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Thread: Humidity and condensation issues with Mitsubishi minisplit system

  1. #1
    Join Date
    Aug 2020
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    Humidity and condensation issues with Mitsubishi minisplit system

    Hey all,

    New here so please bear with me.

    My wife and I purchased a 4 story townhouse (~500sqft/floor) this past winter in Virginia. It has a traditional HVAC system for the first two floors, and a Mitsubishi ducted minisplit (p/n PEAD-A30AA7) for the 3rd and 4th floors, connected to an outdoor Mitsubishi split-system heat pump (p/n PUZ-A30NHA7); the air handler system is installed in a 3ft crawl space between the 3rd and 4th floors. The ducts are, from what I can tell, fully insulated.

    The past few months, we have had non-stop humidity issues. The humidity on the first two floors will be ~60%, but will be 80+% on the 3rd and 4th floors.

    The original thermostat included with the Mitsubishi system would only go down to 65F, and even at 65F, the system would never actually go below ~68F. I installed a transformer and Mitsubishi third party wired thermostat adapter (p/n PAC-US444CN-1), followed the wiring diagrams, and installed an Ecobee which allows for much better control.

    We have been having large amounts of condensation on the return HVAC vent in our master, to the point where there is drywall damage around (only this specific one) that return vent. The system is set up with one return vent per room and one supply vent per room, except the master where one return vent supplies both the master bedroom supply vent and the supply vent in our master bath. There isn't condensation issues anywhere else in the house, except the return vent in our master bedroom.

    I have noticed that if I close the supply vent in the master bath, the condensation issues go away on the return vent (making me think the steam from hot showers isn't being blown away correctly by our bathroom fan and somehow being condensed on the return vent in the master), but having no A/C in the master bath is pretty uncomfortable.

    We also noticed the A/C on the 3rd and 4rd floors doesn't work very well. It will be 70F, and I'll set the A/C to 65F; it will run constantly, and never reach 65F (as an example). It will cool down to maybe 66-67F. Installing the Ecobee helped as I can set it to 60F, which will result in the system getting down to ~66-67F.

    We had an hvac tech out, who said there was a refrigerant leak and we were low on refrigerant. They said it was due to loose flare fittings, which they tightened, performed a sniffer test on the 4' of refrigerant line that you can access in the crawl space, said it didn't identify any leaks, refilled the refrigerant, and went on his way. This did nothing to resolve our humidity issues, but the A/C would cool correctly for 2-3 days (and then the same issues started again, making me think there's a refrigerant leak somewhere else).

    I called the hvac company again, who said before they come out again they want us to pay for a duct leakage test, to see if there's air infiltrating somewhere (the weather here has been 85+, very humid for the past several weeks).

    So I wanted to ask the the experts here:
    1. Assuming I did all the wiring correctly, is there any issue with using an Ecobee with my Mitsubishi system?
    2. Is there a way to adjust for the humidity issue? I posted this on reddit, and people recommended adjusting the temperature adjustment on the handler unit itself vs. the Ecobee (need to figure out how to do this).
    3. Should we dish out for a leakage test?
    4. Is there actually a refrigerant issue, or is this just issues with this style of ducted minisplit system?
    5. Should we dish out for the duct leakage test? Does anyone have any guesses on what our next steps should be beyond the leakage test?

  2. #2
    Join Date
    Aug 2002
    Southold, NY
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    If im reading that right 30K BTU/H for 1000 Sq Ft.

    Seriously oversized

  3. #3
    Join Date
    Oct 2009
    God's country - Shenandoah Valley, VA
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    Did the tech remove the refrigerant that was in the system and weigh the correct amount back in? That's the only way to correctly charge an inverter mini split. You can't get it right by just topping it off.

    Where in VA are you located?
    Bob Boan

    ​You can choose to do what you want, but you cannot choose the consequences.

  4. #4
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
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    Cooling any interior space below the outdoor dew point will cause condensation on ducts and exterior sheathing with outside dew points exposed to the drywall on exterior walls and attic dew points.

    In your area, keep interior temperatures +73^F. If indoor %RH is +55%RH, remove excess %RH by adjusting the a/c to remove max moisture y adjusting air flow to make the cooling coil 30^F colder than the return air flow temperature.
    The a/c is unable to remove moisture during evenings and rainy days. Removing moisture after the home is cooled requires supplemental dehumidification from whole house dehumidifier like an Ultra-Aire.

    Keep us posted.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
    Join Date
    Feb 2010
    Metro Atlanta, GA
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    I'm gonna have to agree with oversized for the space and needing a dehumidifier. The Mitsubishi will run mostly fine using the thermostat interface, but you won't get an indoor temperature lower than 66-67° because the unit won't allow it. Once the return air sensor hits 67° the unit will shut down regardless of whether the thermostat is calling or not.

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