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Choosing a Modulating Minisplit

Scottie_Miller | Posted in Mechanicals on

My wife and I are building a small off grid cabin in Leavenworth, WA, climate zone 5, 2,560 elevation, 300 ft2 + loft, 1 small bedroom, one tiny bunk room, small kitchen/living room and 3/4 bath.  The cabin will be used on weekends, holidays and possibly weeklong stints with our two small children and close friends/family.  We ultimately landed on a double stud wall, dense packed cellulose heated with a PV powered mini-split backed up by a solar thermal system we will utilize for DHW. Manual J calculation puts our max heat load @ 9 deg F between 2,200 and 2,900 BTU depending on final insulation level, window placement and SHGC/U Value, etc.  Nov – Feb average temps are in the high 20’s to low 30’s.  We’ll spend 5+ months per year almost entirely between the high teens and mid 50’s.  Heat load @ 47 deg F will be in the 800 BTU range. We’ve thought about buying a cheap mini-split on Amazon, (think 6k-9k BTU, $700 to $1,200 DIY type option) dropping some more batteries into the ole battery bank and crossing our fingers that the COP remains high enough at low temps that the unit doesn’t suck the batteries dry before 3 AM.  The load is so low that the unit would also need to modulate pretty dramatically to keep from cycling constantly in 20-35 deg temps.  This solution has the added benefit of lots of 110V options, which will allow me to do most of the wiring and probably saving us some money on an inverter.  I wouldn’t call this an….elegant solution. Ideally we’d find a mini-split that is easy to install, runs on 110V AC, (or even better, 24v or 48v DC) modulates down to a very low BTU heating output and maintains an excellent COP at low temps.  This may not exist.  It’s also pretty hard to verify some of these details.  Has anyone run into a similar set of circumstances, (off grid, minimal load, using PV for heat)?  Any recommendations on products?  Where to look?  From whom to buy?  For context, in case it matters a Mr Cool DIY install would be pretty easy for me…not so sure about a line set that needs to be charged, etc.  Thanks!

UPDATE: 10/27/21 – Thanks to everyone that contributed their time/expertise to this thread!  After much research, we’ve decided to move forward with an air source heat pump for primary space heating as planned.  We decided against electric resistance, (need for high efficiency) wood, (for safety reasons with young children and difficulties maintaining clean low BTU combustion) and a heat pump water heater, (due to many fewer options, degree of disruption when equipment needs to be replaced and difficulties confirming expected COP at low temp operation).  After going through this process, I suspect the following info will be helpful to other homeowners:

 -If you want to buy and install your own mini-split and low temp efficiency isn’t of primary concern, (grid connected, mild climate) buying a unit from Amazon, ecomfort, acwholesalers or HVACDirect with charged lines probably makes sense and will save you the trouble of buying gauges/pumps/specialized tools to commission the unit.  There are many  good options.
-If you need a unit with substantial modulating range, (500-2000 BTU minimum on a 6k to 12k BTU rated unit) with excellent cold weather COP, the options are significantly more limited as of October, 2021. 
 The lowest modulating unit with good cold weather COP was an LG product that was discontinued in 2018.  The replacement has terrible cold weather COP. 
 The best option available today for a DIY type install appears to be the 6k BTU Mitsubishi model (Outdoor unit: MUZ-FH06NA[-01 to include the condenser heater], Indoor unit: MSZ-FH06NA-U1) which modulates down to 1,600 BTU at 47degF (4.26 COP) and provides between 8,700 (5degF) and 14,000 (47degF) BTUs with COP close to or above 3.  This unit is available through multiple online retailers in the $1,500 to $1,700 range plus additional parts bringing the total cost just North of $2k.  This will require charging lines, etc.  Other options that may work here with similar amounts of skill to DIY include TOSOT and Innovair units currently being sold by Signature Solar, (the TOSOT appears to be AHRI Cert # 10062018 which you can search for on the NEEP database and the Innovair specs appear to match the TOSOT unit).  Fujitsu (AHRI Cert # 204752937) is the other pretty good unit easily available to consumers at AC Wholesalers and AC Outlet.
 If you’re not interested in DIY’ing a mini-split install, the best options are from Mitsubishi, American Standard & Trane.  If you don’t have an installer in your area that carries these products, the 47degF modulation will be in excess of 2,000 BTU.  Great options include Fujitsu, TOSOT, Friedrich, Gibson, Gree, Ameristar, Direct Air, LBG, Lennox, AZUR, Direct Air and Bosch.  Availability and market penetration of these products appears to differ dramatically by location.
 You can check cold weather efficiency at https://ashp.neep.org/#!/ and cross reference information at AHRI https://www.ahridirectory.org/NewSearch?programId=41&searchTypeId=3&productTypeId=4611.  These databases are kept up to date and should remain great resources for years to come.  The advice above is obviously just a snapshot in time, but hopefully proves helpful to anyone with a similar design goal in the coming few years.  Good luck with your project(s)!

