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Multiple Problems with Minisplits

lj_builds | Posted in Energy Efficiency and Durability on

In Boston area, I’m 1 year post gut renovation of my house and have had all kinds of minisplit issues – sorry for the long post. Happy to engage any professionals to get this sorted, let me know if you any recommendations. Roughly 2500sf conditioned space including 600sf basement. Desnsepacked 2×4 with intello inside and proclima outside, triple glazed windows, 7 lunos units. Mitsubishi multizone system compressors are 30k and 42k. 7 interior units – Big compressor has 4 units: 9k basement, 9k 1st, 2nd floor: 9k Bed1, 9k Bed2. Small compressor has 3 units: 6k basement office, 9k 1st, 9k Bed3. My electrical bills have been between $600-700. I’ve got emporia system to monitor electrical usage. Set at 68 degrees and all auto settings the big compressor cannot use less than 3-6 kwh per hour regardless of the number of units on or the ambient temperature in the room. The small compressor is able to use 1-2 kwh running it’s 3 units. We also had humidity issues this summer in cool mode, basically turning on the units would both cool and add humidity to the room, adding between 5-10% RH. The units on the small compressor didn’t have this issue. Lastly, interior vibrations and exterior noise of the large compressor have become unbearable, it sounds like it is permanently in defrost mode. In an urban area, unit is 5′ from property line, neighbors want to kill me. It is wall-mounted on the wood-frame which was my mistake, so the installer is coming to move it. In the meantime I’ve shut off the big compressor so that we can get some sleep. Happy to elaborate more. I’ve upload plans and load calcs (done by installer) etc.
UPDATE: the installer thinks the compressor is bad in the large condenser and will replace it.

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  1. jameshowison | | #1

    Interested to follow this thread.

    For the outside unit mounting you might consider these stands, they have vibration reduction and lots of space under the unit to avoid leaves etc collecting under the unit. Also give you more options to tweak tension in how the unit is anchored.

  2. rondeaunotrondo | | #2

    Levi, I’ll shoot first with the least experience but from looking at your manual J, you are completely oversized. Manual J total is 42K (probably inflated) and you have a 42K and a 36K Outdoor units tied to multiple indoor units which are all calling at the same time. Unbelievable that HVAC people thought that you were undersized. What was your blower door number? You’ll likely need a dehumidifier because unfortunately these mini splits do not do dehumidification very well especially as it become more efficient. Interesting that you went with 7 lunos. These help with fresh ai (CO2/VOC) but do not help your humidity issues in the summer. What windows did you buy and from where?

  3. charlie_sullivan | | #3

    So starting with basics, you have 72k total compressor capacity, 60k total head capacity, and a 42.5k calculated load. So your compressors are 1.7X oversized and the heads 1.41 oversized, vs. a recommended 1.4 maximum oversizing. So I would agree that you have some oversizing problems.

    I'm also skeptical of the 6436 BTU/h duct loss number in the heat load calculation. Are any of these heads ducted? And even if they were, they wouldn't have anywhere near that much loss. Without that, your heads are 1.66X oversized and your compressors 2X oversized.

    And that's taking the rest of the load calculation at face value--likely that's an overestimate too.

    1. lj_builds | | #5

      No none of the units are ducted. And oversizing usually causes these kinds of issues?

      1. paul_wiedefeld | | #9

        Yes, Mitsubishi themselves explicitly advises against using multi-splits this way. The oversizing leads to awful performance, often barely exceeding electric baseboard. There could be a solution here with the 30kbtu unit: if possible, swapping the 9k head for a horizontally ducted indoor unit and try to split the second floor's load. They make a 9,12,15 and 18k horizontal unit. It might also be worth substituting electric baseboard in some rooms (9k/bedroom will never work with a multi-split) and figuring out a different AC source. No particularly great options remain unfortunately.

  4. Expert Member
    PETER Engle | | #4

    Without digging too far into the numbers, a top-level review shows some serious issues:

    Your whole-house heat load is listed in the Manual J as 42,522 Btuh. Assuming that's correct, the entire house could be heated by the 42k unit, and the 36k unit is a complete waste. But the calculated heat loss is probably way high itself. If you were diligent about air sealing during your renovations, the construction quality shouldn't be set at "average." Your dense-packed walls and air infiltration rates will be much better than average. Also, it's not obvious in the printout, but I suspect that the analysis assumes standard glass double pane windows, not the low-e triple panes you installed.

