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Community and Q&A

Oversized Minisplit System and High Humidity

Rioman | Posted in Mechanicals on

We are remodeling our 2300 sq ft 2-story home in Charlotte, NC (built in 1998). All new windows, 60% new plywood/wind wrap/r15 insulation. It should be complete in 4-6 weeks.

Our duct work needed to be replaced on both floors so we replaced (2) Trane 2-ton AC/gas furnace units with (2) Mitsubishi MXZ 36k Hyper Heat units from a local company with a good reputation.

First floor (MXZ 36k + 3-port branch):
1. ~800 sq ft open living area – Mitsubishi Gl 18k wall mount
2. ~400 sq ft 2-car garage (very little insulation)- Mitsubishi Gl 15k wall mount

This system was installed first. It works great, easily keeps the living area and garage heated/cooled with humidity in low 50’s (entire house was low 50’s before)

Second floor (MXZ 36k + 5-port branch)
1. (4) Mitsubishi Mlz 9k ceiling mount units in Master ( 300sq ft + 150 sqft attached bath), Bonus (192 sq ft), Spare (130 sq ft), Spare ( 144 sq ft).

The system cools well, but is barely regulating humidity. It can average in the 60’s and can easily jump to the high 70’s. We added portable dehumidifiers to the rooms until we can find a permanent solution.

I read a few posts here and it sounds the upstairs system is oversized and the mini splits are short cycling? I relayed this to the hvac company and they opened a case with Mitsubishi and changed jumper settings for coil temp and fan speed = no difference. They are already trying to say it’s normal and trying to find non-existent air leaks as the source of the humidity.

What options do I have to correct the issue?
1. Fix the current system?
2. Add a ducted dehumidifier (no ducts currently)
3. Replace the (4) mlz ceiling mount units with (2-3) mini-ducted units + ducts
4. Replaced (4) mlz ceiling mount units with (1) SVZ full sized ducted unit + ducts
For 3/4, assuming the 36k outdoor is likely oversized as well.

Thanks in Advance!

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  1. paul_wiedefeld | | #1

    72,000 btu for a house that size in North Carolina? With insulation and window upgrades? In a 24 year old house? That’s one of the worst multi-split installations on here yet, you have my sympathy. There aren’t easy fixes from here, but 3 and 4 are strong options. I hope you don’t have to pay for their mistakes here, this is truly egregious. If you’re going with a ducted system, go with a different contractor too. It seems Mitsubishi needs better contractor quality control.

  2. Rioman | | #2

    Thanks. It's been very frustrating dealing with the company. They are already blaming invisible air leaks for the humidity and claim Mitsubishi says its not oversized. I'm going to call Mitsubishi on Monday, but not looking good for me.

    For options 3/4. The 36k condenser is still oversized for a smaller load right? Assuming the SVZ would need a single port condenser, and the mini ducted units would likely need a 24k or smaller multi-port?

    1. paul_wiedefeld | | #3

      Yes oversized. You’re looking at a new system unfortunately. No need to stick with Mitsubishi for a two ton heat pump (although I have the two ton SVZ/SUZ and I like it). If they’ve screwed up the cooling calculation this badly, throw out their heat loss too.

      Running an independent blower door test isn’t a bad idea.

    2. aunsafe2015 | | #4

      6 tons for 2300 sq ft is really, really bad. No idea how Mitsubishi could possibly be defending it.

      There's no question that both 36k condensers are oversized for 2300 sq ft. An actual manual J would probably return 1.5 tons for each floors -- maybe even only 1 ton per floor. So you are probably 2-3x over sized.

      Which actually makes me think... one option might be to eliminate one of the condensers all together and see if you can have all of your indoor units (both floors) connected to one of the 3-ton ODU. Net result would be single 3-ton ODU serving both floors. Even that might be oversized but it would be a heck of a lot better than 6-tons.

      1. user-2310254 | | #7

        I have a similar system with a single outdoor unit serving two zones (3.5 tons for almost 3,000 square feet). The design uses a single branch box to allocate refrigerant to each zone.

        FWIW, this system has enough leftover capacity for a third zone. My installer said I could put a wall head in the garage if I wanted to.

        If it were my house, I would try to get the installer to reconfigure one of the systems and take back the surplus equipment.

        1. Expert Member
          Akos | | #9

          +1 on re-zoning your main floor unit. This will also mean downsizing some of the main floor wall mounts as you can only install up to 130% indoor capacity on the outdoor unit. 2 tons of cooling for 950sqft is way overkill. Probably closer to 1 ton.

