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  1. #1
    Join Date
    Mar 2009
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    Humidity Control - Getting New Cond./Have Trane XB80 (Houston)

    Hello -

    I have a 100 yr old, 1200 sq ft house in Houston. I have no insulation in the walls, 10' ceilings, and every room has several original 3' x 6' and 4' x 6' windows on each wall. I have fully insulated the attic, added a radiant barrier to the underside of the roof, and sealed the windows as I best I can.

    I currently have a relatively new Trane XB80 furnace and an ancient 3-ton Janitrol A/C condenser. The ductwork in the house is mess - a sloppy job from the late 80s, I would guess. I have improved improved it somewhat by adding a second return box and trying to seal the ductwork up better.

    However, as you might expect, I have a big problem with humidity in the summer, given that I live in a not-very-tight old house in Houston. I run a large portable dehumidifer and have to empty it twice a day in the summer

    I plan to now have the condenser, coil and all ductwork replaced, but due to budget would like to keep the existing furnace.

    With the existing furnace, what is my best alternative for improving humidity control? One installer told me that, with a new condenser and ductwork, just changing the speed setting on the air handler fan would probably fix my problem. (It is currently at max).

    Is there an "intelligent" condenser/controller system (of any brand) that would work with the existing furnace/air handler? What I would like to do is have a system that would be able to keep the house in the summer at 78 F and 40% rh.

    Any advice would be most appreciated. I've gotten bids from 3 guys so far (all 3 ton units - 2 Carrier Comfort, 1 Ruud UAPL-JAZ), and none of them really wants to provide me any logical explanation of how I can do a better job of effectively controlling humidity in the house.


  2. #2
    Join Date
    Mar 2009
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    Honeywells visionpro IAQ thermostat has some dehumidification capability in that it will overcool the space by 2-3 degrees to create the longer run time. Depending on the installation specifics, you may be able to install a whole house dehumidifier which works in conjunction with the existing Trane. Aprilaire makes a nice product.

  3. #3
    Join Date
    Jan 2004
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    If you want 40%RH, get a whole house dehumidifier.
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    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    May 2000
    Indianapolis, IN, USA
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    Don't oversize!!!

    Also keeping it pretty warm means it doesn't run as much meaning it doesn't dehumidify as much.

    I keep my house cool and tend to run around 55% RH in the summer, sometimes down to 50%. been is right, you want it 40% you'll need to talk to Teddy Bear about a Thermastor or the like.

  5. #5
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
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    To achieve 78^F, 40%RH without excessive energy cost, the a/c is set-up to maintain the desired condition during highest cooling load condition. The whole house dehumidifier maintains 40%RH during the low/no cooling load conditions.
    Best to setup your a/c with a very cold coil. For 78^F, 40%RH, You need to slow your air flow down to have a 40^F-45^F a/c coil temperature. Any exposed air ducts/equipment in uncondition space will require enough insulation to avoid condenstion on their surface. An air tight vapor barrier on the exterior of the insulation to prevent moisture accumulating in the insulation. The whole house dehu needs an independent return from the house to the dehu with the dry air from the dehu put into the a/c supply ducts for dry air distribution. Do not operate the a/c fan when not cooling. Operating the a/c fan quickly evaporates the cooling coil moisture back into the home. If you are unable to cold a/c coil temperatures the size of the dehumidifier is larger.
    As an option, I recommend 60-80 cfm of fresh air during the no-heating closed window homes when the home is occupied. We find that during the warmer times of the year even leaky homes are not getting enough fresh to pruge indoor pollutants. Mechanical make-up fresh air stops random infiltration, supplies make-up air for clothes dries, bath fans, kitchenhoods. Whole house dehumidifiers like Ultra-Aire/Honeywell are capable of providing minimal fresh air when the occupants are routinely in the home. The fresh air is filtered, blended, and dried when the a/c is unable to keep the home dry.
    Homes in green grass climates with adequate fresh air need supplemental dehumidifcation during low/no cooling periods to maintain <50%RH. If a home is dry during damp cool weather, inadequate amounts of fresh air is entering the home. You should have an air change every 4-5 hours when the home is occupied. Thanks for the support.
    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 2000
    Magnolia, Texas
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    The first thing to address is the tightness of the house. Get a blower door & duct blaster tests performed. You'll want to eliminate infiltration and prevent a negative pressure from being created.
    "You must be prepared for the opportunity to succeed."

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