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  1. #1
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,467

    Out door dew point determines inside dew point

    Unoccupied home in FL is wet (+55%RH) or dry depending depending on the outdoor dew point.
    The a/c is off and the home is unoccupied. Following quickly indicates an air change in 3-5 hours. Adding occupants raises the indoor humidity. Raising the indoor temperature lowers the indoor %RH. Significant cooling lowers %RH. Ultrimate control of the indoor %RH requires a good dehumidifier. Second graph shows same house occupied and dehumidified
    Regards TB

    Nov FL %RH dehu.pdf
    FL DEC 12 no ac occupants RH.pdf
    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. #2
    Join Date
    Dec 2012
    Location
    Daphne, AL
    Posts
    60
    One could also put in reheat coil or strips in duct to control humidity.

  3. #3
    Join Date
    Jun 2007
    Location
    Minneapolis, MN
    Posts
    735
    Quote Originally Posted by teddy bear View Post
    Unoccupied home in FL is wet (+55%RH) or dry depending depending on the outdoor dew point.
    The a/c is off and the home is unoccupied. Following quickly indicates an air change in 3-5 hours. Adding occupants raises the indoor humidity. Raising the indoor temperature lowers the indoor %RH. Significant cooling lowers %RH. Ultrimate control of the indoor %RH requires a good dehumidifier. Second graph shows same house occupied and dehumidified
    Regards TB

    Nov FL %RH dehu.pdf
    FL DEC 12 no ac occupants RH.pdf
    Thank you for sharing.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,467
    Quote Originally Posted by waregl82 View Post
    One could also put in reheat coil or strips in duct to control humidity.
    Of course. This is common in commercial systems. A 2-3 ton a/c coil collects 2-3 lbs. of moisture before dripping to the drain. Removing 2-3 lbs. of moisture lowers the humidity 10%RH in a typical home. The dehumidistat satisfies and a/c shuts off. The moisture on the a/c coil revaporates back into the space humidifying the space +10%RH. This triggers the reheat cycle all over again. This is an energy intinsive cycle that is illegal in most building codes. An effecient dehumidifier will maintain the desired %RH for a fraction of the cost.
    Thanks for the post.
    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
    Join Date
    Dec 2012
    Location
    Daphne, AL
    Posts
    60
    Thanks for the input teddy bear. Always learning. Was thinking commercial when I made the statement. I have never seen a dehumidifier in a commercial system, only reheat. I live in the south (Alabama). Do they use dehumidifiers in commercial buildings? Perhaps depending on the size. Just curious and learning.

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,467
    Quote Originally Posted by waregl82 View Post
    Thanks for the input teddy bear. Always learning. Was thinking commercial when I made the statement. I have never seen a dehumidifier in a commercial system, only reheat. I live in the south (Alabama). Do they use dehumidifiers in commercial buildings? Perhaps depending on the size. Just curious and learning.
    The larger models from Ultra-Aire handle a 7.5 ton a/c or about 5,000sqft. of conditioned space. They can also handle the make-up air ventilation needed meet most codes. They remove upto 8 lbs. of moisture per hour. The lbs. per KW are +7lbs. While a/c removes 2 lbs. per KW. ultra-aire.com
    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"

  7. #7
    Join Date
    Dec 2012
    Location
    Daphne, AL
    Posts
    60
    Sounds a heck of a lot more ecomomical than reheat!!!! I wonder why I haven't seen it in commercial. Could be a lot of the AHU's I've worked on are older technology? Maybe before dehumidifiers became more economical, or perhaps the large tonnage of the hydronic systems don't lend themselfves to the technology. Gonna check out their website and learn a little more.
    Thanks, waregl82

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Larger commercial dehumidification needs can opt for desiccant wheel technology vs. merely DX dehumidification. Especially appealing if waste heat can be used to reactivate the desiccant wheel.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
    Join Date
    Dec 2012
    Location
    Daphne, AL
    Posts
    60
    Now that I think about it, most have outside air units (Desert Aire for one). Everything control wise we do is moving toward "demand ventilation". The CO2 in the building is monitored, and the system calls for outside air only when necessary. All of the outside air units I've worked on have energy wheels.

  10. #10
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,714

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