          Make sure the 2nd floor ducted unit is inside conditioned space, never in the attic. Might be a bit more work and take up a bit of space but saves a fair bit running costs and simplifies maintenance down the road. Chances are what you would save in mounting the unit in the attic you would have to spend in extra labor if the unit ever needs to be serviced down the road.

          The ducting is best also inside the house but if this is too troublesome, running up through the attic is not terrible as long as it is well sealed. This means spray foaming where they go up to the attic and the register boots in the ceiling. Checking the install with a duct blaster is also a good idea. Once it is all sealed up, burry the ducts in insulation.

          1. Rioman | | #11

            (Moving reply to thread and editing response)

            They originally quoted a 24k SVZ for the upstairs with new duct work or a 4/5 zone mini split setup. They were consulting with a supplier on the mini split design which seems to be the issue (bad advice).

            If I moved to a single 36k condenser, I would likely end up replacing all 6 heads, as the 9k ceiling mount mlzs
            are oversized for the bedrooms, and the two wall mounts downstairs are already at 100% capacity. At 130% capacity I'm at ~46.8k total. So likely 18k ducted upstairs ( 1 - svz or 2 - mini ducted) and 16k/12k walls units (or smaller here and larger upstairs).

            The downstairs system works pretty well, so it might end up being cheaper to install a single port condenser/SVZ ducted with new ductwork ($$).

            I'll see what they are willing to do this week after talking to Mitsubishi.

            I did use a credit card for cash flow reasons (large renovation), but not sure if a chargeback is even possible. The quote does say the system was designed to meet 50% humidity and it's not even close.

  3. Deleted | | #5


  4. walta100 | | #6

    You may try turning off the down stares unit and keeping the doors open enough cool air may find its way down stairs to keep it comfortable and the upstairs unit will run more so it will remove more moisture.

    Are the discharge air temps similar for both the upstairs and downstairs units? It is possible the upstairs unit had a small leak making it underperform.


    1. Rioman | | #8

      They checked the pressure/temps last week and said everything look good.

      1. aunsafe2015 | | #24

        You should measure the supply air temp, though. Turn the setpoint down to like 65 and stick a meat thermometer in the vent closest to the air handler. See what temp the outcoming air is. I would think it should be 20+ degrees colder than room temp.

  5. ianrking | | #10

    You said you “changed jumper settings for coil temp and fan speed.”

    I assume you mean your tech lowered the target coil temp - an option on Mitsubishi multi-split condenser units. I’d recommend reversing this setting back to the factory default. Yes lowering the target coil temp will increase latent capacity and remove more humidity but only while the coil is cool. Lowering the coil temp will increase the rate of cooling, which on your oversized system is already too short, so what you’ll experience is actually even more coil off time, which is when the indoor humidity is climbing.

    If you haven’t done so already I’d recommend cutting the JRRE jumpers on the indoor unit PC control boards so that the indoor fans cut out when the unit is no longer calling for cooling. This way the condensation left on the coil does not get reevaporated into the room.

    1. Rioman | | #12

      Yeah, that's what they changed. I told them the same thing (already short cycling, not improving that issue). The air is definitely colder, but we noticed higher humidity as you mentioned. I can ask them to cut the JRRE jumpers, but do you think it will make a meaningful difference?


      1. ianrking | | #13

        Yes I think cutting the jumper so that the fan cuts out while the coil is not on will make a considerable difference.

        Also the 15 and 18k wall mounts have different fan speeds and auto fan behavior than the lower capacity models. The fan speeds are higher. And for some reason the auto fan mode does not use all of the fan speeds (it uses low instead of quiet as it’s lowest setting) so you may want to force the thing into “quiet” so it doesn’t cool the spaces as quickly.

        1. Rioman | | #17

          It looks like this would require external thermostats? The ones I saw were $200-$300 each. Maybe we could test on one unit.

          1. ianrking | | #19

            Correct. If you have Kumo, you can add the external temp/humidity sensor and it'll be much less per unit.

      2. aunsafe2015 | | #25

        I agree with ianrking that cutting the JRRE jumper could well make a noticeable difference.

  6. walta100 | | #14

    Are you the type of person to accept the fox’s opinion of the health of your chickens? I say check the temperatures yourself.

    Seems to me you already own more equipment than you need and you need to redeploy it in a way to heat & cool the room you use.


    1. Rioman | | #18

      Fair point and I agree

  7. Expert Member
    Akos | | #15

    This is a rely to #11.

