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  1. #1

    Humidity and mini splits

    We recently converted part of the garage into a bedroom w/bath with a total area of 355 square feet. They installed a Mr. Slim Mitsubishi mini split with a rated capacity of 12000 btu/h and a minimum capacity of 6000 btu/h. SEER is 13.5 btu/h. When it is very warm outside and the unit is running it is always very humid inside, even when the "dry" mode is on. The humidity remains between 55 and 65 percent. Is it because the unit is too big? The a/c guys said they don't make a smaller unit so they installed this. Actually, they need to replace it anyways because they inadvertently installed a unit without a heater. Should I ask for a different type/size unit? Any other ideas?

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  3. #3
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    Quote Originally Posted by Michele333 View Post
    We recently converted part of the garage into a bedroom w/bath with a total area of 355 square feet. They installed a Mr. Slim Mitsubishi mini split with a rated capacity of 12000 btu/h and a minimum capacity of 6000 btu/h. SEER is 13.5 btu/h. When it is very warm outside and the unit is running it is always very humid inside, even when the "dry" mode is on. The humidity remains between 55 and 65 percent. Is it because the unit is too big? The a/c guys said they don't make a smaller unit so they installed this. Actually, they need to replace it anyways because they inadvertently installed a unit without a heater. Should I ask for a different type/size unit? Any other ideas?
    Slow the air flow as much as possible. Get a small dehumidifier for <50%RH when low/no cooling load. If a/c operates for several hours continuously during hot weather, smaller unit not the answer. The Ultra-Aire SD 12 removes more 2X moisture with less sensible cooling. No heat through.
    Regards TB
    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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    Quote Originally Posted by Michele333 View Post
    We recently converted part of the garage into a bedroom w/bath with a total area of 355 square feet. They installed a Mr. Slim Mitsubishi mini split with a rated capacity of 12000 btu/h and a minimum capacity of 6000 btu/h. SEER is 13.5 btu/h. When it is very warm outside and the unit is running it is always very humid inside, even when the "dry" mode is on. The humidity remains between 55 and 65 percent. Is it because the unit is too big? The a/c guys said they don't make a smaller unit so they installed this. Actually, they need to replace it anyways because they inadvertently installed a unit without a heater. Should I ask for a different type/size unit? Any other ideas?
    Dry mode is actually to keep the evaporator dry, which helps keep crap from growing on it, but in the process puts that humidity into the structure.

  5. #5
    thanks for the input. TB, regarding that dehumidifier, would it be a whole house operation (it says it does up to 4,000 sq feet) or could I just put it in that one room. I wouldn't mind having it for the whole house, but can it work with two different a/c systems? Ar_hvac_ man...that makes sense, since it never dehumidified the room. I need to tell my a/c guy since I don't think they have installed many of these.

  6. #6
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    If it gets humid inside when it is humid outside, you probably don't have an equipment problem, you have a house problem. You likely have some air leakage that needs sealed.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  7. #7
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    Quote Originally Posted by Michele333 View Post
    thanks for the input. TB, regarding that dehumidifier, would it be a whole house operation (it says it does up to 4,000 sq feet) or could I just put it in that one room. I wouldn't mind having it for the whole house, but can it work with two different a/c systems? Ar_hvac_ man...that makes sense, since it never dehumidified the room. I need to tell my a/c guy since I don't think they have installed many of these.
    Choices, you could add a small 30-40 pint dehumidifier to the room to maintain <50%RH in that room.
    Or you could a small whole house dehumidifier, Utra-Aire 70H of 90H, to the serve the whole house dehumidifier connected to air handler serving the home with a small duct to the single room that is being discussed. Sizing has to do with location of the home, size of the home, and number occupants.
    The UA SD 12 is a one ton a/c only that also can be split between this space and the rest of the home. It has an outside condenser.
    Using the 70H-90H is going to be more cost effective.
    Regards TB
    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"

  8. #8
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    Quote Originally Posted by tipsrfine View Post
    If it gets humid inside when it is humid outside, you probably don't have an equipment problem, you have a house problem. You likely have some air leakage that needs sealed.
    Tell us more about a/c keeping space <50%RH while the outdoor dew point is +60^F and occupants in place. All spaces need minimal fresh air infiltration of an air change in 4-5 hours and occupants add .5lbs per hour typically. This reqires 1-2 lbs. of dehumidification when the a/c is not cooling?
    Dehumidification or live with 60-70%RH?
    Regards TB
    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"

  9. #9
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    One drawback to minis I've seen is the blower runs constantly. That puts moisture back into the air during the off cycle.

  10. #10
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    Quote Originally Posted by teddy bear View Post
    Tell us more about a/c keeping space <50%RH while the outdoor dew point is +60^F and occupants in place. All spaces need minimal fresh air infiltration of an air change in 4-5 hours and occupants add .5lbs per hour typically. This reqires 1-2 lbs. of dehumidification when the a/c is not cooling?
    Dehumidification or live with 60-70%RH?
    Regards TB
    No need to crack wise on me TB. My advice to the OP is not at odds with your advice. The OP mentioned a direct correlation between indoor humidity going up when outdoor humidity goes up, so I thought it was a good idea to mention the importance of air sealing a home. Or, can "Do tell us..." why my advice is bad advice.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  11. #11
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    Quote Originally Posted by tipsrfine View Post
    No need to crack wise on me TB. My advice to the OP is not at odds with your advice. The OP mentioned a direct correlation between indoor humidity going up when outdoor humidity goes up, so I thought it was a good idea to mention the importance of air sealing a home. Or, can "Do tell us..." why my advice is bad advice.
    Say it like this. When your a/c is operating continuously with a 48^F cooling coil temperature and your indoor %RH exceeds 50%RH, check for excess air infiltration. Another angle. Two occupants are in an air tight home. The outdoor dew point is 55^F. The home is 80%RH. Is the home leaking too much or not enough? I am trying to help people understand the moisture problems.
    Regards TB
    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"

  12. #12
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    I'm well aware that you are trying to help people understand about moisture problems, but you don't have to make a snarky comment my way in order to do it. The OP has made an addition to a home by annexing part of a garage, and then posed a question as to why they have high humidity in that room while the minisplit is running. Advising someone in this situation to go straight to installing a dehumidifier without addressing possible air infiltration issues is not the best advice, especially when they have pointed out a direct correlation between a rise in outside humidity and a rise in indoor humidity. First addressing a homes pressure & thermal boundaries is always the best first advice; then equipment.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  13. #13
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    Quote Originally Posted by tipsrfine View Post
    First addressing a homes pressure & thermal boundaries is always the best first advice; then equipment.

    It's a good point. Even TB has admitted that outside of low sensible load conditions, a properly sized AC can deumidify adequately. For example today it's 78F, 69F Dewpoint where I am. That's jsut enough sensible that I should maintain descent humidity levels in my home if I don't have excessive air leaks.

    TB does have a point also, that as you tighten a home, you liekly no longer have adequate ventilation, and adding that ventilation would add enough moisture to a home to need dehumidification. His charts are impressive. I don't know what the total power comes out to, but proper equipment sizing becomes less of an issue with a WHD. But it definitely not a cheap option.

    I know I'd really, really like to see a lower efficiency ductable dehumidifier in that's abotu 1/3 the price of the Ultra-aires. I guesss you could tear apart a cheap-o unit from Target/Walmart and use a small inline fan and make it ducted. Hmmmm.

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