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  1. #1
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    Aug 2014
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    Oversized AC unit?

    I just purchased a Whynter ARC-110WD 11,000 BTU dual-hose portable air conditioner. It cools my home office very well, but I'm concerned about humidity.

    When I start work in the morning, it's about 76 degrees and 61% RH. As it gets hot outside, the AC unit starts running, and the humidity eventually rises to 68%. I maintain the room at 78 degrees or so. The unit is a type that blows the fan all the time, but starts and stops the condenser for efficiency, even before the temperature reaches the desired level.

    I live in Durham, NC. The temperatures outside get up to the mid-80s to mid-90s on an average summer day. The average RH outside has been between 70 and 85 during the past week while I've been operating the AC unit.

    The AC unit advertises being able to cool a room up to 350 square feet. My room is 130 square feet, the ceiling is 9 feet high, and the room gets direct sun in the afternoon. My laptop fan blows hot air pretty continually, and I'm the only one using the office. The ventilation in the room is awful. The ceiling vent doesn't blow out a whole lot of air, even when I keep the house AC fan on.

    I have a few questions:

    1. Is my AC unit too large for the room?
    2. If the unit is too large, what humidity levels can I expect if I bought a smaller unit?
    3. If the unit is too large, would decreasing the AC unit's fan speed help the RH levels in the room?
    4. Is it worth purchasing a separate dehumidifier to help bring the RH down?

    I'd like to keep the room at around 50% RH, if possible.

  2. #2
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    Thread Starter
    Two more things. First, I made a related post about the ventilation in the room. That's another concern of mine. I wonder if inadequate ventilation may be contributing to the temperature and humidity issues.

    Second, the relative humidity in the office was pretty high, even before I bought a portable AC unit.

  3. #3
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    Aug 2014
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    Thread Starter
    The more I think about this, I wonder if I'm solving the problem the wrong way. Shouldn't it be reasonable to expect that all the rooms in the house could be the same temperature, and have the same level of ventilation, if the ducts were adjusted correctly? Assuming there is nothing wrong with the ducts themselves, could I just adjust the dampers to achieve the right balance of ventilation and temperature? I keep the office doors shut all day, so maybe it would make it difficult to regulate the temperature and humidity.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    First things first. You a/c needs to be setup to maintain 75^F, 50%RH during high cooling loads. This requires the a/c to supply air that has been cooled down to 50^F to remove the excess moisture. Measure the in/out temp/%RH of your a/c after 30 min/60 min of cooling. We are looking for <50^F dew point from your a/c. Operate your a/c blower in the "auto" mode to minimize the re-evaporation from the a/c coil. Is there a crawlspace/basement involved in this home?
    Your a/c will supplemental dehumidification from a large dehumidifier during cool wet weather but should be able to handle the home during peak cooling loads. The setup is the key.
    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"

  5. #5
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    Thread Starter
    Thanks, TB! I measured the %RH after setting the thermostat to 75^F and running it for a while. It was between 55-57%, depending on which room I was in. I'm not sure how to measure the in/out temperature of the AC. Do I need to purchase a pocket thermometer for that? There is an HVAC contractor coming tomorrow to take care of some other issues, so I could ask him to take some temperature readings.

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by mstalker View Post
    Thanks, TB! I measured the %RH after setting the thermostat to 75^F and running it for a while. It was between 55-57%, depending on which room I was in. I'm not sure how to measure the in/out temperature of the AC. Do I need to purchase a pocket thermometer for that? There is an HVAC contractor coming tomorrow to take care of some other issues, so I could ask him to take some temperature readings.
    I assume that the 75^F, 55%RH is the room temp or the temp of the air going into the a/c. With the same meter, measure the air temp/%RH coming out of the closest to the cooling coil vent. We can tell how the coil is working from that.
    We looking for the info that will determine the cooling coil temp.
    Keep us posted.
    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"

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