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  1. #1
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    Do I Need A Whole House Dehumidifier?

    During the summertime, the humidity levels in my house run between 48 to 54% with 50% being the average. The HVAC company which installed my AC unit says these humidity levels are too high and recommended that I install a whole house dehumidifier.

    What would be an optimal humidity level in my home during the summer? Wouldn't a thermostat with a humidity/dehumidity control on it do the job for a lot less money? I live in central Massachusetts. My house is 2100 sq. ft. and I have a 3.5 ton AC unit/coil on my furnace (with floor registers). Thanks!

  2. #2
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    Does your furnace have a VS blower?
    3.5 tons sounds like its a bit oversized.

    What temp do you keep the stat set to.

    Does your house have a lot of infiltration.
    If so, can you tighten it up so you don't have as much humidity entering your home.

    A whole house dehumidifier never hurts.(other then your wallet)
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  3. #3
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    Quote Originally Posted by pfrankk View Post
    During the summertime, the humidity levels in my house run between 48 to 54% with 50% being the average. The HVAC company which installed my AC unit says these humidity levels are too high and recommended that I install a whole house dehumidifier.

    What would be an optimal humidity level in my home during the summer? Wouldn't a thermostat with a humidity/dehumidity control on it do the job for a lot less money? I live in central Massachusetts. My house is 2100 sq. ft. and I have a 3.5 ton AC unit/coil on my furnace (with floor registers). Thanks!
    Why such a huge A/C? That could be part of the problem. An oversized A/C doesn't run long enough to provide proper dehumification.

    I'm a homeowner who just faced your situation.

    My variable speed air handler with optional humidistat (or an IAQ thermostat would also work) can control humidity in most of the house just fine- provided it runs enough. We had a mostly cool, damp spring and the upstairs two floors were usually OK, the basement just climbed into the high 50's RH and stayed there into the summer. Buying a cheap dehumidifier brought it right back down again, and lessens the load on the A/C. My combined bill WILL be larger, but it is an energy star DeHumidifier and only draws 3.88 amps (Kill-a-watt P3). Plus, are basement stays at 44-46% RH now, making it quite enjoyable on hot days and nights.

    Like most people, I find 45% RH (relative humidity) the most comfortable.

    45-50% isn't too bad (comfortable). Anything above 50% and I don't feel comfortable, but add fans and I could go to 55%. You may be different.

    For dust mite control you want to stay to 50% or lower.

    http://lancaster.unl.edu/pest/Resour...Mites311.shtml

    But anything under 70% is really OK.

    http://hgic.clemson.edu/factsheets/hgic2551.htm

    There may be other considerations, books, expensive wood furniture and collectibles, etc. Humidity cycling (RH going up and down) can be more harmful to expensive wood than a single high humidity.

    You didn't tell us what type of A/C + air handler/control board you have. If it is relatively modern and multi-speed or variable speed, it should have the ability to control humidity with the proper thermostat. If it is a single speed fan the 'stat will give you nothing new.

    Most people overbuy dehumidifiers based on bad "expert" advice. A big bad 70 pint dehumidifer may bring down the moisture quickly, but chances are it will only have to do that once in its lifetime (first time run) or a few times (large crowd boiling corn and crabs). 35 pints/day will do most homes fine, especially if you are just battling ambient humidity and not a leaky basement with a creek running through the middle of it.

    If you don't have a basement, the humidistat/IAQ Thermostat may be a better option.

    -HF

  4. #4
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    Jun 2006
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    36
    I have a Concord Air 90+ furnance and a American Standard Alligence 14.

    The HVAC contractor who put the heating system in, came back and put the AC system in. They claim it was properly sized.

    I keep the thermostat at 75 (for both zones-upstairs and downstairs). It is set to that temperature for the entire summer.

    My house was built in 2003 so it is tight. However, I do have an Aprilaire 8126 ventilation unit going into the supply side of the furnace/AC coil.


    Quote Originally Posted by beenthere View Post
    Does your furnace have a VS blower?
    3.5 tons sounds like its a bit oversized.

    What temp do you keep the stat set to.

    Does your house have a lot of infiltration.
    If so, can you tighten it up so you don't have as much humidity entering your home.

    A whole house dehumidifier never hurts.(other then your wallet)

  5. #5
    Join Date
    Jul 2008
    Location
    Rydal,Ga
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    84
    Quote Originally Posted by pfrankk View Post
    During the summertime, the humidity levels in my house run between 48 to 54% with 50% being the average. The HVAC company which installed my AC unit says these humidity levels are too high and recommended that I install a whole house dehumidifier.

