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  1. #1
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    Jun 2005
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    Indoor humidity in Houston, TX

    Is is possible to get indoor humidity below 50% in Houston during the summertime using conventional equipment (no add on dehumidifier or dual stage fans).

    I'm in Houston with a current outdoor temp at 79 F/63% humidity and and indoor temp of 74 F/58% humidity.

  2. #2
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    Apr 2009
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    Santa Ana, CA
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    Your best bet is investing in a portable dehumidifier - which run pretty cheap and can be room specific.

    http://www.air-conditioner-home.com/faq_dehumidifiers/

  3. #3
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    Quote Originally Posted by aw View Post
    Is is possible to get indoor humidity below 50% in Houston during the summertime using conventional equipment (no add on dehumidifier or dual stage fans).

    I'm in Houston with a current outdoor temp at 79 F/63% humidity and and indoor temp of 74 F/58% humidity.
    I expected that you would get more help from our group. On a hot day, your a/c setup properly with fairly air tight ducts should get you down to <50%RH. During typical evenings with minor occasional cooling cycles, the level of %RH will depend on the fresh air leakage and the number of occupants. More fresh air and more occupants equals increase humidity levels. So for low humidity moderate cooling loads, avoid fresh air and keep the number occupants to a minimum. During wet cool weather, expect the %RH to rise directly with the amount of fresh air and occupant numbers.
    The dehumidifier is good suggestion but the resulting humidity control depends on the dehumidifiers drying capacity and all of the above. I prefer intentional fresh air to purge the indoor air pollutants like an air change in 4-5 hours. During wet cool weather with 4 occupants, expect to remove 40-70 lbs. of moisture per day (5-8 gallons), while maintaining <50%RH. Residential dehus struggle with this load and use a fair amount of electricity. Check the Ultra-Aire, Honeywell, Santa Fe and etc. for more efficiency and durability. These units can be ducted and will introduce fresh air if you want the best indoor air quality and comfort.
    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"

  4. #4
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    Quote Originally Posted by aw View Post
    Is is possible to get indoor humidity below 50% in Houston during the summertime using conventional equipment (no add on dehumidifier or dual stage fans).

    I'm in Houston with a current outdoor temp at 79 F/63% humidity and and indoor temp of 74 F/58% humidity.
    I'm near Houston too, a homeowner about 30 miles west. I don't believe you can overstate the importance of infiltration. Some houses are leaky and some are tight, you might look into getting a blower door test to measure what your house is. The results are measured in ACH50, the number of air changes per hour (ACH) at pressure of 50 Pascals, my house test number was 5.3 a couple years ago when I had the test done. That translates to about 0.24 estimated ACH due to natural infiltration in the summer.

    Mostly it is hard to get below 50% RH indoors but I have spoken to people who do it. They have a natural advantage when they work on HVAC for a living. There are several things you can get done which will help your AC lower humidity. Probably make your house feel distinctly more comfortable.

    But... as was famously stated on this board awhile back, no AC on earth can be tricked up to remove humidity when it isn't running. The past few days have had only warm weather with very little call for AC. Last night the outdoor temperature only got down to about 73F, I checked dewpoint and it was 69F -- that's humid!

    Hope this helps -- Pstu

  5. #5
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    Aug 2003
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    Fort Worth, TX
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    Quote Originally Posted by aw View Post
    Is is possible to get indoor humidity below 50% in Houston during the summertime using conventional equipment (no add on dehumidifier or dual stage fans).

    I'm in Houston with a current outdoor temp at 79 F/63% humidity and and indoor temp of 74 F/58% humidity.
    You ask if it is possible to get indoor humidity levels below 50% using only your a/c system in Houston, Texas, in summertime.

    I say it is. Requirement? A cooperative effort between your a/c system and your house. Your a/c system must be charged correctly, flow the correct amount of air through the system, and have duct work that leaks very little, especially if these ducts are located in the attic. Your house needs to be better at containing the air you pay to cool and dehumidify. This is accomplished by identifying where the walls, windows, doors, ceiling, and floors leak air into the house from outdoors.

    Many central a/c systems, as they are installed, are crippled in performance by subpar installation techniques. Even if you do not a thing to the house to make it more airtight or increase insulation levels, if the a/c system is sized correctly and installed with care, it should keep your house comfortable on hot, humid days. However, optimizing the a/c AND improving your home's "thermal boundary" (walls, ceilings, etc.) is a win-win combination. You reduce energy consumption while increasing comfort levels.

    Last summer my own home a/c system kept indoor humidity levels in the mid-forties with our occupied thermostat set point of 75 degrees. DFW tends to run a bit drier than Houston, but we have our share of high dew point days as well. This past winter I undertook additional weatherization of the home. Our recent spell of warm (but not hot), humid days accompanied by gusty winds from the south saw my indoor conditions, with t-stat adjusted to 74 degrees, holding humidity at around 50-52%. Humidity levels tended to spike in the morning due to showering, etc. and decline as the day warmed and the system ran more often. The wind blowing against the house had some effect but not as much as it would have previous to my air sealing efforts.

    During this upcoming summer I hope to obtain data loggers to create consistent monitoring throughout the house under various outdoor conditions. While our friend Teddy Bear speaks at length regarding dehumidifiers (which some homes may require for various reasons), not all of us are in a position to purchase and have installed a whole-house humidifier, Optimizing the a/c system already in place, along with weatherizing the house, is often the only options available to many.
    • 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.

  6. #6
    Join Date
    May 2002
    Location
    Houston Texas
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    In a word no!