    What I meant is change the garage wall mount to a 12k head and install an 18k ducted unit for the 2nd floor. This can still go onto your 36k main floor unit with the 3 port branch box. The 36k with the 5 ports and all the upstairs heads can be pulled.

    The ducted unit can be either an SVZ or a slim ducted unit. The slim ducted unit is easier if you have ceiling space you can hide it in (hallway ceiling above a stair or the ceiling of a larger closet). The SVZ is simpler for an HVAC tech as it is pretty close to standard air handler.

    Leaving the main floor as is and putting the upstairs SVZ/PEAD/SEZ on its own outdoor unit is the better option especially if you can reuse some of the existing lines. This gets you higher efficiency and better modulation range.

    1. Rioman | | #16

      Makes sense. Thank you

  8. Jon_Lawrence | | #20

    +1 for ducted upstairs. I am using ducted Fujitsu mid-static units for my 2nd floor and they have great turn-down ratios, they keep the temperature within a degree or two of the set-point and they are quiet. You may still want to add an inline dehumidifier upstairs.

    1. Rioman | | #21

      Appreciate it

  9. Rioman | | #22

    I called Mitsubishi and the rep said it sounds oversized, and escalated to their consumer relations team. The sales rep at the HVAC company is looking at the original load calculations, system design, and potential solutions. I pointed them here as a starting point. Appreciate all of the input.

    1. aunsafe2015 | | #26

      If the sales rep at the HVAC company comes back and tells you that they stand by the original 6-ton recommendation, do not accept that. No reasonably performed manual J could possibly result in 6-tons for 2300 sq ft in NC. 2-3 ton much more reasonable. I could even believe that somebody could say 4 tons with a straight face. But 6 tons? No. Just no.

  10. jameshowison | | #23

    Run your own Manual J at that gives you a proper comparison point.

    I think you will be happier with a dedicated dehumidifier. There are times that even an appropriately sized AC system won't manage the humidity as you would like.

    Of course that takes ducts (but so does fresh air distribution), can combine with the SVZ. SVZ also has simpler setup for large filter. Make sure you get a 4" filter box (or multiple filter registers).

    1. relevante | | #27

      A nice whole house dehumidifier really is an amazing feeling. It may not take all that much ductwork either. I had one (UltraAire XT155h) in my last two story house. Just stuck it in a closet at the end of the lower level and poked a vent through the upstairs floor. The stairway in the middle of the house acted like a return. You could absolutely see the humidity drop everywhere in the house. It took longer at the end that wasn't part of the obvious flow loop, but it happened there too. Definitely made the whole house feel noticeably nicer. Went from being cool but a little damp to feeling like walking into a casino in Vegas or something.

  11. Rioman | | #28

    Mitsubishi called me back and let me know they will work with the contractor to identify/resolve the issue. They are concerned the unit is oversized, and asked to review the manual j with the contractor. I should have an update this week.

    The contractor reversed the previous settings for coil temp and fan speed. I tried a Ceilo wifi remote in one of the rooms. It's basically a wifi IR remote w/ temp+ humidity sensors. It has a comfy mode that can power the unit on/off at low/high temp points to stop the fan from blowing 100% of the time. It did help a little but the humidity still jumped into the 60's so I'm not sure if cutting the fan jumpers will help.

    The contractor seems to know it's oversized, but assumes the system should be able to ramp down low enough to meet the btu requirements for each room, likely (3k, 3k, 4k, 6k) on 9k heads. It looks like the 36k condenser has a min (18k) and I read the multiple head configuration is also a limiting factor?

    Thanks again

    1. Expert Member
      Akos | | #29

      The min on most multi splits goes up as you add zones. Mitsubishi doesn't mention this in their current models but do show it for the previous generation. Seems to be about 1000BTU/zone. I doubt that a 24kmin is anywhere near your cooling load, even if it is, the chances of all zones calling for cooling at the same time is pretty small so the heads will run at short bursts at full tilt then off. That won't work for dehumidification.

      1. Rioman | | #30

        That makes sense. Thank you

  12. greenright | | #31

    ... just off the top of my head you are at about a ton and a half per floor... maybe. Like two ton upstairs and a bit less downstairs.

    No matter what you do you are so oversized that short cycling, lack of dehumidification and overall efficiency will be an constant issue. Add the issue that most MItsus that folks buy are not true VRF solutions so their turn down is pretty bad and this only makes oversized installs worse.

    1. Rioman | | #32

      Appreciate it

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