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  1. #1
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    Relative humidity and air conditioner

    I live in Austin, TX (moved here not too long ago). Here is our average relative humidity (RH) data, month-by-month: http://www.cityrating.com/cityhumidity.asp?City=Austin

    I like the internal air to be somewhere around 40-50% RH and during the hottest months, outside RH here is close to my normal range (59%-53% for the summer months). So my question is, how much humidity an AC removes from the air? Because cooling season is close, the AC will be working all the time and if it drops RH by say 20%, then shouldn't I be thinking about getting central humidifier + humidistat installed? But whoever I talk to, say that it is very rare to have a central humidifier in Central Texas...

  2. #2
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  3. #3
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    Quote Originally Posted by golhon View Post
    I live in Austin, TX (moved here not too long ago). Here is our average relative humidity (RH) data, month-by-month: http://www.cityrating.com/cityhumidity.asp?City=Austin

    I like the internal air to be somewhere around 40-50% RH and during the hottest months, outside RH here is close to my normal range (59%-53% for the summer months). So my question is, how much humidity an AC removes from the air? Because cooling season is close, the AC will be working all the time and if it drops RH by say 20%, then shouldn't I be thinking about getting central humidifier + humidistat installed? But whoever I talk to, say that it is very rare to have a central humidifier in Central Texas...

    Its about the outside dew point verses the desire temp/%RH inside. Outside the daily varience is near 100%RH at sun rise and <50%RH during the hottest time of the day while the dew point varies onlt a few degrees through out the day.
    Outside air slowing infiltrates the home depending on wind and air leaks. the occupants add moisture to the infiltrating air. A/c, humidifiers, dehumidifiers remove or add moisture to proved +-50%RH at our desired temperature. Familarize yourself with these relationships. Homes are comfortable with 50-55^F dew points. When outdoor dew points are below 40^F you may need humidification, while when the outdoor dew points are +60^F, dehumidification is needed to maintain <50%RH.
    The a/cs do not operate enough during cool wet weather to maintain <50%RH. During very hot weather, your a/c should be able to maintain <50%RH. A whole house dehumidifier makes the home comfortable during low/no cooling load conditions. Check out the Ultra-Aire/Santa Fe whole house dehumidifiers.
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  4. #4
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    You wont need a humidifier during the summer , if your ac is oversized a dehumidifier is nice .

  5. #5
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    Teddy Bear is the humidity expert.

    If you have a humidity problem, have the blower set for a lower 350-cfm per ton of cooling &, then I'd install a Temperature/time "SWING" time adjustable room TH.

    Depending on the system you have installed a low cost stat is an ACE ATX 1500, that you can buy at an ACE hardware store.

    I'd set it so it cycles on at say, 78 or 79-F & off at around 75-F.
    Experiment with settings until you get it where it is most effective...

    If the system is oversized too much, depending on air infiltration rates & other factors, during cooler high humidity weather, the off portion of the cycle might be so long that humidity cannot be effectively controlled by the increased runtime length of each cycle.

    Do what you can to reduce air infiltration & the other internal factors...

  6. #6
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    Quote Originally Posted by udarrell View Post
    Teddy Bear is the humidity expert.

    If you have a humidity problem, have the blower set for a lower 350-cfm per ton of cooling &, then I'd install a Temperature/time "SWING" time adjustable room TH.

    Depending on the system you have installed a low cost stat is an ACE ATX 1500, that you can buy at an ACE hardware store.

    I'd set it so it cycles on at say, 78 or 79-F & off at around 75-F.
    Experiment with settings until you get it where it is most effective...

    If the system is oversized too much, depending on air infiltration rates & other factors, during cooler high humidity weather, the off portion of the cycle might be so long that humidity cannot be effectively controlled by the increased runtime length of each cycle.

    Do what you can to reduce air infiltration & the other internal factors...
    I think he is saying that his unit will dehumidify more than he needs and wants to ad a humidifier.

  7. #7
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    Quote Originally Posted by second opinion View Post
    I think he is saying that his unit will dehumidify more than he needs and wants to ad a humidifier.
    Looking at the linked pdf Relative Humidity of Austin, TX it appears he would need dehumidification; I thought Austin was a dry area...

    I looked up the Summer Design; figured the psyc & it appears to have RH at only 31% with a 99-F dry bulb, 74-F wet bulb. Grains of moisture is 87.6 gr/lb dry air.

    Indoors at 75F, 62.5F wet bulb, 50% RH, is 65.48 gr/lb dry air.
    Therefore, it appears with a lot of A/C runtime you maybe right; he could be getting too low a humidity in his home?

    I should have read his post more carefully...

    He needs to specify what he needs under differing temp & humidity conditions...

    Dew point changes as humidity gets lower which limits how low the A/C will get it...
    Last edited by udarrell; 03-16-2012 at 10:24 AM. Reason: linked pdf...I should have read his post more carefully...

  8. #8
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    Doubt you'll need a humidifier. Adjustments can be made to the blower to keep the humidity from getting to low.
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  9. #9
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    Quote Originally Posted by golhon View Post
    I live in Austin, TX (moved here not too long ago). Here is our average relative humidity (RH) data, month-by-month: http://www.cityrating.com/cityhumidity.asp?City=Austin

    I like the internal air to be somewhere around 40-50% RH and during the hottest months, outside RH here is close to my normal range (59%-53% for the summer months). So my question is, how much humidity an AC removes from the air? Because cooling season is close, the AC will be working all the time and if it drops RH by say 20%, then shouldn't I be thinking about getting central humidifier + humidistat installed? But whoever I talk to, say that it is very rare to have a central humidifier in Central Texas...
    If this is your first full summer to spend living in Austin, all discussion about using a humidifier in summer will vanish quickly once you're a few weeks into the season.

    I live north of you in Fort Worth, which is a bit drier, but consequently a tad hotter in summer. But I've been in Austin at all times of the year, as my brother lives there, and my region of RSES likes to hold conferences there. Austin is more humid than Fort Worth because it is closer to the Gulf of Mexico and farther away from the West Texas deserts.

    So, since you are making your home in a hot, humid climate, Teddy Bear gave you good advice. Learn the significance of dew point, and how that is a better metric for measuring humidity and comfort than relative humidity (although the latter explains why you can be uncomfortable at higher dew point and dry bulb temperatures...the higher the RH, the less able you are to be cooled by perspiring). Dew point is significant pertaining to your house in that whenever outdoor dew point temperatures exceed ~ 55 degrees, you may want your house dehumidified. Outdoor dew point below ~ 40 degrees, you may want your house humidified. Both of these factors also depend on how leaky your house is....tight homes need little humidity added but may need humidity removed year round.
    • 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.

  10. #10
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    Look a little closer at your data

    Austin Relative Humidity (Morning) 83% based on am temp
    Austin Relative Humidity (Afternoon) 59% based on pm temp

    The reason that the PM RH is lower is because even with the small increase of absolute humidity, the higher afternoon temp reduced the RH.

    There may be other reasons for not controlling your internal RH but seems to me your weather pattern is not that demanding.
    I think that is why the TV weather seems to be reporting dew point vs RH
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  11. #11
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    Quote Originally Posted by beenthere View Post
    Doubt you'll need a humidifier. Adjustments can be made to the blower to keep the humidity from getting to low.

    +1. There's also the possibility that your ductowrk is undersized and/or equipment is oversized, so therefore it's short on airflow. Lower airlow = more dehumidification. So before anything is added, I'd find out how much airflow your moving and start from there. If your removing too much mosture, then you may want ot just increase your airflow ot as much as 450CFM/ton. But, your ductwork will need ot be large enough to handle that.

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