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  1. #1
    Join Date
    Jun 2015
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    Outside Humidity

    New to the forum here, hopefully this isn't in another thread...but I have a couple of questions regarding my HVAC.

    My ac has been keeping the house cold, but it is incredibly humid upstairs (one unit for entire house...about 1000 sq.ft.). It hasn't been like this the past 7 years that we've lived here. However, it's also been raining off and on showers (& hight humidity) everyday for the past week.
    1) Could the outside humidity level and frequent rains cause this humidity inside my house? The house windows are not very airtight...which makes me think it's a result of so much outside moisture.
    2) Would the upstairs naturally tend to be more humid than downstairs due to heat rising?

    Sorry...I really know nothing about HVAC, so you'll have to explain things simple! Thanks!

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by saxsellers View Post
    New to the forum here, hopefully this isn't in another thread...but I have a couple of questions regarding my HVAC.

    My ac has been keeping the house cold, but it is incredibly humid upstairs (one unit for entire house...about 1000 sq.ft.). It hasn't been like this the past 7 years that we've lived here. However, it's also been raining off and on showers (& hight humidity) everyday for the past week.
    1) Could the outside humidity level and frequent rains cause this humidity inside my house? The house windows are not very airtight...which makes me think it's a result of so much outside moisture.
    2) Would the upstairs naturally tend to be more humid than downstairs due to heat rising?

    Sorry...I really know nothing about HVAC, so you'll have to explain things simple! Thanks!
    This is a common problem with certain weather sequences. As you keep your home cool and the outdoor air dew points are near or above your indoor temperature, the inner sheathing on the outside walls may have moisture condensing on the surface. The sun on the outer walls drives the moisture toward the inside. This also happens in the attic.
    Another source is the infiltration of outside air into your home. This is mostly driven wind and operation of air moving devices, like clothes drier/bathfans/kitchen hood/gas water heaters. Keep in mind it is important to allow fresh air infiltration to be able to live in the home. In fact you need an air change in 4-5 hours to purge indoor pollutants and renew oxygen. If the dew point of outside air is +65-70^F and you want 75^F, 50%RH inside, you need to remove 2-4 lbs. of moisture per hour.
    The last common moisture source is the occupants themselves. A single occupants adds .5 lbs. per hour from respiration, perspiration, and activities.
    During high cooling loads, a properly set up a/c removes 3-6 lbs. per hour of moisture. During low/no very little moisture is removed.
    You guessed the rest, moisture builds up over several days of no significant cooling. Near drought, low dew points and hot, no problem. During damp cool day big problem with %RH buildup. Mold and dust mites are sure to follow.
    A neat solution is a small whole dehumidifier with enough capacity to maintain 50%RH when the moisture loads are high and cooling loads are low.
    A 70-100 pint dew will handle most homes. Check out one of the sponsors of this site, Ultra-Aire's web site or ad on this site.
    Another benefit of supplemental dehumidification a warmer inside temperature to avoid condensation on the outer surface of the home. Lower indoor %RH makes warmer temperature more comfortable also.
    Hope this helps. Keep us posted if you need clarification.
    Check my past posts as usually deal with various moisture issues. As you probably guessed, I am a semi-retired employee of Ultra-Aire and dealing with moisture issues for +25 years.
    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
    Jun 2015
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    2
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    Thread Starter
    Thanks man...I appreciate the advice. Went downstairs and sure enough, I had the furnace fan in the "on" position, as well as regular fans blowing in the house. Will the regular fans help/hurt/indifferent with the humidity? We popped open a window and shut off the air for awhile. We'll see what happens. Thanks again...what you said makes total sense, especially since the fan was constantly running.

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