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Community and Q&A

Minisplit sizing question #138

alan_swanson | Posted in General Questions on

Really appreciate all the great info on the site.  Most thoughtful and civil comment sections on the entire internet!  I have read all I can and think I know the answers to my questions, but would like some validation before I spend a whole bunch of money.  Here is my situation:
– 600 sq ft 2BR new construction with above code insulation and tight construction.
– L-shape with some inefficient glass.
– climate zone 6, not concerned about A/C.
– ran coolcalc and got 6k BTU at 0 degree design temp.  I think this equates to 4 btu/ft^2/hdd.
– have 1500W of backup resistance heat in each bedroom and kitchen.
– open floor plan and good central location for the indoor unit.
– trying to have net-zero consumption with rooftop solar.

Given all this and the information I have found on this site, I have focused in on the mitsubishi m-series 9k unit with hyperheat.   Should provide plenty of heat down to zero and modulates down to 1700 BTU, which is the approximate heat load at 47 degrees.  Enter the local HVAC contractors.  Bids were as high as $7k ($4500 install?!?) and they all think I’m woefully undersizing.  Latest guy thinks I need 28k BTU but he didn’t do a load calc, now I’m questioning my logic.  My questions:
– do my numbers sound like they’re in the right ballpark?
– does the 9k unit seem like a good fit?
– is it worth spending $1500 on an install to have warranty support when mine is relatively straightforward and I’d sadistically enjoy learning how to use a vacuum pump and manifolds?
– is it worth spending however much extra to buy from a local distributor (if they’ll stoop to talk to me) to have local warranty support vs online?

Thanks in advance!

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  1. Expert Member
    Dana Dorsett | | #1

    The heat load sounds slightly low for a 600' building with "...some inefficient glass...", but not insanely low- it's in the ballpark. You have margin with a single FH09 in the open space. A load of 28K might be the heat load of a 600' uninsulated building with single panes, one of which is left open.

    What are the individual heat loads of the doored-off bedroom spaces? The 1500W backup PER ROOM is ridiculous oversizing. Size the backup no more than 1.2x the room load design temperature and it'll be more comfortable if/when it ever needs to be used.

    USD$7K might be in the right range (maybe slightly high) for TWO individual FH09s, cheap for three, but crazy for just one.

    I believe the warrantee is longer and covarage broader with Mitsubishis if it's installed by a "Diamond" contractor. But if you're willing to go ahead with refrigeration tech training get certified the basic warrantee should still be valid.

  2. Trevor_Lambert | | #2

    Your "back-up" heat is more than 2.5 times your total load. 4500W = 15300BTU/h. At 600' total, your two bedrooms must be pretty small. I'd be surprised if you needed more than 300W in those rooms. 1kW in the main living area and 500W in each bedroom would completely cover your load. Get thermostats that modulate (I think they are pretty common), then if your heaters are oversized it won't matter.

    1. kbtstone | | #15

      I agree with Trevor on the over-sized baseboard and the modulating thermostats. I am only running 5300 W for a 24' x 32' cape built in 1957. R13 fiberglass 2 x 4 walls, R19 in attic, but low-e double pane windows with storm windows and storm doors. I replaced all thermostats and the baseboard never runs full tilt. Much better temperature control and no annoying noises from the thermostat or baseboard. I used these:

  3. Jon_R | | #3

    Re install difficulty, consider either an inverter PTAC/PTHP or a mini-split with a pre-charged line set.

  4. alan_swanson | | #4

    We like the bedrooms to be cooler anyway, and are fine with open doors, so haven't tried to calculate those loads. The bedrooms are small! Good point about how oversized the back-up units are, hoping we never need them. The point about needing a licensed tech to have the warranty honored is a good one. I went a little crazy on the insulation with 2" of rigid foam over the walls and 5" over the roof, but skimped on the windows and have 3 full-light glass doors. In retrospect I should have paid relatively more attention to the windows and doors. Are there PTAC/PTHP units that work well at low temps? Willing to pay more to achieve our net-zero dreams so leaning towards the FH09 and hiring a tech to hook up the lines.

    1. Expert Member
      Dana Dorsett | | #8

      >"Are there PTAC/PTHP units that work well at low temps? "

      Not really. Some don't switch over to their onboard resistance heaters (turning off the heat pump) until ~20F or colder but many switch over at 25F.

