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Community and Q&A

Request for more technical Mini-Split articles on GBA

maine_tyler | Posted in Mechanicals on

There’s lots of discussion here on heat pump sizing, but all of the specs regarding minimum, maximum, and rated btu/h outputs come from the outdoor units (as far as I can tell).

What does the sizing of the indoor unit really tell us?

When I look at the submittal sheet for two differently sized indoor units (otherwise same model #) what jumps out at me are different airflow rates. And sometimes the gas pipe size. There’s no BTU/h specs for indoor units other than the nominal rating.

You can open the below PDF submittals and scroll to the same spot in each and then click back and forth between tabs to easily see which numbers are different. *(I guess you’ll need to copy paste)\M_SUBMITTAL_MSZ-FS15NA_FOR_MULTI_ZONE_SYSTEMS_en.pdf\M_SUBMITTAL_MSZ-FS09NA_FOR_MULTI_ZONE_SYSTEMS_en.pdf

For a 1:1 mini split system, this is all kind of a moot point since we can use the system submittal (outdoor unit included), but what significance does indoor unit sizing have on multi split systems?

I recently had an installer recommend installing two 6k btu/h indoor units paired with an MXZ-3C24NAHZ3-U1 outdoor unit. That’s a 24k btu/h outdoor unit. I honestly have no idea what they’re thinking, but it got we wondering about indoor unit sizing and it’s significance.
(that 24k btu/h outdoor unit has a minimum btu/h output at 47F of  11,400 btu/h… is this installer thinking they should match outdoor minimum capacity to total indoor rated ? Seems nuts as the low end modulation will be terrible)

If someone wants to keep going on this question, part b is:
Are multi splits EVER better than a 1:1 on efficiency/performance (assuming correct sizing for both)? I know total system install can be cheaper, but otherwise? It’s hard for me not to want to push installers to just install 1:1’s, but is that too simplistic? (CZ 6a- small structure)

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  1. maine_tyler | | #1

    Well it seems like a thread tried tackling this. Some clarity, but still a bit hazy.

    I would love to see GBA bring in some bona fide heat pump experts to write a comprehensive and detailed (technical) article on this issue. (Cold climate please!)

    It seems like its a giant black box, and my sense is that installers all over the place are as ignorant as to what's inside that box and any of us. They're just installing equipment and collecting a bill— most likely they have no clue how well the systems are operating.

    We need experts and data to verify the success of mini-split systems in their various configurations.

    It has been written enough times here that the 1:1 mini splits work better than multi splits (the only reason I have picked up is that it's due to better low end modulation) but it's hard to convey such a general and blanket notion to an installer and not sound like some arrogant chap that 'knows better' but then can't actually explain why they know better.

    Are people like me that are 'arm-chairing' the crap out of this pushing back against installers for no reason?

    Back to the issue: Would it be fair to say that a 2:1 (Indoor to outdoor) would be the most problematic given that the minimum output for the multi-style outdoor units is similar regardless of number of IDU's? Thus those 2 indoor units would have some of the worse turn-down compared to if there were, say, 4 indoor units to split the minimum output? (assuming 4 units isn't itself overkill).

    1. Expert Member
      Michael Maines | | #2

      Tyler, a mechanical engineer I work with installed a multi-split at his own Pretty Good House a few years ago, just before the news came out about the lack of efficiency in those systems. I think he has four indoor units on a single outdoor unit, all Mitsubishi. He was aiming for net zero so he pays close attention to his electrical usage and says that the system performs at about half the rated efficiency. He has done everything he can to improve the situation but now recommends sticking with 1:1 systems until Mitsubishi starts offering multi-split outdoor units that actually ramp down as intended. That's totally anecdotal but representative of what many others are reporting.

      I see a fair number of proposals from installers doing multi-splits, sometimes on my own projects where I have already spec'd 1:1 systems.

      I don't have an answer for your main question but hopefully somebody more knowledgeable will weigh in.

      1. maine_tyler | | #3

        Thank you Michael, I really appreciate hearing that experience.

        My sense is that installers push the multi-zones because it somehow seems like the more intuitive approach (and maybe because Mitsubishi is pushing it?). The multi systems seem more like old traditional distribution systems (central power unit with piping or ducting to distribute) the difference just being it's refrigerant instead of air or water. Why install all those superfluous outdoor units, right!

        I'm not convinced the capacity and modulation component is understood or considered all that well, but I should be careful not to throw all installers under the bus with blanket statements.

        Hence the request to bring in some true experts to help sort some of this. I heard Dana Fischer on a podcast warn against improper installation of multi-splits, but he did NOT imply multi-spits are never appropriate. But he didn't get super into details to qualify how one makes the distinction (other than to say they need to be sized correctly).

        If project managers or homeowners are going to push back against installers, it is helpful to know the 'why.' Or having data that shows which systems aren't working.

        1. Expert Member
          Michael Maines | | #5

          Dana's great but as an employee of Mitsubishi he can't exactly speak freely. Here's one of many GBA articles and Q+As with more information on the multi-split issue:

          1. Deleted | | #11


          2. maine_tyler | | #13


            No different Dana. The GBA Dana is Dana Dorsett. I am referring to Dana Fischer.

            Here is the podcast I am referring to. (note the actual link to the podcast seems to be gone, but you can search it out)

            Another mystery to me is why Dana F (in that podcast) implies that the multisplit oversizing issue would more adversely affect colder climates than warmer climates— as if the warmer the climate the more cushion there is for oversizing?

    2. Patrick_OSullivan | | #8

      For a reasonably sized house (for some definition of reasonably sized), the more likely it is that the "oversizing errors" accumulate disadvantageously.

