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  1. #14
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
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    Quote Originally Posted by Shophound View Post
    Yep. 70F dry bulb @ 43%RH yields 47 grains per pound and a 47F dew point. 54F dry bulb @ 83% RH is 52 gr/lb and a 49F dew point. Supply dew point must be lower than return dew point for system to actually dehumidify the air. Same for grains.

    And my students wonder why I beat them up with psychrometrics?

    Readings are best taken at supply and return plenum at air handler, not a considerable distance from same.

    I also would not trust that return ductwork beneath the foam to not be leaking, especially if foam is open cell. Or there are voids in the foam, which are common on most foam jobs if not caught and patched. Or the underside of the ducts never saw any foam sealing at all, and are pulling attic air in from a yet to be discovered path. At any rate, measuring at the return air plenum and at the return air intake in the house, and then comparing the two readings to see if any significant change in dew point occurs, would indicate if there's noteworthy return air duct leakage to affect dehumidification performance.
    You make a good point about measuring the dew point of air at the return of the a/c. Also consider that if the dew point of the air at the a/c return, then outside air is leaking into the return between and the house pickup.
    Good to here from you.
    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"

  2. #15
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,844
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    Quote Originally Posted by teddy bear View Post
    You make a good point about measuring the dew point of air at the return of the a/c. Also consider that if the dew point of the air at the a/c return, then outside air is leaking into the return between and the house pickup.
    Good to here from you.
    Regards Teddy Bear
    Thanks, Teddy. Wish I could make Summer Camp this year so I might see you there, but accepted a new role with my employer and will be very busy. New research building currently under construction with chilled beams, total energy transfer wheels, cold rooms, labs, etc. And I'm now the manager of all of it.
    Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.

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