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Replies

  1. Keith Gustafson | | #1

    What is your plan for cooking fuel?
    DO you plan on needing AC in the summer?

    I ask this because it seems for that low a heat load there might be other ways to do it, and save your battery capacity for lights etc.

    ACwholesalers.com has had pretty good prices over the years. A vacuum pump and gauges are good enough for a single install, and even if you had to pay a guy to come out to do the refrigerant install, it would not be the biggest deal

    1. Scottie_Miller | | #8

      @Keith We’re currently planning on an all electric cooking setup indoors, induction burner, pressure cooker, kettle, etc. and an exterior wood burning oven. We’re definitely open to changes here, but trying to stay away from gas/propane.

      We’ll give AC Wholesalers a look for a heat pump or heat pump water heater/radiator, etc. The total annual cooling load is roughly an order of magnitude less than the heating load, but there are a few days per year that get into the mid/upper 90’s with lows in the 70’s, so a bit of cooling will be necessary. I'll do some YouTubing re: the vacuum pump and gauges to see if I can handle charging the lines on a less DIY friendly unit. Thanks for the feedback and recommendation!

  2. emma_vt | | #2

    For that low of load at a vacation spot, I would dial back the sophistication and go with a ER panel radiator for your base temp and a tiny wood or pellet stove for real coziness while you’re there.

    A cord of wood holds as many kWh as 333 Gen 2 Tesla powerwalls, which in my mind makes an excellent solar “battery.”

    This pellet stove even qualifies for the tax credit: Mr. Heater PS20W-CIW Mini Pellet Stove, WiFi Enabled, Black https://www.amazon.com/dp/B07MY6TKF8/ref=cm_sw_r_cp_api_glt_fabc_FHZ2YGX9F5P5HSYA2E20

    1. James Someone | | #21

      Emma vt,

      Your video was entertaining to watch. I agree, you don't get much stored electricity by installing a Powerwall in a residence-a tax credit maybe-but not a lot else. I have a portable generator for this reason.

      The pellet stove you mentioned appears to be very low quality and made in China. I would be cautious recommending heating equipment sold on Amazon. I don't believe the stove is equipped with a thermal shutoff safety feature to prevent over firing like Harman and Quadrafire brands offer.

      Either way, a pellet stove in 300 sq ft living space is silly. The mechanical noises alone would drive a person crazy along with constant short cycling.

      1. emma_vt | | #24

        There are two low-btu pellet stoves on the EPA Wood Heater database that qualify for the tax credit and Amazon has some easy links - but buy local when you can! :)

        Scottie could also consider one of these Cubic mini stoves, https://cubicminiwoodstoves.com/collections/cub-cb-108/products/cb-1008-br-cubic-mini-wood-stove. I don't have any direct experience with them but they're pretty popular. They aren't EPA certified though and the tiny fuel stoves always strike my as challenging to keep going.

  3. Charlie Sullivan | | #3

    Your needs are so poorly matched to what's available that I'm tempted to think of crazy hacks like buying a refrigerator and using its heat pump to heat your cabin.

