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  1. #1
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    May 2019
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    Humidity increases when HVAC is run

    Hi all. Its odd, but when my AC system (one of 3 units) runs, the humidity increases. Temp comes down fine, but its the humidity increases that puzzle me. Any suggestions? Thanks in advance!

  2. #2
    Join Date
    Aug 2002
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    Southold, NY
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    How are you measuring it?

  3. #3
    Join Date
    Aug 2013
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    Visalia California 93291
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    A boat load of water sitting in the pan

  4. #4
    Join Date
    May 2019
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    How are you measuring it?
    Measured from a TH8321R1001 thermostat, as well as an AcuRite 01083 Indoor Thermometer & Hygrometer with Humidity Gauge. They are within 2% of each other in terms of humidity measurement, so I assume its accurate?

  5. #5
    Join Date
    May 2019
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    Thread Starter
    [QUOTE=lkapigian;25717359]A boat load of water sitting in the pan[/QUOTE

    No water in the pan. Is there another place I should be looking?

  6. #6
    Join Date
    Aug 2002
    Location
    Southold, NY
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    If your measuring at supply grills that would explain it!

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    How about some real numbers? Temp/%RH in the space and at the supply?
    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"

  8. #8
    Join Date
    Aug 2007
    Location
    Rochester, NY
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    Sounds like the system is oversized and is dropping the temp without running long enough to dehumidify. As the temp drops the RH will go up.


    Sent from my iPad using Tapatalk

  9. #9
    Join Date
    Mar 2018
    Location
    Chico, Ca #StateofJefferson
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    Are you saying that your uncomfortable, or that's just what the instruments say?

  10. #10
    Join Date
    Sep 2002
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    Sure - the cooling coil temperature is not far enough below the dew point of the air. It decreases the the air temperature but the moisture volume in the air stays the same. So the Humidity - Relative to the temperature - increases.

    Or; that's my first guess anyway. <g>

    PHM
    ------------



    Quote Originally Posted by vantage78 View Post
    Hi all. Its odd, but when my AC system (one of 3 units) runs, the humidity increases. Temp comes down fine, but its the humidity increases that puzzle me. Any suggestions? Thanks in advance!
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  11. #11
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    Rare to have an installer measure the static pressure of the duct system and set the blower accordingly. They just leave it on high for cooling where the factory puts it. So we see 5 ton drives (2000 CFM) on a 3 ton A/C (which needs no more than 1200 CFM) and a humid house.

    No idea why it increases though. Unless it is moving way too much air. Or big duct leakage bringing in humid outside air.

  12. #12
    Join Date
    Sep 2002
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    The moisture content stays the same, the air temperature decreases, so the Humidity Relative To The Air Temperature increases.

    Could that be it?

    PHM
    --------


    Quote Originally Posted by BaldLoonie View Post
    Rare to have an installer measure the static pressure of the duct system and set the blower accordingly. They just leave it on high for cooling where the factory puts it. So we see 5 ton drives (2000 CFM) on a 3 ton A/C (which needs no more than 1200 CFM) and a humid house.

    No idea why it increases though. Unless it is moving way too much air. Or big duct leakage bringing in humid outside air.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  13. #13
    Join Date
    Jan 2012
    Location
    Arrowhead Stadium
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    Have your airflow tested and adjusted for less sensible heat removal and more latent heat removal.

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