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  1. #1

    Existing home with 3 HVAC zones and looking to install a whole house dehumidifier

    Learned a lot over the last couple of days reading this site, thanks to all! I did a bunch off searching but didnt find anything that applied to to my situation /set up and the question I have.

    We are in Savannah Georgia and recently moved into a 3200 sf single story home on the water surrounded by even more water. The house has HVAC 3 zones; 1. master br and bath, 2. main living area with kitchen, and 3. two additional bed rooms and a over the garage bonus room. Slab contruction and all are at ground level. We have 3 new HVAC systems of the same size. The problem we have is that the humidity is running very high, reaching 70% at times and the humidity differential from inside to outside averages only about 10%. I believe we need to install a whole house dehumidifier.

    +Can a whole house dehumidifier be installed on one of the 3 systems and have an effect of lowering the humidity throughout the house? If so, my wife’s major concern is with the MB and bath at one end of the house. Or would it be better to put it in the middle system?

    +Or - Would it be better to install ductwork between the systems so that a single whole house dehumidifier could service all areas? If so, would ducting between 2 systems be sufficient or would we have to incorporate all 3.

    I understand that humidity flows from one area to another and will eventually equalize but for a single unit on one system to work the humidity would have to equalize better that temperature does.

    I am trying to avoid the increased cost of three separate dehumidifiers.

    Thank you in advance for your time.

  2. #2
    Join Date
    Jun 2002
    Location
    Dacula, GA
    Posts
    12,617
    Quote Originally Posted by rackeu View Post
    Learned a lot over the last couple of days reading this site, thanks to all! I did a bunch off searching but didnt find anything that applied to to my situation /set up and the question I have.

    We are in Savannah Georgia and recently moved into a 3200 sf single story home on the water surrounded by even more water. The house has HVAC 3 zones; 1. master br and bath, 2. main living area with kitchen, and 3. two additional bed rooms and a over the garage bonus room. Slab contruction and all are at ground level. We have 3 new HVAC systems of the same size. The problem we have is that the humidity is running very high, reaching 70% at times and the humidity differential from inside to outside averages only about 10%. I believe we need to install a whole house dehumidifier.

    +Can a whole house dehumidifier be installed on one of the 3 systems and have an effect of lowering the humidity throughout the house? If so, my wife’s major concern is with the MB and bath at one end of the house. Or would it be better to put it in the middle system?

    +Or - Would it be better to install ductwork between the systems so that a single whole house dehumidifier could service all areas? If so, would ducting between 2 systems be sufficient or would we have to incorporate all 3.

    I understand that humidity flows from one area to another and will eventually equalize but for a single unit on one system to work the humidity would have to equalize better that temperature does.

    I am trying to avoid the increased cost of three separate dehumidifiers.

    Thank you in advance for your time.
    There is no comparison between temperature and moisture equalizing. The amount of grams of moisture in the air will equalize through out the whole house. The humidity level will depend upon the temperature. That is why you have to super seal a room where you want to maintain a curtain humidity. One dehumidifier would work in the basement. Thank you, thank you very much
    "I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
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  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,449
    Quote Originally Posted by rackeu View Post
    We are in Savannah Georgia and recently moved into a 3200 sf single story home on the water surrounded by even more water. The house has HVAC 3 zones; 1. master br and bath, 2. main living area with kitchen, and 3. two additional bed rooms and a over the garage bonus room. Slab contruction and all are at ground level. We have 3 new HVAC systems of the same size. The problem we have is that the humidity is running very high, reaching 70% at times and the humidity differential from inside to outside averages only about 10%. I believe we need to install a whole house dehumidifier.

    +Can a whole house dehumidifier be installed on one of the 3 systems and have an effect of lowering the humidity throughout the house? If so, my wife’s major concern is with the MB and bath at one end of the house. Or would it be better to put it in the middle system?

    +Or - Would it be better to install ductwork between the systems so that a single whole house dehumidifier could service all areas? If so, would ducting between 2 systems be sufficient or would we have to incorporate all 3.

    I understand that humidity flows from one area to another and will eventually equalize but for a single unit on one system to work the humidity would have to equalize better that temperature does.

    I am trying to avoid the increased cost of three separate dehumidifiers.