    1. Scottie_Miller | | #10

      @ Charlie I’m concerned I’m more macgruber than macgyver, but hacking a fridge to heat/cool the house sure would make a great YouTube video! If you show me how I’m game to try. :)

  4. Steve Knapp CZ 3A Georgia | | #4

    It seems you can buy 12/24 VDC minisplits with as little as 6,000 BTUs of cooling. I noticed there are units designed to work with solar panels as well. Of course, YMMV on any of these things.

    https://www.amazon.com/ACDC-Conditioner-Refrigerant-Mini-Split-Installation/dp/B08DHP82LG

    1. Scottie_Miller | | #11

      Hi Steve, if these are modulating units it just might fit the bill! I’ll do some research on lowest BTU output, COP at low temps, etc. and see if the unit could work. Thanks for the suggestion!

  5. Kyle R | | #5

    What about a heat pump water heater fed by your solar system? You could have it ducted to both the inside and outside and have dampers to open and close depending on the season. Vent to outside during winter and inside during summer. This would give you some AC and dehumidification. Run a low temp baseboard off of the water heater.

    1. Scottie_Miller | | #12

      @Kyle This idea has a lot of appeal, aesthetically and practically, (why separate the DHW and space heating if an efficient option exists to combine them). I’ll need to do some research on HPWH COPs at lower exterior temps, (it won’t make sense to steal heat from indoors during the heating season) and the inherent complications of a system like this may hamper friends/family use of the cabin compared to a set it and forget it air source heat pump. There may be solutions to these potential issues. At the very least you’ve given me another rabbit hole to jump down. My wife will be annoyed that I’m going to spend multiple hours on this, but I’ll enjoy it! Thanks for the suggestion!

  6. AlanSZ | | #6

    Doesn't meet all your requirements, but looking through the notes for my personal HVAC project, I know the top end Mitsubishi wall unit can do the following modulation range in heat mode.

    [1600-14000 for 6000 rated BTU]
    [1600-18000 for 9000 rated BTU]

    I pulled this data out of the submittal PDFs. IIRC Mitsubishis have been independently lab tested across many temp ranges for COP.

    MSZ-FS09NA MUZ-FS09NAH is the pair for the 9000 BTU one, with HyperHeat outdoor unit.

  7. Jon R | | #7

    Don't overthink cycling. Note that a 2.2-2.9k design load with a 6k unit and a 3:1 turn down ratio will run more continuously than a perfectly sized single speed unit. And that in Z5, my somewhat over-sized single speed system provides good comfort, even in mild weather (46F right now) and proportionally less interior thermal mass than I expect you will have.

    Make sure you have enough capacity to cover the various forms of ventilation.

    For such little use, I wouldn't rule out propane (for stove and space heating). If not that, then I'd look at PV solar, resistance heat and water storage for space heating and DHW. Water is cheaper than batteries and no expensive HP or thermal solar is needed. Tank size for a weekend isn't excessive - but a cold sun-less week long visit and you would probably have to burn something.

    1. Scottie_Miller | | #14

      @ Jon R. Thanks for sharing your own experience! This gives me some hope. :) We’re definitely looking for a modulating unit, so hopefully we’ll have a similar experience. Even at a COP of 2, we should have 20+ hours of battery storage at 99% design temp. A higher COP plus hot water backup should get us to 3+ days in any scenario. I suspect most people would go wood or propane here but part of the whole fun of this project is to stay away from gas…hopefully I won’t curse the decision in hindsight!

    2. BFW577 | | #26

      I recall seeing some LG minisplits that could go as low as 800 btu/h.