    What would be an optimal humidity level in my home during the summer? Wouldn't a thermostat with a humidity/dehumidity control on it do the job for a lot less money? I live in central Massachusetts. My house is 2100 sq. ft. and I have a 3.5 ton AC unit/coil on my furnace (with floor registers). Thanks!
    Frank
    No! You have a great dehumidifier... your AC unit
    You are oversized, you mention floor registers, is it a crawl or basement??
    If it is a crawl, foam seal up the walls and band.... laminate a comm. vapor retarder about 3 inches up the crawl wall and seal the ground off....
    If you live in a termite zone leave about a 3 inch strip off exposed wall and paint it black than start the foam... Then CONDITION the crawl.... your done!

    If it is a basement, seal it, insulate it CONDITION It....

    The Aprilaire 8126 is supposed to be in the return this could be a huge problem... 1st your by-passing the filter.... second your by-passing the coil

    Go to my web page and see how it is done...

    WYL
    Last edited by kenny mac; 07-09-2008 at 02:26 PM. Reason: Addind stuff

  6. #6
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    The fresh air may be causing some of your trouble. Depends if they installed the OD sensor or not.
    Do you know how many CFM it is bring in.
    Not all new homes are as tight as people thing.

    Your design conditions are below my area. Unless you have a lot of glass, and no shade, your most likely over sized.

    Sounds like your A/C was sized by rule of thumb using 650 sq ft to the ton, and then rounded up.

    If you don't have a VS blowr in your furnace. The IAQ isn't going to do you any good as far as humidity control.

    During your hottest days, does it run constant to maintain temp, or does it cycle on and off.

    With an oversized A/C, and no VS blower, you will most likely need a whole house dehumidifier. Wspecially with that fresh air intake.

    Also, the Aprilaire 8126 is just a motorized damper and control.
    if its hooked into the supply. It isn't bringing in fresh air, its exhausting air.
    Its suppose to be hooked into the return duct.
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  7. #7
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    Rydal,Ga
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    Tushai Beenthere... hadn't thought of that...it is exht.

    Quote Originally Posted by beenthere View Post
    The fresh air may be causing some of your trouble. Depends if they installed the OD sensor or not.
    Do you know how many CFM it is bring in.
    Not all new homes are as tight as people thing.

    Your design conditions are below my area. Unless you have a lot of glass, and no shade, your most likely over sized.

    Sounds like your A/C was sized by rule of thumb using 650 sq ft to the ton, and then rounded up.

    If you don't have a VS blowr in your furnace. The IAQ isn't going to do you any good as far as humidity control.

    During your hottest days, does it run constant to maintain temp, or does it cycle on and off.

    With an oversized A/C, and no VS blower, you will most likely need a whole house dehumidifier. Wspecially with that fresh air intake.

    Also, the Aprilaire 8126 is just a motorized damper and control.
    if its hooked into the supply. It isn't bringing in fresh air, its exhausting air.
    Its suppose to be hooked into the return duct.
    I put a ton of these in (8126) and had not thought about the results of being on the supply... it will exhaust.... your good
    How do you spell Tushai ?????

  8. #8
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    Quote Originally Posted by kenny mac View Post
    I put a ton of these in (8126) and had not thought about the results of being on the supply... it will exhaust.... your good
    How do you spell Tushai ?????
    Not sure. but I think its Touchι.
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  9. #9
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    I doubled checked and confirms that the Aprilaire 8126 is on the return side of the furnace (air going into the furnace). The Aprilaire 8126 does have the outdoor sensor on it.

    On the hottest days it does cycle on and off.

    If my system is oversized, what are my options? Do I install a variable speed blower? Does that mean I should get the whole house dehumidifier as recommended by one HVAC contractor or just get the thermostat with the humidity controller in it as recommended by another? Any suggestions?

    Quote Originally Posted by beenthere View Post
    The fresh air may be causing some of your trouble. Depends if they installed the OD sensor or not.
    Do you know how many CFM it is bring in.
    Not all new homes are as tight as people thing.

    Your design conditions are below my area. Unless you have a lot of glass, and no shade, your most likely over sized.

    Sounds like your A/C was sized by rule of thumb using 650 sq ft to the ton, and then rounded up.

