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  1. #1
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    Oct 2019
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    Mini-Split cooling too quickly

    I recently built a small vacation cabin in SC. It is tightly built with SIP panels and has a Mitsubishi ductless mini-split system. It is virtually two separate buildings connected by a porch. Each building has its own mini-split head.

    The main building is 450 sf and has a 12,000 btu unit. It does a good job of keeping the temperature and humidity levels around 60% in the hot summer. Water drips out of the drain pipe outside regularly.

    The small building is 200 sf and has a 9,000 btu unit. The humidity level in there is always over 70% when the air conditioning is on. Water NEVER drips out of the drain pipe outside. My question is regarding this unit.

    When we turn it to dry mode, water will condense out and the humidity level goes down. But when we turn on the AC, the humidity spikes.

    I believe the unit is oversized and asked the contractor if we should switch to the 6,000 btu unit. He agreed that the unit was cooling the room too quickly but he said that the 9k unit and the 6k unit have the same lowest speed, so there would be no benefit to going smaller because it wouldn’t cool the room more slowly. This seems surprising to me, because this implies that the 6k unit isn’t actually made to cool a smaller room. His suggestion was to put it on dry mode all of the time and never use AC mode. He said dry mode will go 3 degrees below the temp setting, so if we want it 75, set it to dry 78.

    I called the Mitsubishi hotline for their advice and they said they don’t recommend dry mode for more than 4 hrs per day, but they didn't answer my questions about whether or not the 6k unit can cool a room more slowly than the 9k unit. They thought I was asking them to size my unit for me, which I wasn’t. I just want to understand if it’s true that there would be no reason to switch to the smaller unit.

    Logically, it seems like the smaller unit would cool a room more slowly than the larger unit, thus allowing the moist air to condense out the drain pipe. But my contractor says they are the exact same unit at low speeds, the only difference is that the control panel allows one to go as high as 9k, but the other one is limited to 6k. I understand that the fan speeds are the same, but are the coil sizes different between the units? Wouldn’t this affect the cooling speed?

  2. #2
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    Mar 2018
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    Chico, Ca #StateofJefferson
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    Going be honest, I dont know how Mitsu is controlling capacity, be it different coil size and slower blower motor or by controlling the refrigerant flow. Either way the 6k head will absorb 33% less btu/hr. What is the condenser and head unit model numbers? Why is your humidity so high in the other room, short run times?

  3. #3
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    Feb 2010
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    Metro Atlanta, GA
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    Judging by your humidity levels I would say that both units are oversized. Did anyone do a load calculation for the space? If your 450 sq ft space has a 1 ton head why would your 200 sq ft space, less than half the size, need a 3/4 ton head?

    Do you have a multiport outdoor unit or are there 2 outdoor units?

    This only pertains if you have one outdoor unit:
    Your contractor is right that the air delivery on the 6K and the 9K are the same. Parts wise the only the difference between the 6K and the 9K indoor heads is the control board, which would let a multiport outdoor unit know what size it is. If it sees a 6K indoor then it's going to set expansion valve positions and compressor frequency rates for a 6K indoor head and the indoor fan will adjust accordingly.

    Post your indoor and outdoor models so we can get more specifics on them, please.

  4. #4
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    Thread Starter
    Thank you so much for this advice. I've heard that SIP panel built homes require lower loads, but my thought was even if this wasn't such a tight home, I'd still need a smaller unit than they put in. In the summer, the humidity levels outside are sky high, so we might need a dehumidifier for both buildings, but I certainly want to first get the AC to do its job before I add a dehumidifier. I assume since water is draining regularly from the bigger unit, that it's at least removing humidity, but the smaller unit isn't removing any water. The bigger building never feels too humid, but the small building is cold and sticky when the AC is on.

    Mitsubishi Outdoor unit – Split System Heat Pump Model # MXZ-2C20NA2
    Mitsubishi in smaller building (200 sf) – Split System Heat Pump Model # MSZ-GL09NA
    Mitsubishi in larger building (400 sf) – Split System Heat Pump Model # MSZ-GL12NA

  5. #5
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    Feb 2010
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    Metro Atlanta, GA
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    Per the operational performance on your configuration the indoor units get a slight derate in capacity - the GL12 derates to 11,400 BTU and the GL9 derates to 8,600 BTU. If you drop the 9K to a 6K the larger unit will give the full 12K capacity, which may result in even poorer humidity removal for the larger unit. I'd strongly recommend doing a load calculation before making any decisions on swapping indoor heads. The performance data for your unit is at Mitsubishi's website here:

    http://meus1.mylinkdrive.com/files/M...erformance.pdf

  6. #6
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    Oct 2019
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    Thread Starter
    This is all so helpful. I will ask him to calculate the load before we make any changes.

    I need a little more information in order to discuss this with him because the technician had said there wouldn't be any difference if we drop to the 6k unit because they both run the same at low speed (ie the 6k can't cool the room any slower than the 9k). You mentioned that the expansion valve and compressor rates are different on the smaller unit. Would this effectively slow down the cooling, or is the fan the only way to slow down cooling?

    I feel like if the 9k unit is oversized, then the 6k unit would be a better fit, but he's basically saying both sizes are oversized and I need to buy a humidifier or put it on dry mode 24/7, and skip AC completely.

  7. #7
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    Mar 2018
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    Chico, Ca #StateofJefferson
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    Why bother putting btu ratings on equipment if it doesnt mean what it says? I'm sure the 6k and the 9k share a lot of the same components but there has to be differences in operation to to lose 3k btu an hour, I would think that the airflow would have to decrease or it wouldn't dehumidify at all. With a 0.25 ton decrease your only talking maybe 100 cfm less airflow. You might want to try your own manual j for free online (Loadcalc.net) as I dont think the installer is going to if he didn't the first time

  8. #8
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    Oct 2019
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    Thread Starter
    He said that the components are the same in both units, but that board in the 6k tells it to stop at 6k, while the 9k unit is allowed to go up to 9k when needed. I had assumed that this meant that when not running at full power, they both run at less than 6k, but when it's really needed, the 9k unit would be able to run above 6k. Maybe I'm wrong and that isn't how they work? Do all units use the full btu all of the time? Or is the btu variable based on the need? (Kind of like a generator that pulls less energy with a lower load, vs one that runs full-on all of the time.)

    He admits the 9k is too large, so I just need to understand why he thinks the 6k won't be a better fit. Thanks. I'll check the load calculator.

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