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  1. #1
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    Apr 2011
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    7

    Dehumification on the Texas Gulf Coast

    I live in SE Texas on the Gulf of Mexico in a new 1600sf ICF home with a Trane 16i-3ton AC. Humidity has been running almost 70%. The ac apparently doesn't tun long enough to dehumidify properly. Outside humidity is 60-90%. I don't want portable dehumidifiers so am looking for a whole house unit. I am concerned about hooking up a whole house unit because the discharge air is so hot on the units I'm studying. Help.

  2. #2
    Join Date
    Mar 2011
    Location
    Louisiana
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    119
    Is you unit cycling at all. In theory a properly sized piece of equipment will run 100% of the time at design conditions and maintain design set-point.

    At what ambient temperatures are you seeing these 80% indoor RH valves?

    As far as the whole house dehumidifiers and high discharge air temperature, remember this. The airflow thru the dehumidifier is going to be in the neighborhood of 250 CFM while a 3 ton AC system for your area should be in the 1000 CFM range. So if you see a 100 discharge air temperature off of the dehumidifier when mixed with the 75 central system temperature you will only see a net gain of approx 5 to the central system airstream.

    J
    Last edited by Jarreau; 04-06-2011 at 09:24 AM. Reason: typo

  3. #3
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,317
    Quote Originally Posted by Mvanza View Post
    I live in SE Texas on the Gulf of Mexico in a new 1600sf ICF home with a Trane 16i-3ton AC. Humidity has been running almost 70%. The ac apparently doesn't tun long enough to dehumidify properly. Outside humidity is 60-90%. I don't want portable dehumidifiers so am looking for a whole house unit. I am concerned about hooking up a whole house unit because the discharge air is so hot on the units I'm studying. Help.
    Teddy Bear, our resident dehumidifier guru, will likely be along shortly to give you a full rundown on whole house dehumidification.

    Our dearly departed Carnak once mentioned a dehumidifier that has both an inline and remote condenser, switchable depending on weather conditions. With a remote condenser the dehumidifier is more like a small a/c in that it will dehumidify and slightly cool the air in the home. With an inline condenser there is a little reheating of the air that occurs.

    Overall if your home is ICF and 1,600 square feet, 3 tons sounds considerably oversized. That is why it won't run long enough to dehumidify. I have an 1,800 square foot conventionally built house with considerable work done on its shell to retain its interior environment better, and at 3 tons I'm a tad oversized. For you it's overkill. Whoever sized your system did you no favor.
    • 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.

  4. #4
    Join Date
    Apr 2011
    Posts
    7
    Thanks, It probably is oversized, but I have a metal roof and blown in insulation in the attic covering up 49- 6" downlights, 9' 4" walls and a large open ceiling living room. The Trane 16i supposedly is variable speed and ramps down to two tons or so in low. Outside air is 70-78 degrees. I keep it at 70 degrees inside.
    When the humidity is above seventy outside, the inside humidity craws up treal close to the outside humidity. Clammy cold at 70/70.

  5. #5
    Join Date
    Apr 2011
    Posts
    7

    Jarreau

    HVAC seems to run fine. Just not very much. Highest RH inside was 75% when outside was 75 degrees and 90% RH.
    Right now, it's 70 degrees and 59% RH outside, thermostat is set on Auto at 72, It's 72 inside and 61% RH. Unit has been running on and off on low speed.

    Yesterday, a from blew thruough, and we lowered the humidity to 45 by opening up the house.

    Glad to know the hot air from the dehumidifier won't hace too much effect, but I think they are wired so they only run when the HVAC is off. If I buy a 400 cfm humidifier, it ought to work with my blower.

