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  1. #1
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    Mitsubishi Mr. Slim and dehumidifying

    We live in Portland, Oregon. In the winter, outside humidity is typically 90%. I live in a three story house, less than 10 years old, reasonably well insulated and fairly well sealed. Heating is primarily provided by a forced air system. It's generally warm enough that the furnace doesn't run much, so it is not circulating air all the time.

    The first floor is the entry, garage, bedroom, bathroom, no issues. The second floor is kitchen, living area, back door, thermostat for the forced air system. Humidity stays in the 25% - 35% range most of the winter. No issues.

    The third floor is two bedrooms, laundry, bath, fans in the usual locations. The room I use for my office stays in the 40-45% humidity range. The bedroom we sleep in has humidity that is at or above 50%, and we're concerned about dust mites, mold, and mildew. I've seen some black mold on the window sills.


    We have Mr. Slim units in the bedrooms and living area. I tried using the Mr. Slim in the bedroom in Dry mode, I ended up lowering the temperature about 5 degrees (expected) and raising the humidity almost 10% (unexpected). Cold and clammy was definitely not the goal. I've also used the Mr. Slim to heat the bedroom, which works well but doesn't impact the humidity much (at least it doesn't raise the humidity).


    I'm willing to experiment with this as much as needed, and can use some guidance as to what to try next. I've read about opening windows to introduce cold air, not sure if that is useful in a damp climate like ours, a brief experiment indicates that it is not. Not sure if the Mr. Slim can do anything for me or not. I'd like to avoid purchasing a standalone dehumidifier if possible.


    Thoughts?


    And thanks for a truly useful forum!

  2. #2
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    Thread Starter
    I've done more research, it's looking like the Mr. Slim Dry mode really is not designed for dehumidifying in winter, and maybe not even that much in summer. Looks like I'll need external help (small room unit or large DampRid).

  3. #3
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    It is about the dew point of the outside air not the %RH verses the indoor dew point when judging the benefit of fresh air.
    When your indoor moisture level is high and the outdoor dew point is below 50^F, use fresh outside air to purge indoor moisture.
    This also an indication that you are not getting enough fresh air change in your home. You need an fresh air change in 4-5 hours when the home is occupied minimum to purge indoor pollutants and renew oxygen.
    Using dry outside air is cheaper than trying to dehumidifier the moisture out. Most dehumidifiers are unable to effectively reduce indoor %RH low enough to keep windows from sweating.
    More fresh air passing through the is critical to indoor air quality during all seasons. When the outdoor dew point is +55^F, you will need a/c and dehumidifier during low/no cooling loads.
    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"

  4. #4
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    Thread Starter
    Thanks for the response T Bear. We're farther into the rainy season, now we're getting warm and raining. External humidity 75-95%, dew point 50-55 degrees. We don't have dry outside air even in the summer, certainly not this time of year. My living area is up to 45-50% humidity, and my bedroom a swampy 65%.

    In an attempt to get more outside air in, I reenabled our Lifebreath heat recovery ventilator on our central FA heating. It definitely freshens the indoor air, though now that it is warm and raining the central heating system does not run very often.


    Would I be better off using AC or Dry mode to dry out my bedroom? Dry mode just seems to make it cold and clammy, and the RH doesn't drop very much.


    I can see a dehumidifier in my future ;-)



    Again, thanks very much for the response!

  5. #5
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    Last post: as a final experiment, I closed up my office and ran the Mr. Slim in AC mode. Similar to Dry Mode, I got cold clammy air and raised the humidity. I've no idea theoretically why that happens, but it clearly does.

    I'm at a point where I can answer my question: no, the Mr. Slim does not function as a dehumidifier in my situation. I've purchased a dehumidifier, and was able to change the humidity in my bedroom from 65% to 49% in a few hours.

  6. #6
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    Quote Originally Posted by Salmineo View Post
    Last post: as a final experiment, I closed up my office and ran the Mr. Slim in AC mode. Similar to Dry Mode, I got cold clammy air and raised the humidity. I've no idea theoretically why that happens, but it clearly does.

    I'm at a point where I can answer my question: no, the Mr. Slim does not function as a dehumidifier in my situation. I've purchased a dehumidifier, and was able to change the humidity in my bedroom from 65% to 49% in a few hours.
    The a/c industry will be surprised to here about this.
    On the other hand, expect the Mr Slim to maintain <50%RH on hot day after an hour of steady cooling load. If not get a good tech to set the unit correctly. With a correct charge of refrigerant, proper air flow, and continuous sensible cooling, expect about 3 lbs. of dehumidification per ton of continous sensible cooling.
    Instead over-cooling, get a good dehumidifier like the Ultra-Aire/Santa Fe to operate along with any a/c. You get the temp/%RH with minimal operating cost.
    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"

  7. #7
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    Quote Originally Posted by teddy bear View Post
    The a/c industry will be surprised to here about this.
    On the other hand, expect the Mr Slim to maintain <50%RH on hot day after an hour of steady cooling load. If not get a good tech to set the unit correctly. With a correct charge of refrigerant, proper air flow, and continuous sensible cooling, expect about 3 lbs. of dehumidification per ton of continous sensible cooling.
    Instead over-cooling, get a good dehumidifier like the Ultra-Aire/Santa Fe to operate along with any a/c. You get the temp/%RH with minimal operating cost.
    Regards Teddy Bear.
    Yeah, a 15,000 BTU Mr. Slim doesn't dehumidify very well in a 6,000 BTU bedroom.
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  8. #8
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    Your issue is that your trying to do something that mini split can't do. No mini split is made for drying out the air in the winter. De humidification is a function of cooling. The only system I know of that can dehumidify in heating is the Daikin quaternity and the Mitsubishi single wall head FH system.
    Part of the reason the temperature is lowering and the humidity is going up is because humidity is RELATIVE. Meaning that at 100% humidity thats the maximum amount of moisture the air can hold... at that temperature. Moisture can be measured in grains. So lets say its 45 degrees here in Portland and 90% humidity, well at 70 degrees that same amount of moisture in the air may only be 45% humidity( i wish i had a chart on this). If your house has balance the moisture between inside and outside(even though outside is 90% and inside is 50% its the same amount of water in the air). So if you lower the temperature in the room for 70 to say 65 and RELATIVE humidity was at 50% at 70 degrees it may go to 53% at 65 degrees because no moisture was removed, but the temperature dropped and in relative terms that amount of moisture in the air is now 53% of the capacity that can be held.
    You need a dehumidifier if you are having an issue. I rarely see a need for a dehumidifier in portland so you could probably pick up a portable one and remove the water pan daily as a whole home solution will be difficult without ducting and being in a 3 story home.
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