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Community and Q&A

Minisplit for small home in Arkansas

SimpleNurseGuy | Posted in Mechanicals on

I’m building a 600 sq ft 2 bed 1 bath house in northwest arkansas. 18′ cathedral ceilings w zip sheathing, slab on grade r15 mineral wool walls, r30 mineral wool ceilings maybe r38. No slab insulation, house orientation is east to west with 8′ porches on each end. There will be only 3 windows front and 2 windows back, w 2 exterior doors 1 front and back. 1 window in the bathroom on the north side. All low E. I think most people around here have minimal experience with mini splits and most hvac is oversized in general from what I’ve read here. I would like opinion s/suggestions on appropriate sized mini split for heating and cooling this space as well as areas to improve on before I start building. The majority of the volume of air would be in the great/living/kitchen/dining area w an upstairs open loft and I also wonder if 1 head will heat and cool this adequately as the bedroom downstairs is only 110 sq ft and I’m sure even a 1/2 ton head in there is way too much. Any help is appreciated! Thank you

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  1. Expert Member
    Akos | | #1

    I would run your place through one of the online load calculation programs (coolcalc or loadcalc).

    For smaller places, your cooling load will depend a great deal on your window size, orientation and shading. Make sure you think about this before sizing the equipment, oversized windows on the wrong side of the house can easily add a ton or two of cooling load.

    If you don't mind open doors is bedrooms, you can probably cool the whole place with a single wall unit mounted high in the living space. This would also do a reasonable job of heating the living space but it won't cover the more tucked away rooms. Generally in a well insulated place, you can heat those with a small amount of resistance heat (floor heat/baseboards/panel) for minimal operating cost.

    A better setup would be a single ducted unit mounted in the loft area with ducts to the main floor. With a bit of careful register placement, this can heat and cool your place without needing to leave bedroom doors open. This does mean running ducts, which can add a fair bit to the cost unless you can DIY it.

    1. SimpleNurseGuy | | #2

      Thanks akos, the glass on east and west sides should have enough overhang to minimize gains vs not having them. I did intend to mount the head high in the living space and use a radiant cove heater in the bedroom to supplement heat. Ill try n work out wether I think the ducted system is something I can swing but is a good idea. I've tried a few versions online but got figures like 14k cooling and 26k heating. From many things I've read here I simply didn't know wether to trust this or not.. Thanks again Akos. Materials have gone up here and it may be a few more months before I can get started so I thought I'd bounce a few ideas off everyone. I've been suggested to use 2 or 3 head systems but as I read they do not modulate as well and would likely short cycle a lot in that smaller 110 sq ft bedroom w a 9k head .. I thought a 18k unit would be enough for the entire house though was hoping to use a 12k cooling 16k heating unit.. lg specs are great.. Mod down to just over a thousand btu/hr.. Just not sure..

  2. Jon_R | | #3

    > lg specs are great.. Mod down to just over a thousand btu/hr

    While this sounds good, it's likely that at this low output, efficiency will be poor and dehumidification will be zero. I suggest sizing right and ignoring modulation range specs beyond 2:1.

    Also consider Daikin Quaternity for much better dehumidification.

    1. SimpleNurseGuy | | #4

      Thanks Jon R. I believe I may have misunderstood in regards to modulation. I thought the more it modulated the less it short cycled and thus the longer it ran which would provide more dehumidification and better energy savings. However I'm new to this and maybe need clarification in these regards. Thank u sir

      1. Jon_R | | #5

        That's widely believed but is often false. At some common operating conditions , mini-split cycling improves dehumidification and at others, efficiency. For specifics, look at curves of SHR vs load and COP vs load (data that manufacturers almost never make available).

    2. Expert Member
      Dana Dorsett | | #6

      +1 on the recommendation for using the Quaternity series, most likely the 1 tonner, barring lots of unshaded west facing glass The FTXG12HVJU / RXG12HVJU delivers about 13,000 BTU/hr in heating mode at your +15 t0 +20F 99% outside design temp, and about 10K @ +5F, which should be enough if you at least have the code-required slab-EDGE insulation:!/product/25237

      That's over 20 BTU/hr per square foot at design condition, odds are your actual design LOAD would be 15BTU/hr per square foot, unless it leaks a lot of air or the windows are large.

