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Minisplit efficiency – multiple single zone vs single multi-zone

Bdgray | Posted in Mechanicals on

Hello all,

I am strongly leaning toward installation of a ductless minisplit system to replace an archaic air handler used for heating and cooling. I’ve done a lot of research on mini-splits and would definitely prefer one of the ~-15* low-temp heating options offered by Fujitsu or Mitsubishi (this just makes sense as I’m in Chicago burbs). Currently from what I can tell these models are only available in single zone models. However, for our home I’m certain the sqf and layout of our home would mandate more than one interior unit. (2700 sqf ranch with bedrooms that are pretty well shut off from the living space).

I have several questions related to these products:
1) Are there any known/concrete plans to offer low-temp heating/ high SEER units within the multi-zone product set? In other words, would it be wise to wait another year or two to make this investment?
2) If there are no short-term plans by these providers to make these units available in the multi-zone platform, is there any objective way to evaluate whether it makes sense to go with a lower efficiency multi-zone vs multiple single zone units (meaning I would have up to three outdoor units)?
3) Separate but related question. We have a detached garage that gets good sunlight during the day. I’m curious whether it would be advantageous to install the exterior units so that they draw air from the garage (while still exhausting outdoors)? I’ve not actually tested this theory but I suspect the temperature inside the garage is always a few degrees higher than outside (because of solar gain & whatever heat comes up from the ground). The flip side is this would add ~20 feet of tubing to each run.

We live in Chicago so these units would be used extensively for heating. Even with a low-temp heating unit I know we will need supplemental heat. I do have a plan for this.

Lastly, I have an HVAC contractor with minisplit experience coming next week to bid this. My goal is to be as educated as possible prior to that meeting.

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  1. Expert Member
    Dana Dorsett | | #1

    Efficiency is relative, but multi-splits are automatically at a slight disadvantage since the compressor has to serve multiple masters, making it harder to optimize. A 2.5 ton multi-split serving a single half-ton head when only one head is calling for heatng/cooling at a fraction of the head's capacticycan never be as well matched & optimized as when the outdoor & indoor units are matched. When all zones are calling for service at the same time, or the combined loads of the heads calling is a sufficiently high fraction of the compressor's capacity it can be pretty good.

    Mitsubishi M-series H2i multi-splits already have a rated output @ -25C/-13F, just like the single-head "FH" mini-splits.:

    These multi-splits deliver the full-capacity rating down to +5F, and something between 70-75% of the full capacity @ -13F.

    In general three singles will have higher average efficiency than a 3- head multi-split (like the 2.5 ton MXZ-3B30NA pr 2-ton MXZ-3B24NA :,_21-32%29.pdf ) at the same tonnage, provided the design temp loads are at least 2/3 of the rated output of the mini-splits. The smallest mini-split is rated 9000 BTU cooling, but there are 6000 BTU options for multi-splits. If a zone has a design load of 4000 BTU you'll probably get better efficiency (and definitely better comfort) out of the multi-split with a 6000 BTU/hr cassette than a 3/4 ton mini-split.

    It's important to do the room-by-room load numbers to figure all this out. Depending on the floor plan possible to split the output of a mini-duct cassette to cover 2-3 adjacent rooms with short duct runs without taking a huge efficiency hit from the ducts. Splitting the output with short ducts makes it easier to better match the cassette output rating to the load to keep it modulating at part-load rather than short-cycling range. A 6000BTU head or 9000 BTU mini-split serving a 1500 BTU peak-load is probably going to be quite a bit less efficient than a 9000BTU mini-duct cassette serving three 1500 BTU loads via duct runs, even if the latter isn't a great match either- it's the difference between being 2x oversized for the load vs. 4x or 6x oversized. The ideal would be no more than 1.5x oversizing.

    Putting the outdoor unit in the garage will not improve the heating season efficiency, and if it restricts air flow AT ALL, it can cut into heating season efficiency. A the air volumes the outdoor units are pushing it's literally dozens, maybe even hundreds of air exchanges per hour for a normal sized garage. (If your garage is the size of the Houston Astrodome it might help though. :-) ) What you are proposing is to use the garage as a solar collector, and as a solar collector it's both way too small and way too inefficient to be of any benefit.

    A reasonably insulated older 2700' ranch with reasonable air-tightness and U0.34 windows can easily have a design heat load @ 0F (Chicago's 99% outside design temp) as low as ~30,000 BTU/hr, or lower, which is the +5F output of the -3B24NA. If your design heat load at 0F is EXACTLY 30,000 BTU/hr, the -3B30NA (capacity= 36,000 BTU/hr @ +5F) would pretty cover your extreme cold-snaps:

    The economics of waiting until there is a big step-function on multi-split efficiency probably aren't there (but I'm not an industry insider.) Big steps like the efficiency jump from Mitsubishi's "FE" cold climate units to the recently released "FH" mini-splits are pretty rare, but if you're burning propane or heating oil while waiting, putting it off a year or two would not have been cost-effective on a lifecycle basis.

