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Thread: High humidity

  1. #1

    High humidity

    We have a new duct work system total of 4200cfm. One existing roof top unit has 2900cfm output. We step up the evaporator fan RPM to match 4200cfm. The room temperature set to 75-76 degree with no outside air. The inside room feel very sticky. We did some data of the existing unit. When the unit is on, Circuit 1 the head pressure-280 psi, low pressure 77psi. Circuit 2 the head pressure-270 psi, low pressure 80 psi. Return temperature is 68 F and supply temperature is 56 F. Why the unit can't property dehumidify the room?

  2. #2
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,215
    This is in a residence or commercial?

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,368
    With a 68^F return, you need a <45^F coil temp. to deliver 50%RH. Slow the air flow until the cooling coil is cold enough to remove the moisture to the desired level. Over cooling to control the indoor %RH has its problems. Certainly at 75^F, 50%RH requires a <55^F dew point. A 50^F cooling coil is needed minimum. Lowering %RH with low/no cooling loads requires supplemental dehumidification. There are units available for light commercial applications. Suggest checking on the large Ultra-Aire 200H dehus. They connect to the a/c ducts.
    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
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,806
    You have almost twice as much airflow as the unit is designed for? 2900cfm changed to 4200cfm?That would cause humidity

  5. #5
    Residence.

  6. #6
    Join Date
    Sep 2009
    Location
    Hampton, GA
    Posts
    537
    Quote Originally Posted by jtrammel View Post
    You have almost twice as much airflow as the unit is designed for? 2900cfm changed to 4200cfm?That would cause humidity

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,086
    What size unit? 2900 CFM sounds like about a 7.5 ton. Big house!

  8. #8
    Yes, 7.5ton. Is a penthouse apartment.

  9. #9
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,806
    Airflow is way to high. Moisture laden air does not have enough time to hit cold coil to release moisture

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