      It's hard to get decent temperature balance between levels with just one head, especially if the bedrooms are doored off. A ducted mini-split like Fujitsu's 3/4 ton AOU/ARU 9RLFCD with seasonally adjusted balancing vanes on the ducts might be more comfortable that the single head Daikin. It has more capacity than the 1-ton Daikin (higher efficiency too), but doesn't have a settable humidity level of the Quaternity. There will be days when it needs to be run in "DRY" mode in summer to bring the humidity down. Overall it requires a bit more user-input, but can probably provide more comfortable for those willing to tweak the balancing vanes and settings to adjust for the seasonal weather.!/product/25310

    3. Jon_R | | #7

      From another topic, an interesting device that apparently automates fixing the lack of dehumidification caused by low modulation (as a ratio of fan speed).

      The Fujitsu above is a good example of the problem. The compressor has a 4.8:1 range but the fan is apparently only 1.3:1. The resulting high CFM/ton often causes no dehumidification. So you might add a dehumidifier, which kills efficiency and is likely to have distribution problems.

  3. Expert Member
    Akos | | #8

    Folks, we have to be realistic here when it comes to recommending the Quaternity unit. That unit is specifically designed for galleries and art collectors, not what I would call a budget option.

    For the OP, the better option is a correctly sized AC unit with supplemental dehumidification if needed.

    The flow rate on most ducted units can be reduced to lower SHR and get more dehumidification. This can be done by either programming in lower static system static pressure or just simply partially closing vents. It is something I do at home, it works and works well.

    As for the heat loss, a well sealed 600sqft building in a 20F outdoor design temperature climate with R12 walls and R30 roof, you are probably between 10000btu to 15000btu heat loss, which can easily be covered by a single wall mount or slim ducted unit. A hyper heat unit is not needed but it might be worth as it doesn't need to be oversized for heat.

    A multizone setup is a very bad idea in this case as most will be at least 2x over sized for the load which means low efficiency and comfort.

    Dana's suggestion of the AOU9RLFC / ARU9RLF is probably spot on for this application. There are also some Gree and Midea units with similar specs that are about 1/2 the price.

    1. Jon_R | | #11

      > lower static pressure

      One can shift the curve, but the range still won't match the compressor range. Ie, I'm sure it helps humidity, but it can only achieve the correct/efficient CFM/ton for a limited load range. Now if the adjustment were automated....

      All the marketing I've seen for the Quaternity explicitly targets home use. It is expensive.

  4. SimpleNurseGuy | | #9

    thank you gentlemen, it looks like ive got some reading to do ; ) always appreciate what folks have to say here and ive found myself turning to GBA frequently when i have questions i need answers to. i appreciate your time and when ive looked at enough of the above to formulate additional question(s) ill be back for more input. i had wanted to put a 1 ton ductless head on the exterior wall in the living area which would be mounted about 6-8' and would cover the open loft as well, that overlooks the kitchen/living/dining area but would be able to blow air down the short hallway back to the bathroom/lower bedroom area. i had intended also to put a 1000-1500 btu cove heater in the bedroom to help maintain comfortable temps when it got into temps that kept the 1 ton from keeping the room 68-70 degrees. a medium sized ceiling fan for comfort in the summer to move air was also the plan i had in mind. im a firm believer in less is more but i dont want to leave myself wishing id done something differently, considering im in a position to 'measure twice and cut once' with the new construction vs remodeling an older house and being left w fewer options. whats everyone's thoughts on a unit placed in such a way as i mentioned above to cover the space as i described with supplemental heating in bedroom and fan to move additional air? i will look at the daikins and see if the ducted system is something i can make work. its tricky...finding a place for everything. it took me awhile to figure out i could even do it w mini splits. here in arkansas you dont see alot of cathedral ceilings..except in churches..haha. most folks have their hvac up there or under the house so talking to folks about this doesnt always get me very far..they keep building things the same as they did 50yrs ago because in their minds its worked and doesnt need fixed/adjusted. i want to do everything i can to make the place efficient in terms of structure, comfort, cost,looks.. etc..