  7. #7
    Join Date
    Nov 2008
    Location
    Houston, TX
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    77
    75 and 50% humidity would mean the coil is running at 55. Sounds high to me. And if 50% is as good as you can get with the system running, then it's going to feel awful at night time when the temp drops and the humidity increases and your A/C is off much of the time.

    I'd say your air conditioner needs a tech to go over it. I have arguably the worst air conditioner in Houston. And my indoor humidity is lower than yours during the last week or so.

    Outside my front door at 2:30PM today: 80/54%
    Inside I've got 75/49%. System is basically off. Maybe on for 4 minutes once in awhile.

    Portable dehumidifiers work pretty good during the colder months when it's 75 and 75% outside. The heat they generate can be tamed by running the air conditioning. I've actually been running mine daily this last week due to roof leaks and wet floor/carpet issues.

    But during the summer time, I never use the dehumidifier. The heat gain plus the radiant heat coming through the wall and ceiling when it's 95 outside is too much for my air conditioner to deal with. All I end up getting is a burning hot room where the dehumidifier is. And it's in an open dining room/living room.

    One of my friends lived for a few years in Orlando FL, which has similar weather to us. His house was 78/39% all summer long. No dehumidifier. Just A/C. His thermostat had a counter for on use. It would show about 14 hours of runtime per day during the hottest parts of the summer. He did mention that he thought Houston was muggier than Orlando though. I asked him how long it would run to get it to 73-75. He said it would probably never turn off.

  8. #8
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    Apr 2002
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    Quote Originally Posted by aw View Post
    Is is possible to get indoor humidity below 50% in Houston during the summertime using conventional equipment (no add on dehumidifier or dual stage fans).
    It is possible to do this in the 'summer time' in places which are more humid and not as hot as Houston, but you need a tight structure to do it.

    You do not need two stage AC or vairiable speed air handlers either
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  9. #9
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    Aug 2003
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    Quote Originally Posted by Baseboard Gord View Post
    75 and 50% humidity would mean the coil is running at 55. Sounds high to me. And if 50% is as good as you can get with the system running, then it's going to feel awful at night time when the temp drops and the humidity increases and your A/C is off much of the time.
    The coil surface temperature with 75/50 return air passing over it is probably colder than 55 degrees (closer to 50...depends on sensible heat ratio if you want to plot it out on a psychrometric chart). 75/50 gives an indoor dew point temperature of 55 degrees. If the cooling coil is colder than 55 degrees, it will remove moisture from the air. The temperature of the coil for a given amount of air that is passing over the coil will determine the quantity of moisture removed for a given condition of the return air.

    I'd say your air conditioner needs a tech to go over it. I have arguably the worst air conditioner in Houston. And my indoor humidity is lower than yours during the last week or so.
    You have a leaky structure...it's a flat roofed 1960's era apartment building with a 3 ton package unit on the roof, and single pane windows (possibly an old alumimum frame sliding glass door thrown in for grins) about 45 miles inland from the steamy Gulf of Mexico, if I recall correctly. Not an optimal setup for human comfort.

    Outside my front door at 2:30PM today: 80/54%
    Inside I've got 75/49%. System is basically off. Maybe on for 4 minutes once in awhile.
    Outdoor grains/dew point = 85/62, indoor grains/dew point = 64/55. If it's cloudy out there's not a great deal of solar heat gain to the structure going on right now.

    I've actually been running mine daily this last week due to roof leaks and wet floor/carpet issues.
    Roof leaks and wet carpet...not good!!

    Last night I had outdoor temperatures and dew points running close together, and flipped on my a/c to bring indoor humidity levels down. Achieved 74 degrees/52% RH in short order and was comfy rest of night. OP asked if a/c alone can hold 75/50 (a 55 degree dew point) in warm, humid weather. It can, but with cooperation from the building envelope. Our resident dehumidifier dude Teddy Bear speaks of "cool, wet weather" presenting a problem for indoor humidity. I had pretty cool, wet weather last night (outdoor temp 66/dew point 64), but managed to achieve an indoor dew point of 55 with my a/c alone.
    • 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
    Join Date
    Jun 2005
    Posts
    66
    i forgot to mention an important detail....my system is a zoned system in a two story house...i.e. one unit (condenser and evap) covers both floors, a t-stat on both floors. I swear I will never buy a house with this type a system again....the run times are short, maybe 10 minutes until the t-stat is satisfied. 4 Tons of a/c does not take long to cool ~1000 square feet of 2nd floor.

    I think the short run times are what is causing the elevated humidity levels.

  11. #11
    Join Date
    Nov 2008
    Location
    Houston, TX
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    77
    And just think. Somebody paid extra to have it installed that way. Time to switch that over to a one stat, all ducts running system. I'd simply try and close off some vents on the bottom floor aftewords if it's getting too cold.

  12. #12
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    Quote Originally Posted by aw View Post
    i forgot to mention an important detail....my system is a zoned system in a two story house...i.e. one unit (condenser and evap) covers both floors, a t-stat on both floors. I swear I will never buy a house with this type a system again....the run times are short, maybe 10 minutes until the t-stat is satisfied. 4 Tons of a/c does not take long to cool ~1000 square feet of 2nd floor.

    I think the short run times are what is causing the elevated humidity levels.
    Short run times combined with moderate infiltration can lead to elevated humidity levels. By conventional standards of observation my own a/c has short run times, even when outdoor temperatures and dew points are elevated, yet I do not run high indoor humidity levels. If your house is leaky, you will always fight a losing battle with indoor humidity in a humid climate, even if you manage to control it well with a highly tweaked and tuned a/c system. You lose in this sense due to operating cost, among other things.
    • 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.

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