    Thank you in advance for your time.
    If or when the inside doors are all open, yes, humidity equalizes.
    But, there are other considerations. A dehu produces warm dry air that can be annoying if droped in a single sensitive area like a bedroom or near a thermostat.
    There are a couple basic issues to consider. Your a/c should be able to maintain your desire humidity levels during peak cooling loads. Typically, 50%RH inside during peak cooling loads does not require a dehu. If not your a/c is not setup right or you are leaking excess outside air into the home. Using a dehumidifier to fix an inadequate a/c setup or excess air leakage problem will use excess energy and require a larger dehumidifier.
    On the other hand, a home getting required fresh air ventilation, an air change in 4 hours, plus the moisture from occupants, will have a higher dew point than as outside air. This about outdoor dew point, not outdoor %RH. Thus whenever the outdoor dew point is above 50^F, the home needs dehumidification. The a/c will dehumidify when running enough to remove 2-6 lbs. moisture per hour. Otherwise you need a 90-100 pint per day dehumidifier.
    Next there is a big difference in dehus. Capacity and efficiency are important. You need about 100 pint per day to handle upto 100 cfm of fresh infiltration/ventilation and upto 6 occupants.
    Consider the Ultra-Aire, co-sponsor of this site. The UA XT105H is the highest efficiency, 8 pints per kwh and removes upto 105 pints per day. Dehus in your climate operate many hours per year while maintaining <50%RH. A premium dehu is good investment by allowing higher temps with more comfo.
    Glad to answer anyother questions. Nothing like a <50%RH home with the at warmer temperatures. Its more comfortable and takes less energy.
    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
    Thanks Tedybear,*

    I hear that one dehumidifier on a single system *will work if the doors are open. *I would presume that it won't make a difference if it is the right left or middle system as the house will equalize, is tHis correct?


    Didn't know about the heat but after looking at a couple of manuals online I see there are a number of ways to duct a dehumidifier. *The heat would only be an issue if ducted to run outside of the ac. *What's the best method of ducting considering my situation?

    Our humidity varies significantly and appears to be in step with the outside humidity. I would assume we have quite a few leaks. One thing We are looking at is spray foaming the attic which would certainly cause less air leaks. *Any dehumidifier considerations if we do get the foam?

    Thanks in advance

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,449
    Quote Originally Posted by rackeu View Post
    Thanks Tedybear,*

    I hear that one dehumidifier on a single system *will work if the doors are open. *I would presume that it won't make a difference if it is the right left or middle system as the house will equalize, is tHis correct?


    Didn't know about the heat but after looking at a couple of manuals online I see there are a number of ways to duct a dehumidifier. *The heat would only be an issue if ducted to run outside of the ac. *What's the best method of ducting considering my situation?

    Our humidity varies significantly and appears to be in step with the outside humidity. I would assume we have quite a few leaks. One thing We are looking at is spray foaming the attic which would certainly cause less air leaks. *Any dehumidifier considerations if we do get the foam?

    Thanks in advance
    Yes humidity moves rapidely throughout the home. There may be a slight gradient if the moisture source is one end and dehumidification is in the other end, expect a slight gradient. Avoid discharging the air from a dehumidifier into a space like the master bedroom. Best to have a return from the home and dehu supply into a/c supply in a none sensitive area. This allow operation of the dehu without any air handler fan operation. I assume that your home is <50%RH during long cooling runs and that the %RH raises during a/c short cycling periods.
    Foam attic insulation helps reduces excess infiltration. Caution on foam in attics without correct moisture control in the roof decks. Most homes are not getting enough fresh air during calm wind conditions and too much during windy conditions weather. Monitor the CO2 levels to determine the actual amount of fresh air infiltration. Some users provide fresh air ventilation as part of the dehu install. CO2 monitoring can be part of the control package to provide fresh air when the space is occupied and natural infiltration in low.
    Keep us posted.
    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
    Quote Originally Posted by teddy bear View Post
    . I assume that your home is <50%RH during long cooling runs and that the %RH raises during a/c short cycling periods.
    Foam attic insulation helps reduces excess infiltration. Caution on foam in attics without correct moisture control in the roof decks. Regards TB
    Not sure what long verses short cooling runs is all about. Since I have 3 systems and all zones are connected though hallways it’s difficult to assess time on veres off.

    Please explain moisture control in roof decks.

    thanks again

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,449
    Quote Originally Posted by rackeu View Post
    Not sure what long verses short cooling runs is all about. Since I have 3 systems and all zones are connected though hallways it’s difficult to assess time on veres off.

    Please explain moisture control in roof decks.

    thanks again
    Not an expert on roof decks, building science.com may help you with technical info on the variables on foaming roof decks and avoiding moisture problems.

    Are you able to get the %RH down during the hottest time of the day? You t-stat needs fan in "auto" mode to minimize the re-evaporation of the moisture on the cooling coil back into the home. Dehus are needed for maintaining <50%RH during the cool time of the day or rainy days, when the a/c is short cycling.
    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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