      1. Scottie_Miller | | #27

        @ BFW, I looked into the LG mini splits and they appear to have models that drop down into the 800 BTU range. There appear to be current models that modulate down to 1,023 BTU @ 47degF. Based on what I'm seeing online, (NEEP ratings, availability at ecomfort.com and acwholesalers.com) the previous generation of units, (the best options being LSU090HSV4, LAN090HSV4, LAU090HYV1 and LAN090HYV1) had great cold weather (5deg - 47deg F), low BTU (500-1,023) COP, (2-3.5). Unfortunately these models were discontinued in 2018-2020 and the replacements appear to have a couple thousand extra BTU's of heating capability, but horrendous COP (<1) at low BTU output. These are the same model numbers but ending in ...HSV5 and ...HYV3. I'm inferring that LG decided more consumers wanted more heating capacity than high efficiency at very low output, (which is probably true). Thanks for the recommendation though!

  8. Scottie_Miller | | #9

    @Emma It may indeed be worth considering non-heat pump options for space heating needs, (something we’ve gone back and forth on for the past year). If so, we’ll probably go the wood or pellet stove route and add a bunch of thermal mass or PCMs so we don’t boil ourselves alive in our little well insulated box. I’ll check out the recommended pellet stove.

    We also originally planned on going with electric resistance heating, (baseboard, panel, cove, etc.) but with limited winter sun, a higher COP could actually make a dedicated backup unnecessary as long as we can find the right unit. I suspect ER + wood is the natural fallback if we can’t source a heat pump with significant energy savings. Thanks for the suggestions!

  9. Scottie_Miller | | #13

    @Alan This unit does seem like an excellent candidate, (6k BTU modulating to under 2k, great COP at low temps, most respected brand in the marketplace). I’ll do a deep dive into the particulars and see what shakes out. Thanks for the suggestion!

    UPDATE: I looked into the Mitsubishi units, found the NEEP Cold Climate Database and manually input BTU/kW/COP data for all models under 4,100 BTU modulation, with COPs greater than 2.0 at all temps down to 5*F. It looks like the 6k BTU Mitsubishi model is probably the best fit here, (lowest modulated BTU output at 47*F, excellent COPs across the board) and is sold under Mitsubishi, Trane and American Standard brands. There are another 16 models shown in the database with modulated BTU output at 47*F between 2,100 and 2,600 with even better COPs at low temp low BTU output. These may be an excellent fit under the theory that we can simply heat up the structure in the morning at a higher BTU output and then turn off the heating through the day unless temps drop below freezing. My next steps will be pricing the models to see if there is a major difference in relative value. I'll report back here for the benefit of other GBA members with an updated spreadsheet with pricing data and notes. Thanks again!

  10. GBA Editor
    Martin Holladay | | #15

    Scottie,
    Requirements for space heating come at a time of year when (a) days are short, (b) the sun is very low in the sky, and (c) weather is often cloudy. Don't use electricity for space heat if your building is off-grid.

    More advice here: "How to Design an Off-Grid House."

    1. Scottie_Miller | | #17

      Hi Martin, as someone who has relied on Seattle City Light for 38 years, I feel silly arguing with an expert who has decades of lived experience off grid...but perhaps you will indulge my sales pitch? ...and then likely tell me where our logic breaks down from your perspective...

      3.6 kW PV due South on 45* standing seam metal roof. Average daily output in Dec/Jan = 7.0 kWh per NREL/PV Watts. Worst three day output based on three years of solar insolation data (2018-2020) is 146% of Avg Dec/Jan production; 10.2 kWh. Cabin will be used for long weekends in the winter. With a heating load of ~750BTU/hr @ 47*F and ~2,200 BTU/[email protected] 9*F (99% design temp) the Oct-Mar daily heating load will range from ~18,000 BTU to ~52,000 BTU, (5.3kWh to 15.5 kWh). With a mini split rated at 2.0 COP the heating energy required drops to 2.6 to 7.7 kWh. With a COP of 3 the usage drops to 1.7 to 5.1 kWh. Based on the numbers I pulled from the NEEP Cold Climate Rated Heat Pump List last night, (attached) there appear to be dozens of options that maintain COP in excess of 3.0 down to at least 5*F with modulated BTU output in the range needed for space heating this structure. With a 10kWh or 15kWh battery bank, even with no PV production we should have multiple days of heat energy stored even in 99% design scenario. With high quality lithium battery prices down to $0.30/watt or less, and the ability to DIY the PV/electrical, I've priced out the solar system at ~$7,000 including 10kWh battery storage. $1,500 more to increase to 15kWh in battery storage. Add the price of the heat pump + install and you theoretically cover the bases here.

