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  1. #1
    Join Date
    Apr 2008
    Posts
    8

    summer indoors Relative humidity

    Hi,all

    I have just bought a new house in Houston(3200 sq ft, 2 stories, facing south) with two Carrier AC (4 ton for downstairs and 3 ton for upstairs). Recently I bought a Humidity Hygrometer and find that the Humidity indoors is about 53% for indoors temperature 80F in the summer. Do you guys think that the Humidity is a little bit too high? (as I checked in the internet, the recommended indoors Humidity is 30% to 50%)
    My another concern is whether my current AC systems are too
    powerful and reach the target temperature too quickly without dehumidifying enough??

    BTW, here is my old post
    http://www.hvac-talk.com/vbb/showthread.php?t=168432

    Thanks a lot

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,095
    With out using a dehumidifier, you'll have a hard time getting down to 30%RH.
    Under 55%RH is ok.
    If you want lower, call the HVAC contractor back to see if they can slow the blower down, that will help remove more moisture.
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  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,442
    Quote Originally Posted by gqchen View Post
    Hi,all

    I have just bought a new house in Houston(3200 sq ft, 2 stories, facing south) with two Carrier AC (4 ton for downstairs and 3 ton for upstairs). Recently I bought a Humidity Hygrometer and find that the Humidity indoors is about 53% for indoors temperature 80F in the summer. Do you guys think that the Humidity is a little bit too high? (as I checked in the internet, the recommended indoors Humidity is 30% to 50%)
    My another concern is whether my current AC systems are too
    powerful and reach the target temperature too quickly without dehumidifying enough??

    BTW, here is my old post
    http://www.hvac-talk.com/vbb/showthread.php?t=168432

    Thanks a lot
    Keep in mind the range of %RH that sight as recommended means the following. 30% RH is thought tobe the minimal winter humidity. The 50%RH is recommended tobe the summer maximum. The biggest source of moisture is infiltrating fresh air. You need an air change of fresh every 3-4 hours to purge indoorpollutants and renew oxygen. If you can get through the spring/summer/fall season without exceeding 55%, succes is yours. Once the heat of summer arrives, you a/c should be able to provide +-50%RH. Most homes with adeaquate fresh air ventilation, in green grass climates have high humidity during wet <80^F weather. Supplemental dehumidification maybe required to maintain 50%RH during wet cool weather. The concern is avoid mold and dust mites inside the home while getting adequate fresh air . 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
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I am a homeowner in the Houston area too and believe you may not be doing too bad with humidity. Any question of oversizing would be answered more directly by other means. Manual J recommends 94F design temperature and we have plenty of those, is either of your systems running at or near 100&#37; of the time when it's 94 outside?

    Lstiburek did a 2000 study in Cinco ranch, and his metric for satisfactory humidity was what percentage of the time RH was over 60%. That implies a norm for our region which is not all that dry indoors. You may well need a dehumidifier in the spring and fall, if not the summer months.

    Would be nice to say ask your builder for the load calc but I would wager they didn't do one. Sometimes a builder will do one for all orientations of a home design and use the highest number I am told. If that is the case your Southern oriented heat gain will be toward the lowest of all directions not the highest, increasing the likelihood of your being oversized. But that's just S.O.P. everywhere I look in Houston it seems.

    Trane specifically recommends using 350 cfm/ton instead of the usual 400, in humid climates.

    Best wishes -- Pstu

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,346
    Went out for lunch today and happened to have a Testo H2 digital psychrometer with me. Restaurant interior conditions were 73 degrees at 61&#37; relative humidity. Had it not been for the ceiling fan over my head and the lower dry bulb temperature, that dining room would've been uncomfortable to be in. Most restaurants I've measured in the past tend to run higher indoor humidity levels. You think someone would figure out that dehumidification in a restaurant in summer is vital...instead they tend to run the dining rooms so damn cold that women take sweaters and windbreakers with them to eat in the middle of summer.

    80 degrees dry bulb at 53% relative humidity is a bit outside the ASHRAE comfort index. If you were to achieve 75 degrees dry bulb at <50% relative humidity, or perhaps 78 @ 50, you'd probably be more comfortable, and your house would be at lower risk for mold growth, dust mites, etc. Your present equipment can probably achieve these targets, if you think what you have now is oversized. If any design metric went into your system at all, it was likely a 75 degree, 50% relative humidity target. Running it at 80 degrees dry bulb is in effect running an oversized piece of equipment. At 75 degrees on a design day, it may not be oversized.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,095
    [QUOTE=shophound;1898344You think someone would figure out that dehumidification in a restaurant in summer is vital...instead they tend to run the dining rooms so damn cold that women take sweaters and windbreakers with them to eat in the middle of summer.

    [/QUOTE]

    Restaurants are tough to condition. Large amounts of fresh air. Sudden humidity increases from filling up with diners. Plus the sudden load from the food.

    Need units with reheat/heat pipes.
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