      Contrived example: You have a house with a 24,000 BTU/h load. You pick two indoor units, each at 12,000 BTU/h. If instead you say, I need "better zoning", and go with four 9,000 BTU/h unit loads (because that's the smallest your vendor offers), you're now at 36,000 BTU/h aggregate, or 50% oversized.

      1. maine_tyler | | #14

        Maybe the crux here is whether its fundamentally a multi-split problem, or an oversizing problem (or some combination, i.e. the multi-split set-up is more susceptible to sizing issues).

        If we look at 4 scenarios, where do we see the biggest drop in performance:
        1) A properly sized 1:1 system
        2) An oversized 1:1 system
        3) A properly sized Multi system
        4) An oversized Multi system

        1 is the best, 4 is the worse, but the steps in between get less clear (and obviously it depends on many specifics, like how much is it oversized?).

        The reason it matters is because, despite what I've seen written in a few places, going with all 1:1's is more expensive than multis (for comparable indoor heads), at least according to quotes I've received. Albeit not massively, but enough for many people to simply go with the cheaper option with fewer visible mechanicals in their yard.

        I suppose at the end of the day, there is a spectrum of options that vary in their efficiency and performance and sorting out where your system will land on this spectrum gets harder with multi's.

  2. maine_tyler | | #4

    "if you see the FH09 head can do 1500 BTU with paired with MUZ-FH09NA outdoor unit, it does not mean at all you can get as low 1500 BTU when installed it with the MXZ multi zone system. It's nothing but a coil of copper piping with a fixed surface area and variable fan speed. The refrigerant hardware control is all done in the outdoor unit."

    This post from the garage journal thread that was linked in the previous GBA thread says it well. Doesn't answer the finer points, but explains what these indoor units are.

  3. user-5946022 | | #6

    Despite reading GBA several times per week, I was not aware there is any known issue with multisplits. At least this puts the issues with my multisplits in a different context.
    I am fairly confident my multisplits were properly sized & the equipment is properly paired. The design work was done by a professional who is also a contributor to this site. I am not as confident about the installation. My units use multiples more kw/year than projected.
    The question is, how does a homeowner figure out if there is something wrong with the installation that can be improved or if it is just as good as it gets?

    1. maine_tyler | | #12


      Are you taking measurements of heat output and electricity input (to get COP's) or is it just that your electrical usage went up higher than 'predicted'? If the latter, how was the prediction made? Are you accounting for the predicted load based on actual weather data that coincides with your electrical usage?

      This is one of the other major challenges—assessing system performance isn't straightforward and is beyond what most homeowners will/can do. And there is always so many variables.

      I know that some states have inspection programs and they will come check out the system, but I have no idea how thorough such an inspection would be, or whether it would get at the efficiency issue.

  4. Expert Member
    DCcontrarian | | #7

    At the risk of thread-jacking, this is what I was talking about when in a recent thread ( ) I said that air-to-water has fundamental technical advantages over air-to-air. With a modestly sized buffer tank -- 17 gallons is what Chiltrix recommends -- you can separate heat pump performance from the heating load.

  5. MartinHolladay | | #9

    Maine Tyler,
    Here's is a link to a GBA "Category" page with lots of links to articles on minisplits: "Ductless Minisplits and Ducted Minisplits."

    You might be particularly interested in "Goldman on Minisplits."

    1. maine_tyler | | #15

      Thank you Martin. There is indeed a lot of good information compiled here. I forget to use those category pages.

  6. boxfactory | | #10

    I do remember an interesting article from several years ago about a production builder in MA who was building houses with one minisplit head on the ground floor, and one on the second floor. I forget how many outside units were being used (or if the article even mentioned this detail). This article made an impression on me, and I am currently planning on copping this layout on our upcoming build.

    One thing I have been wondering is if the house used 1:1 minisplits, would both units be sized to accommodate roughly half of the heating or cooling load? Would there be two thermostats, one on each floor, positioned somewhere near the indoor unit?


    1. maine_tyler | | #16


      Martin's article that Michael linked to above does a great job discussing the considerations for 'point source heating', which is essentially what you are considering.

      If you're going to copy a distribution layout, make sure the house layout and load is also comparable. The consensus seems to be that the better air-sealed and insulated a building is (and with good glazing) the more likely it is you will be comfortable with fewer mini-split heads.

      Leakier houses with colder walls presents distribution challenges and there will likely be many cold spots.

      "would both units be sized to accommodate roughly half of the heating or cooling load?"

      I imagine it depends on the heating load of the respective areas they are conditioning. It may not be an even split. Room by room loads calcs would be the ticket vs whole house block load.

      By default, most mini-splits have their thermostat in the head itself. Sometimes this works, but sometimes it is wildly inaccurate and may even lead to cycling if the output air is short circuiting back into the units temp sensing region. A remote thermostat can be placed more or less wherever you want, but you probably want it out of the direct flow of the unit and wherever you want the temp to be most accurately matched.

  7. Expert Member
    Akos | | #17

    The basic issue is that manufactuers simply don't provide the data on how these operate. This leaves most of us with the only option of monitoring existing systems to figure this out. There is a lot of Mitsubishi installs out there, it is pretty clear how the MXZ series run.

    As for the rest, it is hard to tell. It seems some of the more commercial based systems with either a branch box or individual expansion valve in the indoor unit do run better. These also tend to be pretty pricey.

    Generally a right sized multi split with a couple of large indoor units will run well. Once you start adding on small zones that are well bellow the min output of the outdoor unit is when you get into trouble. For good heating efficiency, you also want to avoid operating with some of the zones off, when running all zones should provide some heat most of the time.

    When in doubt, go for one-to-one and avoid the unknowns and potential issues.

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