    FWIW, 9k heads are way oversized for any reasonable bedroom. Most bedrooms have loads less than 3k. It sure looks like the system is way oversized and poorly designed for the loads. You will need someone to take a much more detailed look than I can online. Maybe some others here can point you to someone who can do this more formally for you.

    1. lj_builds | | #6

      Air sealing was good everywhere except an basement area where there is a crawlspace it can feel a little drafty there. This confirms my experience because I can keep the house at 68 using the 30k compressor only if it is 30s or 40s outside. Unbelievable

      1. PBP1 | | #8

        Interesting, does appear oversized. I have over 2100 sq ft and just had 30 days (Dec 15-Jan 15) with 1343 Heating Degree Days (65F) and average temperature of about 20 F (included sub-zero temps). House has 6 sliders and clerestory on first floor (double glazed) and the 30k Mitsubishi (rated at 28k heating, 47F) three-zone ducted (15k, 12k and 9k) used about 1550 kWh (about $150 at $0.1/kWh), even with spouse turning her 600+sq ft workspace up to 80F on a couple of days (would only reach about 76F). Computed load was 28k (zone 5, -1 F DB temp).

        For your usage, Boston $0.245 per kWh (?) and total of $650/month so about ~ 2500 kWh/month? And, the Boston heat load zone 3b is at +12 F DB temp. So, I'm at 28k at -1 F and the heat load in the file is over 42k at 12 F.

        Mounting is on a 2x6 garage wall that was spray-foamed (flash-n-batt), which gives the wall extra integrity. I also have vibration damping material on the mount. I get large ice formations under the unit (6 ft x 2 ft and about 10" high), ground is crushed rock. Vibration makes some noise but not too bad. Code had me put it on the wall.

  5. Expert Member
    BILL WICHERS | | #7

    I agree with the other posts about the system being oversized.

    I'd look into air sealing your rim joist, which is probably why your crawl space feels drafty. This is a common place for air leaks, and is often overlooked. On the plus side, it's not usually a super difficult or expensive issue to fix.

    I've detailed some ways to cut down on noise from the outdoor unit. Most important is to get the unit off of your wood framed wall! That wall will resonante and amplify the sound. You want the unit mounted on a stand a little off the wall, with that stand ideally bolted to a concrete slab that is not connected to the home in any way. The concrete's mass acts to damp the sound, the stand keeps it off the house so there is nothing to resonate. Avoid any rigid connections between the house and unit, which means you have to have a loop in the refrigerant lines ("loop" is usually a big "U" shape) to act as a shock absorbing spring. Try to bring the lines through the wall at a rigid connection such as a concrete wall, or as clost on the wood framing to the foundation as possible. More rigid parts of the structure will resonante less, and help to damp the vibrations in the line sets to keep conducted noist down. You need to do the same with any rigid electrical conduit connections, or just use some sealtight conduit in a loop to prevent vibration coupling between the unit and the structure.


  6. Richard_L | | #10

    Yeah, it looks like a cavalcade of poor decisions on the part of the HVAC designer/installer that are leading to your issues. The Manual J is likely overestimated (they usually are, particularly when they ignore the above-average efficiency of the building), duct losses being added to that despite there being no ducts, then they oversized the units by ~1.5x off of their already inflated Manual J, leading to them probably having 2x the capacity you actually need at the design temperature (never mind the other 99% of the hours of the year). And then installing multisplits, which respond terribly to being oversized for a space.

    It's astonishing that HVAC contractors can routinely do jobs that cost tens of thousands of dollars with so little care or understanding of how these systems perform.

    1. rondeaunotrondo | | #11

      Completely agree. How on earth do these contractors get away with this? It’s not that hard.

      Shout out to Nate Adams and the hvac 2.0 process.

      It’s sad to learn that as a homeowner, I know more than the majority of HVAC companies in terms of hvac design.

      1. bfw577 | | #12

        The state efficiency programs are also involved. I would guess Levi got thousands (rough math shows $12k+) in rebates from Massachusetts energy efficiency program Mass Save for his heat pumps.

        The heat pump rebates in New England are some of the most generous in the nation. My neighbor just got a $10k Mitsubishi system for only $4k after the rebates here in CT. I believe Massachusetts incentives are even better.

        The amount of posts I have seen on here and other forums about oversized multi splits in New England with massive electrical bills is staggering. All these oversized multi splits guzzling down electricity seems counter productive to their goal from their state incentive programs. Why isn't there more oversight in these programs considering they are all funded by charges on everyone's utility bills. So the ratepayers are paying for these incentives which in most cases are not even helping the environment or saving anyone money.