    If you don't have a VS blowr in your furnace. The IAQ isn't going to do you any good as far as humidity control.

    During your hottest days, does it run constant to maintain temp, or does it cycle on and off.

    With an oversized A/C, and no VS blower, you will most likely need a whole house dehumidifier. Wspecially with that fresh air intake.

    Also, the Aprilaire 8126 is just a motorized damper and control.
    if its hooked into the supply. It isn't bringing in fresh air, its exhausting air.
    Its suppose to be hooked into the return duct.

  10. #10
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    If you don't have a VS blower. A thermostat will only be able to over cool to dehumidify.
    Which can make the humidity higher, since the cooler it is in side, the less moisture an A/C removes. So save your money and don't get a new stat for the system you have.

    Hate to see a perfectly good furnace pulled just to get a VS blower. But that is an option.

    A whole house dehumidifier, will cover up the fact that your A/C is over sized. And be able to get your humidity down lower.

    A WHD will control your humidity better then a VS blower.

    Your decision on where you spend your money.

    I would just be reluctant to give my money to the company that oversized my A/C and caused this problem.

    WHD is your best option at this time, if its less then a new VS blower furnace with a Honeywell IAQ thermostat.
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  11. #11
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    Quote Originally Posted by pfrankk View Post
    During the summertime, the humidity levels in my house run between 48 to 54% with 50% being the average. The HVAC company which installed my AC unit says these humidity levels are too high and recommended that I install a whole house dehumidifier.

    What would be an optimal humidity level in my home during the summer? Wouldn't a thermostat with a humidity/dehumidity control on it do the job for a lot less money? I live in central Massachusetts. My house is 2100 sq. ft. and I have a 3.5 ton AC unit/coil on my furnace (with floor registers). Thanks!
    What is wrong with this picture? If I put a whole house dehu in your home, I would set the dehumidistat to turn on the dehu at 53%RH and turn off at 47%RH. If your %RH meter is accurate, your home is perfect. During wet cool weather, with 50-75 cfm of fresh air ventilation during occupancy, 50%RH is unlikely. But if you confirm <55%RH with adequate fresh air, document the miracle, relax and enjoy. 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"

  12. #12
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    Teddy bear thanks for the info. One last question: I have my Aprilaire 8126 set to vent 10 minutes every hour. Is this enough? The 6" ventilation duct runs about 10 feet to my return. My home is 2100 square feet.

    Quote Originally Posted by teddy bear View Post
    What is wrong with this picture? If I put a whole house dehu in your home, I would set the dehumidistat to turn on the dehu at 53%RH and turn off at 47%RH. If your %RH meter is accurate, your home is perfect. During wet cool weather, with 50-75 cfm of fresh air ventilation during occupancy, 50%RH is unlikely. But if you confirm <55%RH with adequate fresh air, document the miracle, relax and enjoy. Regards TB

  13. #13
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    Quote Originally Posted by pfrankk View Post
    Teddy bear thanks for the info. One last question: I have my Aprilaire 8126 set to vent 10 minutes every hour. Is this enough? The 6" ventilation duct runs about 10 feet to my return. My home is 2100 square feet.
    Questions like this cause a real s**t storm on this web site. People selling a/cs have been fighting fresh air infiltration for so many years that they can not imagin that a home might not get enough fresh air during mild weather without the stack effect and a strong breeze. In the leaky homes with leaky ducts located outside the conditioned space, and high cooling loads, excess fresh air is entering the home. But tight homes without the stack effect and wind are not getting enough fresh air during mild weather. You need to set-up your ventilating device to provide 50-75 cfm of fresh air when you are in your home. Typically a device like yours will introduce 50-100 cfm. Ideally, setting the unit to operate continously when you are in your home is best. Adjust the flow to 50-75 cfm. This will change the air in a 2,500 sqft. home every 4 hours. This is minimum according to IAQ experts. During typical summer weather your home may only get an air change every 10 hours, not enough to purge indoor pollutants. With 6 occupants, the O2 levels are not optimum without fresh air.
    After getting the fresh air you need, monitor your %RH. Your a/c with a high cooling load should be able to control humidity. If you exceed 60%RH, stop ventilating until you are able to maintain 50%RH. A lack of fresh air is less damaging than excess moisture. A good high eff. dehu will maintain <50%RH when there is low/no cooling load. Regards from a sick bear. Bad case of dehusalesitus, isn't the web great. I was wondering what that was. However it is better than "notinventedhereitis". 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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