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  1. #1
    Join Date
    Jul 2019
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    High Indoor Humidity on Remodel

    Hi - I have read a bunch of posts and am looking for a bit of specific advice. More information might be required - but here is a start.

    Live in gulf coast high humidity environment. We purchased a house built in 1980 and have extensively remodeled. We have put cellulose insulation in the attic and new double pane windows that have caused our house to be much more efficient.

    House is 2700 sqft and has two 3 ton units from 2013 (one brand new and one completely overhauled from 2003). They both cool very well.


    The problem

    As we have improved the house the humidity has risen until last week when we had a particularly rainy week. The indoor humidity upstairs got to 80% and 75% downstairs. As we push the AC lower and lower to compensate (no trouble cooling as low as we'd like..) we cooled below dew point and it felt terrible inside.

    I realized my thermostat (Sensi by Emerson) was making the fan run excessively, and turned the fan to run only when the unit runs, which lowered the humidity some.

    Today it is 80^ with 47% humidity (before 11am) and the system is showing 55% humidity downstairs and 67% upstairs - both have kicked up to 75^ and not run for a little while I'd imagine.

    The question

    After reading and reading and talking to my AC guy it seems like a whole home dehumidifier would be helpful - don't want/need to purchase new systems and want the house to be more comfortable at a higher temp. I have seen mixed messages, but is "bigger better" when it comes to this? Am I going to run into oversizing issues, or should I go ahead and go for a 100+pint unit?

    I would prefer the unit be "whole home" installed in the attic to save space and for noise issues. I am not sure how efficient my duct work is - We have a lot of insualtion as mentioned and a lot of ventilation with soffit vents - should I look at an attic fan as well to reduce humidity in attic (I know attic air should not be getting in the ducts..). I don't have turbines or attic fans or a ridge vent that I know of..

    Also my AC guy mentioned pulling air in from the upstairs return and pushing air into the downstairs return. Does this make sense from an installation standpoint? The downstairs return leads to a hallway near my backdoor and the thermostat - not a bad place to have a little dry heat escape.. Also it would pull the dry air over the downstairs unit coils when the system is in use - The downstairs unit is typically my more efficient unit.


    TLDR

    I understand there is a tone there but tldr comes down to:

    1. Do I need a whole home dehumidifier? If we don't know then what other questions do I need to answer to know.
    2. Do I need more attic ventilation - is that issue even germane to this issue?
    3. If I need a dehumidifier - is that idea of ducting through both system returns a proper solution?

    I know teddy bear is the resident dehumidifier experts - TB I'd be happy to pay you for a 20 minute consult call if you are available for that service.


    Thank you in advance for reading and for your advice and expertise.

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    9,694
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    Here is a post from earlier today.

    Size is the first thing decided when choosing an a/c for a home. Duct work is calculated and installed into a home with air leakage and occupants. This is all exposed to various wind/weather conditions.
    A real "crapshoot".
    It is all hooked together and usually works to the occupants satisfaction. All rather haphazard.
    The size is similar to your neighbors.
    Here is the difference. Your may not be function right, setup right, or your home may leak excessively.
    Setup and function are probably your problem.
    Assuming that your tech knows a how to make the a/c function, leaves the setup.
    The variables of function are mainly air flow. The amount of air flowing through the a/c determines the sensible/latent heat removal ratios. This is shocking info to some including a/c techs. We can blow enough air through your cooling coil that you will not remove any moisture and you would have +80%RH. Or slowing the air flow to the point where the coil will be near freeze-up and 65% of the a/c cooling will be latent cooling or moisture removal. One more concept, without a sensible cooling load, there will be no moisture removal. This is the least understood concept in the industry.
    Explain this to your tech. Adjusting the air flow through a functioning a/c and get the temp/%RH you want. 75^F, 50%RH, a 55^F dew point is most common. Dew point is another poorly understood concept. Most understand that a cold can of pop sweats when the surface of the can is below the dew point. If we want <55^F dew point home, we need a cooling coil well below the desired dew point to remove the moisture well below the space level which faces moisture from outdoors and the occpants.
    Long story short, The cooling coil should be 10^F colder than the desired dew point in the space. 75^F, 50%RH, a 50^F dew point return air to the a/c needs to pass through a 45^F sweating cooling coil to remove enough moisture to maintain a 55^F dew point air in the home. Measuring the dew point of the supply air is key to confirming a/c dehumidifying function. The a/c supply air is ideally 5-7^F lower dew point than the desired dew point in the space.
    Teach this to your a/c tech and he will be able to set up a/c for a life time! He will money to buy fish for the rest of his like...
    One last point, No sensible cooling, no dehumidification. During +50% cooling cycle or a hot day, the a/c removing sensible heat will keep you home <50%RH. During evens and rainy days, no dehumidification from the a/c



