | | #13

      Thanks, Good to know. :)

  6. user-2310254 | | #10

    TNREM,

    One downside to multi-splits (based on GBA posts) is that they don't modulate down as far as individual mini-splits. So be to keep this data point in mind. You might want to consider a ducted mini-split setup (especially if you have ducts already). I have a 2 ton Daikin retrofitted into the ducts that cover my top floor (about 1,000 square feet). It is very quiet and convenient.

    If you can't find a reliable contractor for your project, you might want to consider a DIY approach such as Mr. Cool. It may pencil out with enough cost savings to justify the added risk of a self-install.

    1. TNREM | | #15

      Steve,

      In reverse order, I'd been trying to avoid the DIY approach simply because I already have too many projects going and don't really want or need another one. FYI, I'm not against saving money but the simple fact is in our current situation, my time is best spent doing other things if I can find someone else I can 'offload' this task to for a reasonable cost. DIY is (for me) the last resort on this particular project.

      I take your very valid point regarding the multi-splits vs the individual mini-splits. However I was leaning toward a 'zoned' unit to simplify the electrical end of things. The way this manufactured home is constructed, running additional circuits is a pain, so if I could get by with one new circuit instead of multiples, that would be an obvious plus.

      Anyway, I'm coming to the conclusion that the way to go is likely to research the options a bit closer and to tell the contractor what I want rather than ask him for his recommendation, since the 'default' answer seems to be 'bigger is better.'

      Thanks for the input/pointers.

  7. GBA Editor
    Kiley Jacques | | #11

    Martin Holladay talks about three different sizes and capacity ratings of Mitsubishi units in this article, How to Buy a Ducted Minisplit. I think it might be helpful.

  8. TNREM | | #16

    Thanks Kiley, I'll check it out.

  9. gusfhb | | #18

    Well, if a 14k btu unit cooled the house, you know what the biggest unit you need is

    They really only go down to 9k btu, so...........
    If you put a 9k with the best turn down ratio in the bedroom, and another in the living space, you could shut down the living room one at night, or turn it[or both] to dehumidify you may make it till the sun comes up.

    1. TNREM | | #20

      Thanks Keith, Yeah, I guess I did an 'accidental Manual J' calc. LOL

      And you're correct in that it's tough to get a smaller capacity mini-split. My problem is I really want 4 zones. I initially wanted a multi-split 4 zone unit but after looking at things and kicking things back and forth here, I see the error of that. The 'range' on the 'multi-split' Mitsubishi 4 zone unit I was looking at was 9K to 35K btu/u so even at 'minimum' capacity it would often be more than I'd need. However, I've found that Mitsubishi does make one model rated at 6K btu/h but only in the 'hyper heat' models. I'm not sure exactly what that is, but I need to research it a bit more. Right now I'm looking at 3 - 6K units (one for each of the BRs) and one 9K unit (for the living area/kitchen). Having multiple outside units would be a bit of a PITA, but presuming I can get the cost down that would likely give me the best efficiency and most control. Also there is the added benefit that if one went down, we're not 'dead in the water' so to speak. I'm still waiting to talk to the Mitsubishi dealer/contractor on Monday.

      I've also started to ponder a GSHP using our creek as an 'open loop' but that's the subject of a different post I suppose. :)

  10. gusfhb | | #21

    THe conflict I am seeing is between you wanting to run on the batteries rather than generator and then wanting 4 zones.

    Something has to give.

    Consider ducted units, they are slightly less efficient, but you can get your distribution with fewer heads.

    hyper heat is the low ambient temp heating, which you don't need but you might end up with as the most efficient units may have it

    1. TNREM | | #22

      Keith,
      First, thanks for the clarification regarding 'hyper-heat.' I think you're correct in that I don't need it, but may go that route anyway to get the smaller unit, providing it isn't cost prohibitive. I should be able to sort that out this week after talking to the local Mitsubishi dealer.

      My need for 'zones' is more a 'personnel' issue than a technical one. As in 'keep your own bedroom as cool as you want, but I'm not turning the whole house into a meat locker.' It's a 'peace in the family' sort of thing, which trumps technical/financial issues every time, so if 'something's gotta give' it ain't that. LOL

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