      If you can't find a Mitsubishi installer willing to do it at a more affordable price, you might take a closer look at the LG's cold climate 3/4 ton LAU / LAN090HYV1 . According to their NEEP data it runs a COP of 2.9 even at full-speed @ +5F while delivering 11,220 BTU/hr, and modulates down to 1023 BTU/hr @ +47F at a COP of 4.3. Specs-wise that's probably a better fit, and may more run more efficiently in your house than the FH09, despite the lower AHRI tested HSPF.

      LAU / LAN090HYV1:!/product/25817

      MUZ / MSZ FH09NA:!/product/25894

      1. cody_fischer | | #24

        >"Are there PTAC/PTHP units that work well at low temps? "

        There appear to by two on the market from Innova USA and one from Ice Air

  5. bfw577 | | #5

    One thing to keep in mind is Mitsubishi's 12 year diamond warranty is for parts only. The contractors act like everything is covered for 12 years but labor is not. The warranty in my opinion is kind of worthless if you have to pay a fortune in labor for say a compressor replacement.

    After getting qoutes of 5-7k here for an install i ended up installing mine myself. I bought the unit and all the high quality pro HVAC tools like micron gauge, core remover, pump etc to install it. If it blows up in a few years I could buy 2 more complete units and still be ahead thats how much I saved.

    Is refrigeration certification even required for a new install? The condenser is precharged and you just open the valves after pressure testing and pulling a vaccuum below 500 microns. Its not like your buying tanks of refrigerant and evacuating it or adding it. You can buy mr cool units that have precharged quick connect lineset fittings at Costco now.

    1. Expert Member
      Dana Dorsett | | #7

      >"After getting qoutes of 5-7k here for an install..."

      For a single FH09?

      Where is "here". I that in US-'merican $?

      1. bfw577 | | #10

        It was for Mitsubishi 12k/18k single head units. That included permits, electrical, etc. I am on the CT shoreline so prices are way higher for everything than elsewhere.

        I ended up buying a Midea and all the tools to install it myself for a little over 2k.

        1. Expert Member
          Dana Dorsett | | #13

          An FH12 would run between $3.5-4K in my area (central MA) in competitive bidding, maybe a bit more than that in the busy season. An FH18 could be as high $5K, but not a whole lot more even when the installers are busy. (FH09s usually run about $3K, sometimes as high as $3.5K.)

          I've seen only a few crazy bids like that for installing mini-splits on Martha's Vineyard (another gold-plated location) but also bids comparable to those in central MA & suburban Boston.

          Do you have any feedback on the installation & performance of your Midea? Nobody seems to install or support them in my areas so I've never looked to closely, but now that some models are being re-branded by the bigger US air conditioning companies I'm becoming more curious. I read they partnered with Toshiba for the cold climate compressor technology- not sure how much of their other equipment or components are designed in-house v.s. purchased/partnered.

          1. bfw577 | | #18

            I have been very happy with mine. I agree that there seems to be very few installers of them though Mideas are rebranded by many companies including Carrier/Bryant. Based on my research of cross referencing parts and specs almost all the off brands like Pioneer, Chigo, Comfortaire, etc are all Midea units. I also noticed on a recent trip to Eastern Europe that Midea was definitely the most common installed brand. They were everywhere and some looked really old and beat up but were still chugging along just fine.

            GMCC Toshiba claims to be the largest rotary compressor manufacturer in the world. They are used in pretty much in all the lower priced splits including Midea. I have read if installed properly they are extremely reliable and trouble free.

            I have my eyes on this Midea for adding a second unit to my house. The specs are really impressive for the price. Seems to put out quite the btus for a 12k unit.


  6. Expert Member
    Akos | | #6

    I would avoid PTHP for a house, most don't have an inverter compressor and tend to be significantly louder than a modern mini split (the heat output at low temp is also pretty low so most come with a electric heat strip built in).

    Get some more bids for a wall mount, the price should not be that much. There are some re branded Midea low temp units sold under various brands (Lennox) that might be easier to find an installer for, you would just have to do some digging into the data sheets to make sure the it is the right size at low temps.

    DIY with HVAC tech for vacuum/leak check is usually pretty cheap. If you go with one of the cheaper brands the replacement cost of the entire unit is low enough that warranty is irrelevant (once the lineset + electrical is in swapping out a mini split is pretty easy).