    thanks again guys

    1. SimpleNurseGuy | | #10

      my builder is working on formal blueprints, if i can post them soon i will. i think it will be easier to determine with a floor plan.with the tall ceilings (18' when in great room) and (10' in open loft)which is essentially all (1) space, i was concerned about heating/cooling. i have worked hard to keep ceiling penetrations to a minimum (2 down rods/ceiling fans) and will use site built vent baffles/air seal around seams/joints prior to installing mineral wool batts. hoping for a fairly tight ceiling plain and build overall. i went back and reread the comments and realize im kinda repeating myself..sorry. its easy to get it all mixed up in your head until you write it down. i dont know with joists spacing/size that i can accommodate the ducted head under the loft area but im sure adjustments could be made to do so. to be clear the loft above the lower bedroom is open not walled off and overlooks the kitchen/living area and the hallway the unit would blow down assuming the vanes can blow up/down and left/right is about 12' to the back of the house(under loft). i will likely insulate the interior walls bed/bath downstairs as well as above (under the loft) for sound but will this help or hinder in regards to placement of the unit.? dana in regards to the slab insulation i think the area im in is working on implementing a more updated code.. theyre using min of r13 wall, r30 roof and nothing in the slab. i really wanted to use an inch of exterior foam (xps/r5) for my walls and (2"/r10) at the slab perimeter like the new codes are calling for..however until this gets implemented..theyre just not doing least not because code says so. my fear of having a builder try and do something because its required or because i would like it is..well..lack of experience. you just dont see it here..advanced framing isnt a new concept but it will likely be some time before its the 'norm' here and exterior insulation and the details i fear would be overlooked simply from a lack of familiarity with the assembly and various means to achieve the desired goals by using it. despite moisture problems w crawlspaces (conventional) builders still swear by them here and oversized hvac ducted units are frequently placed under the houses or in the attic and after several years you can see the damage its done to the buildings. lots of moisture problems/mold/rot.. i want to do my part to build a house the is both appealing and comfortable for my sake but also one that lasts but the attention to detail is often overlooked because people build houses the way their daddies did, ya know? and many of them are real nice but moisture is a problem here and they start having problems in less than 10yrs. im trying to understand the importance of dehumidifying and keeping myself comfortable and the assembly dry. i dont however know that even the people designing plans and building these houses around here do.. people will spend 30,000 on cabinets but not pay any attention to how the house itself is built. the plans are drawn up and from everything im trying to read and understand here..are already (houses here) destined to fail. ive really tried to keep things simple in hopes that even if everything isnt done perfectly (which it wont be, im not perfect and dont expect perfection) that my house will still stand and for a long time. its the little attention to detail now that makes a big difference later. to think i never really thought much about an air conditioner, conditioning the AND what that did for both comfort of occupants but also moisture control. most people just trust when they buy something or pay someone to do something that its done. i think or at least it would seem if the builders HERE/architects/engineers AND the clients actually sat down and talked about what they would do and why WE WOULD see better built, longer lasting homes that were more energy efficient among other things. thats my rant, thanks for all your help.!!

  5. Expert Member
    Dana Dorsett | | #12

    >" i really wanted to use an inch of exterior foam (xps/r5) for my walls and (2"/r10) "

    If you end up going with exterior foam, use 3/4" (R6) or 1" (R6, sometimes R6.5) foil faced polyiso rather than 1" XPS. The 1" XPS is only warranteed to R4.5 @ 20 years, and is blown with extremely powerful HFC greenhouse gas blowing agents (predominant component is HFC 134a, with a greenhouse potential ~1400x CO2 @ 100 years). By contrast polyiso is blown with very low impact hydrocarbons (isopentane + others in the mix, running about 7x CO2 @ 100 years.)

    One pound density sheathing polyiso has about half the net impact of HFO-blown closed cell spray polyurethane (similar chemistry, half the polymer per R). This is about 1/10 the CO2e footprint of XPS, guesstimating it by this chart:

  6. Expert Member
    Akos | | #13

    There is no point for exterior insulation in your area. Just bump up your walls to 2x6. It will give you pretty much all the savings in energy cost that you'll get in your climate.

    I cool a 550sqft cottage with 14' ceiling with a 9k wall mount+ceiling fans. The cottage does have larger windows so there is a fair bit of afternoon solar gain, but the wall mount typically catches up an hour or two after sunset. I would probably go with a 12k if I had to do it again.

    The way I would go in your case, is build a closet in the loft area that has one face to the living space. Install the ducted unit there. You can run a couple of supply ducts to your bedroom and bathroom and maybe one more to the furtherst wall in the living space. Most the ducts can be in the floor joist, maybe one or two bulkheads for the rest.

    This would let you heat and cool your place with the single unit, be much more comfortable and less maintenance. Wall mounts need coil and blower cleaning around 5 years, a ducted unit with a good filter (except filter changes) will be maintenance free.

    1. SimpleNurseGuy | | #14

      Thank u guys for the advice

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