        So far the only New England state Efficiency program that seems to be doing anything about the issue is Maine. I believe they recently dropped the incentive for a multisplit to $500 ton while a single zone now gets $1500.

        1. PBP1 | | #13

          BFW577, enjoy your posts. I think a Mitsubishi multi-zone ASHP is OK for some applications and perhaps a single multi-zone ASHP is less likely to be oversized compared to multiple single ASHPs. Two ASHPs means two sources of vibration, two sources of ice, two pieces of equipment to maintain, two circuits, etc. The philosophy of the HVAC team and energy expert (handling various locations in MT, where my city requires HERS ratings) was to undersizing with knowledge that the ASHP would be working nearly all the time at cold temps, with some handing off from zone to zone as each zone became satisfied. The three zones are quite isolated, one being completely isolated from others (separate entrance). The only concern was one zone being in need of heat while another zone being in need of cooling, which has never been an issue. With ducting, it's easy to assure bathrooms (plumbing) and other small rooms are adequately heated/cooled. Of course a properly sized/located single ASPH with short lines to a single wall mounted head is going to be the most efficient (e.g., with use of a PAC for air cleaning).

          To Will, the three zones are ducted, and all bathrooms have a supply as well as the utility room (which has the plumbing manifold and washer/heat pump dryer). How do you heat all bathrooms and other small rooms with a one-to-one wall mounted unit? Bathrooms benefit from additional airflow as do most rooms. I have no complaints with a three-zone ducted single ASHP - rated at 28k. For one-to-one, I would have needed three ASHPs (not a reasonable option, where to put them, three circuits, ugh). And, no arguments as to one-to-one where suited.

          1. rondeaunotrondo | | #15

            I don’t think the HVAC experts here would agree with you. One to one Indoor unit outdoor unit has Greater turn down ratio so that the unit is able to modulate better. There’s also redundancy if one of the compressors fail so one to one is seen as as superior. And I’m assuming you’re not talking about the ducted units which is completely different.

          2. willymo | | #32

            I agree with Will R- An advantage of single zone units is when one compressor goes down, you have the others to protect the house, and the heat is shared with the zones without heat. Also, one to one units allow much greater modulation.

  7. jameshowison | | #14

    We were in a similar position. A head in every room (including bedrooms) which meant that the outside compressor couldn't handle the number of heads. So things were bumped up to two outside compressors. Then the only slightly over-sized outside compressors weren't available (or weren't ordered in advance) and so we ended up with two way over-sized compressors.

    After fighting issues for two years (these were ceiling mounted one-way units), particularly dust and noise on inside units, but also moving the outside units because the defrost wooosh was too noisy, we eventually got everyone (Mitsu, installer, and us) to agree to share costs on converting to a fully ducted system.

    I managed the Manual J like a hawk and loads matched one of the single outside compressors, so we paired that with a matched single multi-position air handler. Installing the ducts was expensive, there is some transferred noise from room to room through the system, but the filtration is excellent (MERV13 4.25" actual filter) and performance has been excellent.

    Sounds like your installer got the Manual J wrong. If that was submitted to an AHJ perhaps that is leverage for having them contribute to the cost of a full system replacement? It's either fraud or incompetence (the duct loses are an obvious error and quite substantial).

    Mitsu and others can say they don't recommend head in every room all they want, but until their marketing material doesn't show it, that's just nonsense. Check out their latest consumer brochure:

    If they want to advertise this stuff, they need 500-3000 btu heads paired with compressors/branch boxes capable of 7-12 units.

    1. rondeaunotrondo | | #16

      Wow, James, that’s an incredible story. How much noise are you experiencing either the ducted units? Which unit was specified? I’m kicking myself a little bit that I didn’t go with a mini ducted unit but just didn’t have the room.

      1. jameshowison | | #20

        Noise is air movement noise (not a problem but probably could have been minimized with sections of flex (ducts are almost all hard pipe). No noticeable equipment noise.

        And ducts mean we can hear a little talking in other rooms, nothing hugely problematic but with multiple people working from home one notices it more. Again, considering noise in the duct design (rather than focusing only on low static pressure and on the impossible task of “burying” ducts beneath cellulose).

        We ended up happy with the SVZ-KP36NA as the internal unit. The multi position air handlers are reasonably small and service access easier than the slimline ducted mini splits.

        1. Deleted | | #22


    2. charlie_sullivan | | #17

      Thanks for the story.

      That's one of the reasons I like the idea of air-to-water heat pumps--you can get emitters as small as you want.

      1. Expert Member
        NICK KEENAN | | #18

        I agree. The problem is that air-to-water is something like ten times as expensive as air-to-air right now.