  7. RMaglad | | #9

    that's crazy high pricing. 14KCAD for 4 separate mini splits (3 mitsu ductless FH06/FH06/FH09, and 1 fujit RLF9D). Heat loss calcs at -15F were 22,500btu/hr, system capacity is somewhere around 30,000, maybe a bit less, since the fujitsu isn't rated to the design temp (but it still puts out). This on a 2000sqft bungalow with conditioned basement.

  8. Deleted | | #11


  9. Jon_R | | #12

    > >"Are there PTAC/PTHP units that work well at low temps? "

    Friedrich claims to have a couple that work down to 0F. I believe that one has a re-heat option to provide dehumidification.

  10. alan_swanson | | #14

    Thanks for all the replies. Feeling validated about my calculations and learning about some interesting alternative units. That LG looks great and $400 cheaper than the mitsu right now. Unsure if there are any distributors/installers working with LG around here, but have seen a lot of mitsubishis around town (Missoula MT). The Mr. cool DIY 12k unit also looks pretty sweet, especially the part about saving $500 getting the lines charged. Having a hard time finding performance specs on that - anybody know where to find those or know offhand the lower temperature limit and/or output at zero degrees? The hspf is significantly lower but isn't that measured at a pretty warm temperature? Performance at 15-40 degrees seems more relevant here.

  11. Expert Member
    Dana Dorsett | | #16

    I don't believe the Mr Cool DIY comes with a pan heater or has a "hyper heat" type compressor extending it's lower ambient efficiency & capacity the way cold climate minisplits do.

    The NEEP cold climate mini-split list is now searchable online.!/product_list/

    If you set the max-capacity @ +5F slider to something like 13,000 BTU/hr and select by manufacturer & configuration type you'll come up with a reasonably short list to research. Pay attention to the pan heater description & operation details on the lower left of the full product descriptions if you think you'll need to squeeze hard to make it under the Net Zero bar.

    Both LG & Mitsubishi seem to run the pan heater continuously when it's below freezing and operating it heat mode.!/product/25817!/product/25894

    Midea's approach is a bit more nuanced, and runs the pan heater in response to both outdoor temperature and defrost mode, turning on at the beginning of a defrost cycle when it's below freezing out, but then turning off 5 minutes after the end of a defrost cycle.!/product/26508

    Fujitsu doesn't really describe their pan heater control algorithm at any level of detail.!/product/25327

  12. Birdo | | #17

    What about a Pioneer? I'm considering one and with my limited knowledge, they seem comparable to the Mitsubishi and a lot less money.

    1. bfw577 | | #19

      They are rebadged Midea units. They are probably the best selling units with they diy crowd. They are about half the price of the big name brands. Their cold weather performance isnt great though. Here is the higher seer Pioneer chart. Capacity really drops at lower temps but for $900 including shipping and lineset the 12k unit its not a bad value.

    2. Trevor_Lambert | | #20

      Aside from the poor low temperature performance, they are a lot less efficient than the more mainstream brands. I don't think these should be considered in zone 5 or higher as a primary heating source. They'd make a nice alternative to a window shaker or portable terminal heat pump for dedicated cooling.

    3. Expert Member
      Akos | | #22

      I've used a couple of the Pioneer splits for A/C only. Works great, can't beat it for the price.

      For a budget heat source for a studio I went with an EMI unit. It had much better low temp performance and also came with a pan heater.

      It is still not as good as a hyper heat unit, the heat output drops significantly bellow -15C (5F), you need to have a backup heat source for those really cold days.

      1. Birdo | | #23

        I will look up EMI units. There are a bewildering number of these things. Mitsubishi doesn't have a hyper heat unit in the 24K configuration that I'm looking for.

        Today I was looking at this one: LG's LA240HYV1

        I don't see a tidy chart to evaluate its efficiency but the HSPF is 12 whereas Pioneer's is 10. About $1000 more but if it's more efficient at those temps when I most need it, that would be made up in the long run.

  13. Birdo | | #21

    Oh, thanks for showing me that chart. I was looking at the 24K model and look at how it dips at modest temp. (At least modest if you live in WI, as I do.)

    I read on their site that the unit would function down to -13 which is similar to the Mitsubishi hyper heating. I think I'll go back and look again. Is there a similar chart for other models?

    I'm just a carpenter so all this HVAC information has me learning. I agree with the original poster who says that this is a friendly helpful site, but then I think maybe he has not been to "Yesterdays Tractors." Another friendly place to visit!

    Thanks so much. I mostly read, rarely comment.

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