    Just basics.
    Before you get the dehu, get your a/c setup to max dehumidification. Getting down to 50%RH during your longest run is ok as long you do not expect any lower out of your dehu.
    Personally, suggest you target a minimum of a 30^F split or drop in your a/c coil temperature verses the return air temp at the desired %RH.
    If you want 75^F, 50%RH, a 55^F dew point, your a/c coil temp should be 45^F measured by your a/c tech.
    Another measure is the a/c supply dew point. To maintain a 55^F dew point in your home, the supply air must be 6-7^F lower dew point to keep the space dry. <50^F a/c supply is important from both a/cs as they cool. This will get a 50%RH home during significant sensible cooling loads( a hot day).

    When the sensible cooling loads decline evenings and rainy days, your a/c will not remove much moisture. If you want 50%RH during this weather, get a good whole house dehumidifier. Like the Ultra-Aire 98H or XT105H. The latter is the most efficient dehu in the world (+8 pints per KWH verses <5pints per
    KWH).
    Your attic concerns me. Attics without ridge vent and with soffit vents tend to build humidity to extreme. The excessive moisture condenses on the back side of the ceiling drywall and migrates into the space below. I suggest monitoring the temp/%RH in the attic peak to decide on what changes to make with venting. Ridge or high roof vents are cure if moisture is excessive.

    Installing the dehu is tricky. We need a return from the open part of the home to the dehu. The dry air should circulated through the most critical areas of the home. The a/c supplies down stream of the a/c coils is ideal. Splitting a dehu between to a/cs works. Attic ok. avoid being over major bedroom because of noise. Also important is a drain and safety pan with safety shut off.

    This is good start on my concerns about your home. Your a/c tech is a big part of this for a/c setup and dehu install.
    Keep us posted on any other issues.
    Regards Teddy Bear
    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"

  3. #3
    Join Date
    Aug 2002
    Location
    Southold, NY
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    Your System was oversized too begin with and now its really oversized.

    You need 2) new properly sized units and possibly duct modifications as well as a whole house dehumidifier!

  4. #4
    Join Date
    Jul 2019
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    Thread Starter
    Quote Originally Posted by teddy bear View Post
    Keep us posted on any other issues.
    Regards Teddy Bear
    Thank you for the info and feedback - I will run the tuning info by my tech. Will keep you posted on where we land.

  5. #5
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    Your System was oversized too begin with and now its really oversized.

    You need 2) new properly sized units and possibly duct modifications as well as a whole house dehumidifier!
    Thank you for your reply.

    I am not positive they were oversized with the single pane windows and no insulation in the 100+^ 80+% summers that go for months.

    I understand they are oversized at this point..

    That sounds like a great solution but I cant justify replacing two working systems because they are oversized.

  6. #6
    Join Date
    Nov 2006
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    Athens, Ohio
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    How about replacing the system that is 16 years old with a properly sized one? Then let that one handle the bulk of the cooling as it will dehumidify better with long cycles.
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  7. #7
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    Aug 2002
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    Quote Originally Posted by beancntr View Post
    Thank you for your reply.

    I am not positive they were oversized with the single pane windows and no insulation in the 100+^ 80+% summers that go for months.

    I understand they are oversized at this point..

    That sounds like a great solution but I cant justify replacing two working systems because they are oversized.
    Do nothing and the short cycling will kill them for you!

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