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,341
    Quote Originally Posted by Mvanza View Post
    I live in SE Texas on the Gulf of Mexico in a new 1600sf ICF home with a Trane 16i-3ton AC. Humidity has been running almost 70%. The ac apparently doesn't tun long enough to dehumidify properly. Outside humidity is 60-90%. I don't want portable dehumidifiers so am looking for a whole house unit. I am concerned about hooking up a whole house unit because the discharge air is so hot on the units I'm studying. Help.
    How many occupants are in the home?
    Each occupant generates about .5 lb. per hour from breathing, sweating, and activities. The occupant generated moisture is added to the air in the home. If the home was perfectly air tight, 3 occupants generating 1.5 lbs. per hour in your home will raise the indoor humidity 10%RH per hour. In a couple days, you would be +100%RH.
    Certainly, you are getting some fresh air in your home which explains why 70%-80%RH. During cold dry weather, your home is drier. During warm wet weather, your home is wetter.
    What are you doing for fresh air ventilation in your home?
    In a good air tight 1,600 sf home, you should have a minimum of 60 cfm of fresh air infiltration/ventilating when the home is occupied. When estimating the wetting or drying effect of fresh air, use the dew point or (grains of moisture per lb. of air) of the outside air. When maintaining 75^F, 50%RH inside of the home, the dew point is 55^F or 65 grains of moisture per lb. of air. If the outside air has a 55^F dew point, the %RH will be 100% at 55^F and 50%RH at 75^F. So refer to the dew point of the outside air instead of %RH.
    Today you have 60^F dew point outdoors as per
    http://www.weather.com/maps/maptype/...nts_large.html
    As the 60^F dew point outdoor air infiltrates your 70^F home, the %RH in the home is 70%RH. Then your family adds their moisture to the fresh air, raising the %RH even higher, Occasionally your a/c is activated, removing a small amount of moisture from the air onto the cooling coil. Until the coil/pan is staturated, no moisture goes down the drain. After 30 minsutes of cooling, most coils are saturated, dripping to the pan, and moisture exits via the drain. If the cooling stops before being saturated, the moisture on the coil evaporates back into the home via the a/c ducts. Also the cooling temp of the coil determines the amount of moisture removed. The coil must be colder than desired inside dew point.
    This is complicated. You need many hours of cooling to maintain <50%RH in your home.
    Going to end of this session, the a/c stops when the home is cooled. Dehumidification starts when the home is +50%RH and cooling is not desiriable. If cooling maintains <50%RH, no dehumidification needed. High efficiency dehu add 1,500 btus of heat per lb. of moisture removed. The house is cooled but we need 3 lbs. per hour of moisture removed. The dehu adds 4,500 btus of free reheat per hour. A 3 ton a/c operates 7 minutes longer per hour and removes 1 lb. of additional moisture. The heat from the dehu helps control the humidity in the home.
    Sorry about all of this detail, but it is important to understand the a/c-dehu interaction.
    Look forward to the fresh air and occupant number and other questions.
    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
    Apr 2011
    Posts
    7
    Teddy Bear,
    Three people and 100 pounds of dog live in our home. I don't know how much fresh air we are getting but we have electric vents in the two bathrooms, 49 six inch downlights in the ceilings that aren't caulked at the trim rings and two 36 inch doors that we are in and out alot. No other fresh air sources except window leakage and they are pretty tight. The house is elevated 12 feet above ground with no obstructions underneath. The slab on the pilings sits on 10 inches of styrofoam. In the winter when the heater was running and the humidity lower, we didn't notive a RH problem. I'm looking at whole house dehumidifiers, particularly the Williams air sponge and the Aprilaire 1750A, leaning toward the air sponge. Thanks for your advice. Here's my weather this evening.

    Galveston, Scholes Field
    Lat: 29.27 Lon: -94.87 Elev: 6
    Last Update on Apr 6, 5:52 pm CDT


    Fair

    72 F
    (22 C) Humidity: 61 %
    Wind Speed: SE 16 MPH
    Barometer: 29.98" (1015.1 mb)
    Dewpoint: 58 F (14 C)
    Visibility: 10.00 mi.
    More Local Wx: 3 Day History

  8. #8
    Join Date
    Apr 2011
    Posts
    7
    Thursday 5:00 a.m.

    Outside
    69 F(21 C)
    Humidity: 84 %
    Dewpoint: 64 F (18 C)


    HVAC set on Auto 70^F, inside temp is 70^F, inside RH 70%. Clammy and cold.

  9. #9
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,341
    Quote Originally Posted by Mvanza View Post
    Teddy Bear,
    Three people and 100 pounds of dog live in our home. I don't know how much fresh air we are getting but we have electric vents in the two bathrooms, 49 six inch downlights in the ceilings that aren't caulked at the trim rings and two 36 inch doors that we are in and out alot. No other fresh air sources except window leakage and they are pretty tight. The house is elevated 12 feet above ground with no obstructions underneath. The slab on the pilings sits on 10 inches of styrofoam. In the winter when the heater was running and the humidity lower, we didn't notive a RH problem. I'm looking at whole house dehumidifiers, particularly the Williams air sponge and the Aprilaire 1750A, leaning toward the air sponge. Thanks for your advice. Here's my weather this evening.

    Galveston, Scholes Field
    Lat: 29.27 Lon: -94.87 Elev: 6
    Last Update on Apr 6, 5:52 pm CDT


    Fair

    72 F
    (22 C) Humidity: 61 %
    Wind Speed: SE 16 MPH
    Barometer: 29.98" (1015.1 mb)
    Dewpoint: 58 F (14 C)
    Visibility: 10.00 mi.
    More Local Wx: 3 Day History
    What were your inside Temps/%RH at after several hours of outside and with the occupants in place? How much does the a/c operate? The 16 mph wind will force some fresh air into the home like 60-80 cfm of fresh air.