      I hope I don't come off as a snotty kid trying to tell you about your business. I'm honestly interested in (and grateful for) any feedback you have on where you think this plan goes wrong after digesting the details. Perhaps it's possible that PV systems, battery storage and cold weather mini-split efficiency have all gotten better/cheaper fast enough for you to reconsider your position on off grid electric heat, at least in certain circumstances? Either way, thanks for everything you do! I've read just about everything you've written in the last 15 years and owe you a debt of gratitude I can never repay. Cheers!

      5.12kWh, 48V, 100AH battery, $1,500: https://shop.signaturesolar.us/products/48v-100ah-lifepower4-battery-by-eg4?ref=cPwLcVc0SW-BjN

      1. emma_vt | | #19

        I made this little video for work during covid lat year, I ought to remake it more professionally because I’m getting a lot of use out of it lately! https://youtu.be/3Bag_fNCY-M

        You’re going to end up with a solar array and battery set up that’s massively oversized for your needs in order to be confident your heat pump won’t run out of power in winter. How’s the snow load in your area? It’s not uncommon for us to go over a week with no PV production at our house because it’s covered in snow.

        ER and wood is the way to go here. Cozy and simple. Generally I prefer wood stoves and pellet boilers, but you would have to have one of the #vanlife sized wood stoves which are trickier to keep going. Hence I think the mini pellet stove I linked before is your best bet. One 40lbs bag of wood pellets contains about 100kwh and costs $3-$6. Just think of all the “battery” storage you could stack up! Let us know what you decide!

        1. Scottie_Miller | | #23

          Emma, if only I looked as good as you do splitting wood I'd have to forego the pellet stove for a wood burner. Youtube would be blanketed with videos of my bulging biceps as I take down trees Paul Bunyan style! It's hard to argue with your practical ER + pellet stove advice with 100kWh of pellets @ $3-$6. I'll chew on this option, (we were leaning toward ER + a tiny wood stove with additional thermal mass before we ran heating load figures that made a cold climate efficient mini-split look practical). My concerns with wood involve preparation, flue cleaning, maintaining a very low level combustion, (will an 800-2,500 BTU/hr fire even produce enough heat to stay lit/burn cleanly?) and general safety with small children, (10 mos and 3 yrs). It also looks like your 4.1kW solar system is producing something like 1.8kWh/day in Dec/Jan. At our location in Leavenworth, WA the daily estimate for our planned 3.6kW array based on NREL/PV Watts figures is around 7kWh/day. This translates into ~430 watt hours of production per kW of panels in Vermont and ~1,900 watt hours of production per kW of panels in Washington. Then again, our estimates are still just that...estimates... Either way, we'll build out the solar system to easily add a ground array in the future if the need/desire arises. I'll let you know what we settle on when construction starts next year. Thanks for the feedback and your willingness to share your hard earned wisdom from off grid living. Your place looks awesome! (...and oh so cozy)

          1. emma_vt | | #25

            Oh the small kids is a new variable - in a small space that could actually be challenging with a wood or pellet stove since the gates people usually use will take up precious square footage.

            You're right that even that mini pellet stove has more oomph that you need - but it was the smallest EPA certified, tax credit eligible one I could find in a quick search. There are these hilariously tiny wood stoves that were originally designed for boats but have become popular with vans, school bus conversions and tiny houses. https://cubicminiwoodstoves.com/collections/cub-cb-108/products/cb-1008-br-cubic-mini-wood-stove

            People sometimes install them on top of (non-combustible) shelves and things, which could help keep it out of reach. I don't have any personal experience with them, but I've just always thought that they would be hard to keep going with such tiny fuel.

