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  1. #1
    Join Date
    Dec 2011
    Location
    TN
    Posts
    40

    Oversized and extreme high humidity

    I have a 15 ton J15P heatpump. This is a new jail engineered building. We are having a major humidity problems the unit runs only minutes until satisfied. This has a tandem compressor design. I believe the unit is oversized but getting some one to admit that is not going to happen!!! We are tring everything to help the customer. Do y'all have any suggestion. I though about unhooking stage 2, but I am worried about oil return because of velocity the lines are sized for a 15 ton. Thought about cutting hole in duct and dump above ceiling to see if helps do y'all have any recommendations thank you

  2. #2
    Join Date
    Sep 2011
    Posts
    85
    Lower the fan speed? I hope that wasnt insulting...

  3. #3
    Join Date
    Apr 2012
    Location
    Paper Street Soap Company
    Posts
    2,304
    You're going to either have to add load or remove capacity. It's sounds way oversized.

    You could add duct heaters for reheat to increase space temp also.



    There is a guy that reps a company that sells dehumidifiers here. He's pretty sharp too.

    Maybe he'll chime in.

  4. #4
    Join Date
    Sep 2011
    Posts
    85
    Can you add a bypass to the conditioned space and connect the supply to the return?

    Would that de-humidify "twice"?

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,451
    Quote Originally Posted by Jeremy325 View Post
    I have a 15 ton J15P heatpump. This is a new jail engineered building. We are having a major humidity problems the unit runs only minutes until satisfied. This has a tandem compressor design. I believe the unit is oversized but getting some one to admit that is not going to happen!!! We are tring everything to help the customer. Do y'all have any suggestion. I though about unhooking stage 2, but I am worried about oil return because of velocity the lines are sized for a 15 ton. Thought about cutting hole in duct and dump above ceiling to see if helps do y'all have any recommendations thank you
    First, you need a basic setup that involves balancing the air flow to max the amount of dehumidification that the a/c will do while maintaining the desire temp. Usually the coil air flow is too high and the coil temp is not low enough to provide the desire inside dew point. Assuming you want 75^F, 50%RH, (55^F dew point) your a/c coil temp should be <45^F. Also minimize your make-up air that is used. There are code minimums that you must supply. Supplemental dehumidification will be required during the low/no cooling load conditions. Also most of these types of facilities require fan "on" mode with required fresh air ventilation. If this is two speed a/c, both speeds must be balanced to maintain low coil temperatures.
    After this setup is complete, calculating the amount of supplemental dehumidification is determined by the number of occupants, the amount of fresh air ventilation, and the desired indoor temp/%RH. 50-60%RH should be do-able. Avoid reheat because extreme high operating cost. The best dehumidifiers like the Ultra-Aire remove 6-7 lbs. of moisture per kw verses reheat which is <1 lb. per KW.
    Keep us posted as these are learning experiences.
    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"

  6. #6
    Join Date
    Dec 2011
    Location
    TN
    Posts
    40
    Quote Originally Posted by wdshea View Post
    Lower the fan speed? I hope that wasnt insulting...
    Done that no change

  7. #7
    Join Date
    Dec 2003
    Posts
    2,412
    Keep the fan on low and lower the stat.

    My home unit is oversized and cuts on/off a lot, but its not humid in my home.

    IDK what the problem is, but prolly not an oversized unit. Maybe your jail is in a negative pressure? Thats what causes most humidity problems.

  8. #8
    Join Date
    Apr 2012
    Location
    Paper Street Soap Company
    Posts
    2,304
    Quote Originally Posted by teddy bear View Post
    First, you need a basic setup that involves balancing the air flow to max the amount of dehumidification that the a/c will do while maintaining the desire temp. Usually the coil air flow is too high and the coil temp is not low enough to provide the desire inside dew point. Assuming you want 75^F, 50%RH, (55^F dew point) your a/c coil temp should be <45^F. Also minimize your make-up air that is used. There are code minimums that you must supply. Supplemental dehumidification will be required during the low/no cooling load conditions. Also most of these types of facilities require fan "on" mode with required fresh air ventilation. If this is two speed a/c, both speeds must be balanced to maintain low coil temperatures.
    After this setup is complete, calculating the amount of supplemental dehumidification is determined by the number of occupants, the amount of fresh air ventilation, and the desired indoor temp/%RH. 50-60%RH should be do-able. Avoid reheat because extreme high operating cost. The best dehumidifiers like the Ultra-Aire remove 6-7 lbs. of moisture per kw verses reheat which is <1 lb. per KW.
    Keep us posted as these are learning experiences.
    Regards TB
    Avoid reheat because Ultra-Aire is the only solution.....

    Ive got a surgical center with electric reheat in every OR room and a community building that runs a boiler for reheat.

    IT IS an effective solution albiet expensive for controling humidity when a unit is oversized AND its been around for decades.

    If its as oversized as he says it is and he's tried all of the tricks then the solution is most likely going to be expensive.

  9. #9
    Join Date
    Dec 2003
    Posts
    2,412
    Quote Originally Posted by Six View Post
    Avoid reheat because Ultra-Aire is the only solution.....

    Ive got a surgical center with electric reheat in every OR room and a community building that runs a boiler for reheat.

    IT IS an effective solution albiet expensive for controling humidity when a unit is oversized AND its been around for decades.

    If its as oversized as he says it is and he's tried all of the tricks then the solution is most likely going to be expensive.
    Is there an OR anywhere without reheat? They need massive amounts of air to meet requirements. No way to move this much air and not have reheat to control humidity.

  10. #10
    Join Date
    Apr 2012
    Location
    Paper Street Soap Company
    Posts
    2,304
    I suspect there isn't but that wasn't the point of my post.

    My point was that reheat is an option albeit expensive option for controlling humidity.

  11. #11
    Join Date
    Feb 2006
    Location
    Atlanta,GA.
    Posts
    893
    where is the stat located and is it a two stage stat? if it is oversized if you only run the first stage it should be ok. if if turns on then back off in a few minutes with only 1 stage that would mean that you are double what you need, unless somebody screwed up big time

  12. #12
    Join Date
    Dec 2003
    Posts
    2,412
    Quote Originally Posted by Six View Post
    I suspect there isn't but that wasn't the point of my post.

    My point was that reheat is an option albeit expensive option for controlling humidity.
    Gotcha.

    My point was that, imho, oversized units are not the cause of humidity problems. If they were, Id be soaking wet right now.


  13. #13
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,451
    Without the correct setup of the air flow verses the coil temperature to assure dry air when system is operating, reheat or dehumidification is useless. Get to the 45^ coil temp, or a 30^F split, first before doing any dehumidification or reheat. Low coil temperatures also slow the cooling process which decreases sensible cooling capacity. This increases the run time. During high cooling loads days, most systems will handle the fresh make-up latent load.
    Be my guest on the reheat verses supplemental dehumidication. Reheat is an energy pig, cosuming 5-10X more energy per lb. of moisture removed. During part load latent conditions, upto 2 lbs of moisture per ton of capacity most accumulate on the cooling coil to get moisture down the drain. Then at the end of the cooling cycle, the moisture on the coil/pan re-evaporates back into the space which makes it damp again. Ultra-Aires remove 8 lbs. of moisture per kw plus supply 11,000 btus per kw of free reheat to extend the a/c run. In some settings, the payback is less than a couple years.
    Anyway, get the cold coil first.
    If you are having trouble during high cooling loads, consider 70^F outdoor with rain. No cooling loads and high latent loads requires dehumidification in all settings.
    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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