    Here is my estimate of the having a good ventilating whole dehu (like Ultra-Aire 90H/Honeywell prefered).
    Adding Fresh Air Only
    Mix in 80 cfm of 72^F, 61%RH, 58^F dew point fresh air to house air, add 2 lbs. per hour moisture from the occupants. The resulting mixed air dew point would be 64^F. With a 58^F OD DP, more fresh air lowers the indoor dew point, less fresh air raises the indoor %RH.

    <50% by Removing Moisture with a/c or dehumidifier
    Maintaining 50%RH at 70^F, 50^F DP from 80 cfm fresh air/2 lbs./hr occupants requires removing 3 lbs. hour of moisture by the a/c or dehumidifier. If the a/c is cycling frequently, the dehu takes over.
    By raising your indoor temp to 75^F, 50% RH, 55^FDP, the dehumdification load decreases to 2.3 lbs. per hour.
    During high outdoor dew points of 75^F, 4 lbs.per hour of moisture must be remove to maintain

    Providing <50% by fresh air ventilation during cold weather
    80 cfm of 22^F dew point fresh air will remove 2 lbs. of moisture per hour in 50^F dew point home. More fresh air, lower inside DP. Less fresh air, higher indoor dew point.
    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"

  10. #10
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,317
    Quote Originally Posted by Mvanza View Post
    Thursday 5:00 a.m.

    Outside
    69 F(21 C)
    Humidity: 84 %
    Dewpoint: 64 F (18 C)


    HVAC set on Auto 70^F, inside temp is 70^F, inside RH 70%. Clammy and cold.
    There's no heat load from outdoors on your house. You're running your a/c not to cool but to dehumidify, otherwise at 70 degrees indoors you should be quite comfortable along the Gulf Coast. You're not because the a/c can't run long enough to extract moisture from the air.

    Whoever designed your a/c system was likely mired in conventional house construction mentality. That is, conventional homes leak, they leak a lot, which along the Gulf Coast means a high humidity load due to this air leakage. Which can account for over half your air conditioning load (a/c spends over half its energy just taking moisture out of the air). In your case with ICF construction you would think it is tight construction, but unless you have already I would get your house blower door tested. If there are leaks occurring in your home's shell that you're not aware of, they could partially account for your elevated humidity levels.

    The 48 can lights in the ceiling would concern me, for certain, especially if the attic above them is ventilated and those can lights are not sealed. I would deem that a HUGE oversight; build an ICF house but punch it full of 48 holes??? One thing I've learned, and I've learned it hard and fast; you can have all the insulation in the world but if the house is leaky, that insulation's effectiveness for saving energy and increasing your comfort level is being largely negated is leaky.

    The ventilating dehumidifier is still on the table even as I mention envelope leakage, because if you find your building shell is contributing to your high humidity problems and you seal it up, you'll need fresh air and humidity control even more. Either way you can't continue as you are going right now as you're heading for some serious air quality concerns, as well as potentially jeopardizing the vitality of your home's structure.
    • 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.

  11. #11
    Join Date
    Apr 2011
    Posts
    7
    Thanks for the advice. I'm moving forward. My 49 can lights are all Cree LR6 LED's with built in trim rings that seal pretty tightly against the ceiling. Blown in insulation over the cans in the attic but the cans themselves aren't insulated. They don't put out much heat. My wife's a light freak and I didn't want a bunch of incandescents.



    Current conditions:

    Apr 7, 8:52 pm CDT
    Overcast 72 F
    Humidity: 91 %
    Wind Speed: SE 15 MPH
    Barometer: 29.85" (1010.8 mb)
    Dewpoint: 69 F (21 C)
    Visibility: 7.00 mi

    Inside temp is 70^F. Inside humidity 70%.

    I talked with my Trane dealer and his techie today. They want me to buy an Aprilaire 1770A -135 pint whole house dehumidifier connected to the HVAC return and supply with a humidistat/controller next to the HVAC thermostat. Pricey solution (about $ installed). I haven't pulled the trigger yet.
    Last edited by beenthere; 04-08-2011 at 05:41 AM. Reason: price

  12. #12
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,317
    Sounds like your can lights are IC rated if they have insulation touching them. And may not be an issue if the trim rings have seals in them.

    Teddy Bear mentioned earlier how much moisture humans produce, as well as your dog(s). Your home may be tight and underventilated, hence the high moisture problem. I think you're moving in the right direction. Hard for me to imagine an ICF home having high infiltration/exfiltration issues.
    • 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.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,880
    Also sounds like a 3 ton A/C was installed where a 2 ton was needed.

    The 16i isn't great at removing moisture from the air in first stage. And in first stage it can be operating at a capacity of 2.4 tons. The 20i is a much better choice for high humidity areas.

    Ask your contractor what CFM per ton it is set for.

    A WHD may be your only solution.
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