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I have a new Mitsubushi 18,000 BTU Mr. Slim ceiling mounted (SEER 13.2) Comfort System. It is for one room only sized 20' by 20' by 12 ' high. The room is on the second floor, has no windows and is insulated on all sides. Like other posts I have seen, the Mr. Slim generally cools the room (even at 100 degree outside days in Texas) to 75 degrees or lower but the humidity immediately goes up within the first 1 to 2 hours to 60 to 70% relative humidity. I have complained and Mitsubishi authorized a new unit to be installed and it was installed. The new unit still is producing humidity at 60 to 70% humidity. Is this an inherent defect in this design? Any solutions? Why doesn't using a Trane conventional air conditioner unit result in this same high humidity problem? or could it?

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  • ccotropia Jul 30, 2009

    I appreciate the last posted advice (to reduce the fan speed and install a dehumidifier) but wonder if there is any way that the current Mitsubishi can or should be able to keept the humidity at 50% or less. I have run the unit on its lowest fan speed but still have the same issue. The room humidity is around 45% when the unit is first turned on and then it rises to 60-64%. I understand this will occur as the room cools down but I would hope a properly desiged A/C unit would keep the humidity in check. My other Trane units (SEER 19 and 6 ton units) don't create this same problem. The humidity stays at 48 - 50%. Is this just an inherent issue with this Mitsubishi unit that I will have to live with unless I install a dehumidifier?

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A/C units do not remove all humidity - that is the purpose of a de-humidifier. Instead they STABILIZE the humidity in the form of condensation when the hot refrigerant is misted into cool vapor. The humidity condensate collects on the coils... thereby stabilizing the humidity at a certain temperature.
There is a drainpipe, usually a white plastic pipe, coming out of your unit that should be dripping water this time of the year at a pretty good volume. This water is the moisture from your house. If it isn't able to discharge this water when your air conditioner is running, it will continually put it back into your home, increasing the humidity level.
Check to make sure the tube is clear and the unit is properly draining.

Thanks :D


Posted on Jul 30, 2009

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    blueextc3221 Aug 06, 2009

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Air conditioners are designed to remove heat from a room or cool it down and it will take some humidity out of the room but not all. Depending on the (SEER) Seasonal Energy Efficiency Ratio as to the amount a 12 and 13 SEER typically remove about 20 to 25% but that is not its purpose to remove humidity.
If you are only interested in removing humidity, then you should get a humidifier, but if you also want to cool the room then the air conditioner will work a little but not as much as what you may want which generally goes for all makes, though this one may be a little worse than similar makes. To help in dehumidifying you would want to lower your indoor air flow over the evaporator (lower the blower speed as low as possible by using the manufacturers minimum airflow recommendations). Since it is so high, I would also get the humidifier like I previously suggested above as that is meant to specifically remove humidity especially if it is so high. Let me know how this turns out.



Benjamin

Posted on Jul 27, 2009

  • Benjamin Patri
    Benjamin Patri Jul 30, 2009

    Yes it is just something that you will live with, this unit sounds like it is running correctly if it is cooling. They won't be made exactly the same as other brands and so a dehumidifier would be in your best interests to help keep the humidity down. Sorry for the bad news, though I would complain to Mitsubishi and see if they would pay for the dehumidifier or towards it since the unit isn't so great.

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