  2. Bdgray | | #2


    Thank you so much for your response. I had a well-respected HVAC contractor come to my house yesterday. While I could tell he was knowledgeable about minisplits (something that seems rare in Chicago), I was pretty dismayed at the premium his firm is asking on these systems. After talking for an hour, showing him the house, and having him take measurements of walls & windows, he informed me that I should be prepared to pay ~$40K for this sort of system. I know what these systems cost. He's essentially tacking on $30K+ in labor and margin which I find difficult to fathom for a 1-2 weeks project. In my opinion and to sound a little jaded, the real premium here is coming from the fact that in Chicago, the only people installing multi-split systems in their homes are the uber-rich who are willing to pay any premium to boast about their state of the art, eco-friendly, green HVAC system (while they make a Latte run in their $120K Tesla). Just to reinforce this belief, the contractor then showed me photos of the $100K project they just completed to install an 8-zone minisplit system plus hydronic radiant heat in a DER house in Winnetka (where homes start at $2M). I'm just floored. Only rich people can make energy efficiency seem so damn wasteful.

    I am now reaching a tipping point of feeling I should do this work myself. I don't really have a question for you but I certainly would appreciate a sanity check if this is way beyond a seasoned DIY'ers head. I already work on my house dawn to dusk on weekends (thank god my wife doesn't mind this obsession). I certainly don't need another project but it sure seems worth the effort if I can shave $30K+ off the cost of this installation. I'd much rather put that money toward retirement or my kid's college.

    Thank you,

  3. corycross549 | | #3

    Did you ever do the mini splits I know a fair amount about them I’ve installed a few I’m not saying I’m a master at it but I learned a few things because I seem to be like you I’m always interested on the best way of doing things one thing I learned is they save energy by running all the time at a slower speed if not needed very much cooling or heating if you do use them for air-conditioning it be best to put a UV light (250) at supply shop. in there to keep them clean inside because they tend to stay moist either that or taking the blower wheel out once a year and cleaning it real good and clean in the unit inside but in my opinion the energy savings is worth this little bit of inconvenience . A couple things I can say is don’t cheap out and get a cheap unit some of them are offering ten year warranty‘s , sometimes they have control boards go out and if you have a 10 year warranty it’ll be free to get the part and put it in yourself if you could learn that once again if you get a good unit maybe you won’t have any problems. The only thing that I’ve heard over and over again with people installing them themselves is when you connect that Freon line set to the unit you have to tighten then loosen it then retighten then loosen it to create a groove so it seals very well a lot of people that do it themselves they just tighten it once and they end up losing the Freon my boss that I worked for actually got a free mini split due to this . Then be 1000% sure you get the wire size and breaker size correct with the electrical it might be worth hiring electrician to do the electrical for you . Hey if you ever need help I for sure wouldn’t be close to 30,000 I come up there and give you a hand just as a helper being that I put in a couple I hate to hear someone want to charge you $30,000 that is insane in my opinion and I know a lot about pricing with air-conditioning . One thing that might help if you’re doing it yourself is to get on the EPA’s website and maybe get your EPA certification teach you a little bit about the Freon regulations . I feel it would be worth the investment and would cost nowhere near 30 grand to do that LOL. Watch yourself a bunch of YouTube videos 10 or 11 different people you know so you get a good idea what you’re doing .

  4. norm_farwell | | #4

    Brian, I'd avoid DIY if you can. There's a lot that's not in the manual, and you'll spend hours you don't have, you'll void the manufacturer warranty, and there's an epa license involved. All doable but I've learned to pick those battles.

    Did you try Mitsubishi's dealer locator?

    Also I've noticed some installers avoid multiheads because they tend to be more complicated, so if you are willing to use the one-to-ones you might have a larger pool to choose from. I'd say you aren't overpaying at around 5000/ head for a larger single, and a bit north of that if you are doing a multihead esp with ceiling cassettes.

    Whatever you decide it would be good to base decisions on a room-by-room heat load calculation. Coolcalc is a free download if you want to DIY. Any competent installer should be able to do one.

    Good luck with your project.

  5. corycross549 | | #5

    Not to be rude or anything just everybody has their own opinion I strongly disagree with not doing it yourself I think mini splits are extremely easy to put in it as long as you don’t oversize the unit for your house you’re good to go you can start with just one and then add more in the future and see how the first one goes I’ve worked in the air conditioning field and if you’re willing to read a couple hours and do a little bit of research you’re smarter than 90% of the people that work in air-conditioning to begin with.

    1. bfw577 | | #6

      I have installed probably a dozen or so units so far for myself and friends/family. They really are pretty simple to install. These things are installed all over the world for cheap. Costco and Walmart sell them on the shelves in Mexico for like $500 and it cost like $100 to install one.

      I actually think a properly done self install might be better than a professional install. I had endless time to spend making sure everything was perfect. I was able to spend days making sure I had no leaks and checking everything over multiple times. Time is money for a hvac company and they are most likely going to install it in 1 day. You really just need a vacuum pump, micron gauge and torque wrench for the flare fittings. You can purchase all HVAC pro quality tools for a few hundred and then resell them on ebay if you dont want to hold on to them.

      I don't think an epa license is required. The outdoor condensers are precharged with refrigerant. Once you know the lineset is leak free and under 500 microns of vacuum you just open the valves and let the refrigerant in. Anybody can buy those refrigerant cans for their cars with no license. You can also buy the r410 by the tank online with no license.

      1. norm_farwell | | #8

        In order to purchase refrigerant you definitely need an EPA license. This is for good reason and fines are steep. Auto cans are exempt for now. More here:

        In order to buy an appliance that contains refrigerant, you do not need a license of course. The units that you are installing are very different than the cold climate ones referenced in the original post, which have much longer and more complete warranties, better efficiencies, better low temp performance and are therefore less amenable to a DIY approach.

        Can it be done? Yes. Is it a good idea? Based on my experience I'd say not if you can find a pro for a reasonable price.

  6. Deleted | | #7


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