        1. paul_wiedefeld | | #19

          Indeed and gets worse the smaller your heat/cooling loads are. Extremely high fixed installation costs.

        2. user-2890856 | | #30

          You don't get what you don't pay for DC . PERIOD . Some folks want a better comfort and indoor experience .

      2. user-2890856 | | #31

        There is no substitute for the flexibility and options of R718 .

  8. Expert Member
    Akos | | #21

    Besides the issues mentioned above, there is something very wrong with your 5C42 outdoor unit.

    Even allowing for oversizing and such, you should see some modulation down to around 1000W in heat mode. It could be the refrigerant charge is way off on the unit.

  9. KevinChan02169 | | #23

    I'm also in Boston area, your units are probably double the size of what you need. I have 18k, 15k, 9k and 9k for my 4800 sf condition space (1300 sf basement), and I was able to keep entire house at 75F even with the extreme cold days we had couple weeks ago. My brother in-law also have the Mitsubishi 42k units installed with 5 heads on a wall mounted bracket, their unit are very quite and don't have any vibration. Your unit probably need service from the tech.

  10. lj_builds | | #24

    Lots of super helpful information and observations in here everyone, thanks for sharing. One more question. Should the indoor units stop blowing or are they continuously on? On the units, the vanes never shut, and the two light for “operation” rarely light up, I’m wondering if this is typical or part of the problem. Even if a room is heated to 68 and you turn it down to 62, it stays on.

    1. PBP1 | | #25

      Looking at the GBA Spotlight and it says "climate zone 5A". I think Boston/Boston area is more like Zones 6b, 7a? I lived in MA for six years, I remember winter 1983-84 as being quite cold in Western MA, January 22, 1984, below -20F (online history -18F in CT weather station). I also lived in Central MA and Natick MA, don't remember it ever getting that cold (and lived even colder in MN, like days w/high of -10F).

      If the Manual J used the wrong zone and design temperature, that'd be a start. I'm happy with my Mitsubishi hyper-heat 28.6 and three zones ducted with design temp of -1F (Western MT), the HVAC guys and the HERS rater/energy consultant both signed-off. At the design temp, no problems with 68 F set point.

      Maybe if more municipalities required HERS ratings there'd be more input on Manual J/equipment sizing?

      1. rondeaunotrondo | | #26

        ??? It’s zone 5.

        1. PBP1 | | #28

          My mistake, not looking at the ASHRAE zone, am I bad, was looking at USDA plant hardiness zone.

          1. rondeaunotrondo | | #29

            No worries.

    2. mjezzi | | #33

      I just spoke with a Mitsubishi installer today and he told me that the blower is on all the time to circulate the air in order to get an accurate reading of the rooms temp. There's a resistor that they cut on the board to stop that from happening, but then you won't get an accurate reading of the room temperature. You would have to manually turn it on and off. He said this works well for some people and overcomes the too much cooling or heating issues.

      1. Expert Member
        DCcontrarian | | #34

        Usually the solution is to get a remote thermostat.

  11. Yeldog | | #27

    With covid moving us to our shore house and a problem with part of the radiant system this winter we had been using the 3 head Mitsubishi system for heat. 3 x 12k on a 30k BTU compressor. Two ceiling and one wall -- all 12k. The ceilings are each up on the 2nd floor -- in the two big rooms of a cape style house. The 12k wall unit w/ eye is in a one story addition off of out dinning room. In the warm summer with them all running it works well -- turn one off and problems start. Since the units have to have some refrigerant flowing to all the heads all the time -- there seems to be a big wast of BTU sending that hot or cold refrigerant around. It also throws off the internal thermostats in the ceiling units ... we had to get wall thermostats and change over where the units get the temp.

    The compressor is mounted on a brick wall ..... we hear it in the summer. Boy -- to we hear it in the winter. I would never put a 30k unit on a wall again ... this should be a no go.

    Two other items. Only the wall units operate like how we expect a mini to. The fixed fans speeds and other limitations would have me looking else ware. We have another house with the same 12k x3 on a 30k .... the compressor is on the ground and the heads all the wall type with the eye. They are also in a barn conversion where they are all working to cool one large space .... this works much better.

    Having these set up with an oversized compressor (like so many are) with long refrigerant runs is common and IMO leads to problems. Most installers just add up the heads and match the compressor and never read the specifications -- the performance is not the same as the single heads. I blame Mitsubishi ... there should be clear install instructions and how these should be matched. In my new house I am using Mini splits -- only singles

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