Question about Mitsubishi Mr. Slim MXZ30TN Air Conditioner

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We purchased 3 Mr. Slim units for an open 1150sq ft. subterranean level classroom with one side walkway. one unit is attached to one condenser and two share another. All three units are set to the de-humidyfy mode. We set the ac to 78. The fan is on the 2nd setting. Problem: high humidity in the space. Difficulty maintaining comfortable temp levels. We just cleaned all the vents. We also have a fresh air exchange unit. What can we do to maintain consistent temp and humidity levels?

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  • jmcinnis171 Oct 02, 2010

    We actually have 3 Mitsubishi Mr. slim split systems. The air exchange system is a Renew Air which was running 24/7 but now we are turning it off at night. The air system has a 6 to 8" diameter flexible duct with the fresh air intake underneath a large deck, pulling in air from deep in the corner (I think this is not a good placement). The room has real humidity problems. Even with all this equipment it is close to 70% humidity. The room had been cleaned prior via a environmental firm that removed mold and all drywall up to 5 feet, and floors. There is real concern that we will have more problems with mold and fungi if we don't get the humidity down to 44-55%.

  • jmcinnis171 Oct 02, 2010

    It is an ERV.



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First make sure a proper manual j is done on the space, then make sure your not bringing in fresh air while dehumidifying. Your either grossly over sized or slight under, you'll know if its over it will feel clammy, you can have mold issues and the equipment will most likely short cycle. If its under sized it will stubble to meet tempetures

Posted on Oct 01, 2010

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  • jmcinnis171 Oct 01, 2010

    Thank you. What is a manual j?

  • Zachary Oct 02, 2010

    a "manual j"is a load calculation. (all apologies for the industry slang). i have spread sheet i can scan in and send you but i would suggest call in a Free second opinion call with a company that employes Nate certified techs and ask them to run a load. Some companies will do this for free, some companies will charge a flat rate.

    Ideal humidity is between 45-55%Rh.

    each of your units is rated for 28400 BTU/hr meaning you have about 7.1 tons of cooling with 3 condensers but you said you have two, so you have about 4.7 tons of cooling capacity in your space.

    in regards to your fresh air system, were is this space located (the south, midwest, etc.) and is it a ERV or an HRV?
    Also is this space sealed, and what material is its construction?

  • Zachary Oct 07, 2010

    if you had a split system i would say to use a reheat coil such as a Lennox humiditrol (only certain systems can use these) , in your case you might want to invest in a stand alone commercial dehumidification unit. You should dehumidify without having to over cool your space. Mini splits can dehumidify but not the same as true dehumidification units or a forced air split systems with reheat.



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