      2. GBA Editor
        Martin Holladay | | #20

        Scottie,
        I haven't double-checked your math, but if you are confident in the figures, go for it. Then, in two years, please write an article for GBA called "Electric space heat for an off-grid house." I'll convince Kiley to publish it.

        Here are a few things to remember: a finely balanced mathematical equation gives you little leeway for unusual events -- events like long stays at your cabin, unusually cloudy weather, or cold snaps. Remember, too, that batteries don't work well when they are cold, and your batteries will always be cold at the beginning of every winter visit. (Batteries are heavy, and they take a long time to warm up -- so after a three-day visit to your cabin, the batteries may still be cold.)

        1. Scottie_Miller | | #22

          Martin, thanks for the additional thoughts! This is very helpful context for someone with some theoretical knowledge but no hands on nor practical experience. Assuming we find the right combination of cost effective equipment and decide to move forward with the electric heat pump (air source or DWH to radiator) I'll log all of the data and share it with GBA via "Electric Space Heat for an Off-Grid House." Somebody needs to be the guinea pig. Cheers!

  11. dlfdk | | #16

    As a European I'm not too familiar with Manual J calculations, so excuse my ignorance here, but have you accounted for internal loads in those heating figures? In the various standards I've come across, internal loads are often set as a certain number per unit area. For houses of regular size this approach makes sense, but it can give misleading results at the "extremes".
    In a regularly sized (leaky) house the influence of body heat is probably negligible, but with just 300 square feet it's certainly worth accounting for. Especially when you have friends and family visiting too. If that 800 BTU/hr at 47F figure doesn't account for body heat, then you'll break even when it's added. And you might even have to open a window when there's company. That'll also save on the power needed for ventilation.

    1. Scottie_Miller | | #18

      Hi dlfdk, the Manual J calculations do indeed take into account internal loads. That said, we used very low estimates for this structure, just to be conservative. Manual J usually ends up with # of bedrooms +1 for occupancy, (if I remember correctly). We roughly halved this from what I recall and used very low estimates for other internal heat loads. I suspect your gut instinct is right, that at 47*F we'll be opening windows, but this area can get pretty cold overnight and drops into the teens-low 30s for a significant portion of the year. One more reason I'm hoping we can get away with an efficient heat pump and low cost battery storage space heating on this build. Thanks for the feedback!

      1. Expert Member
        Dana Dorsett | | #28

        >"I suspect your gut instinct is right, that at 47*F we'll be opening windows, but this area can get pretty cold overnight and drops into the teens-low 30s for a significant portion of the year."

        A heat load of 800 BTU/hr @v +47F results in a net cooling load whenever there are 3 conscious people indoors, even without running any plug loads or any solar gains.

        Manual-J isn't a good enough model for a house like this. With modest solar gains the indoor thermal mass + thermal diffusivity of thick dense packed cellulose hitting the low 30s or even the teens overnight might not require additional heat overnight to stay in the high 60s or low 70s indoors when the days are sunny, which is about half the winter days in Leavenworth, and nearly all days that drop below 25F at night. (I some familiarity with the local climate since I have relatives in the area- a dedicated backcountry snowboarder/rock climber based in Leavenworth, as well as others in nearby Wenatchee.)

        1. Scottie_Miller | | #29

          Eek! The godfather himself responding to our mini-split question. I'm giddy with excitement! Thanks for the feedback re: heating/cooling expectations, and wonderful to know you're familiar with Central WA East of the Cascades. This doesn't surprise me since I've seen you correct Martin on his ERV vs HRV advice for the area. :) We've completed Manual-J and WUFI calculations, but as you note above I suspect the heating load will be even lower than the modeling projects. Seems to me this is all the more reason to find the lowest modulating cold weather heat pump we can find. The winter sun you reference should help make off grid electric space heating more feasible here. I've never heard of anyone implementing this type of system, but I'm excited to give it a go. If you have any other advice, I'm all ears. Otherwise enjoy the slopes and the epic views at the top of your next big pitch in the Cascades!

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