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  1. #1
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125

    Will humidity drift upstairs?

    Hi,
    Im thinking of removing the upstairs aprilaire humidifier to make space for a larger return. The system is for the 1500 sq ft, and is in a tight closet. The first floor has a separate system, 2500 sq ft, with a separate humidifier.

    How mobile is humidity? Would the moisture tend to diffuse upstairs at a reasonable speed?

    Steve

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,283
    Humidity follows pressure and temperature gradients; i.e. greater to lesser in both cases, verified by measuring absolute humidity, or more simply dew point.

    Your downstairs humidifier may keep both levels humidified if it can keep up with or stay ahead of winter stack effect in the house. In fact, the more airtight a house is, the easier it is to maintain comfortable humidity levels, 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.

  3. #3
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    Quote Originally Posted by central nj View Post
    Hi,
    I”m thinking of removing the upstairs aprilaire humidifier to make space for a larger return. The system is for the 1500 sq ft, and is in a tight closet. The first floor has a separate system, 2500 sq ft, with a separate humidifier.

    How mobile is humidity? Would the moisture tend to diffuse upstairs at a reasonable speed?

    Steve
    The rate of diffusion will depend on the circulation rate of the air. The moist humid air will rise to the upstairs naturally due to the lighter constituents of the air, but if you leave the fan on and the air down stairs is warm enough to allow additional grains of moisture and the house is tight you should not have any problems maintaining a comfortable moisture level.

  4. #4
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    1,688
    Humidity will disperse throughout the air. Provided there is no barrier between the upper and lower levels.
    Years ago we installed two systems in a large house. We installed a humidifier on the downstairs system and had a water line installed in an upstairs closet so we could install a humidifier on the attic system later if it was needed. I was reluctant to put in a humidifier in the attic for obvious reasons. They never needed one.
    Just make sure the humidifier on the downstairs system has sufficient capacity for the entire house or it won't do the job anyway.

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I've generally found in my home at least, that it seems to mostly move with the air movements. Cool dryer air tends to fall and warm more humid air tend to rise. FUrther stack effect a reverse stack effect push cool dry air form the upstairs to hte downstairs and out and hot humid air leaks in upstairs.

    Also consider that vented attic are extrmely hot and don't have a vapor barrier. That hot moisture loaded outdoor air can be driven into ceiling assemblies unless you have closed cell spray foam insulation. Loose fill and cellulose insulation or even open cell does nothing ot stop moisture migration. Any air leaks form ceiling penetrations only makes it worse.



    I only have a humidifier on my downstairs system. No need or one upstairs.

    If I had a whole house dehumidifier, I would only need it upstairs. Cool dry air is constantly "Falling" down the central stairway. I rarely even need to run a dehumidifer in the unfinished basement wit hattached garage most of the summer. Although this summer is dryer than usual.

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,850
    I have experienced how humidity works in the summer; I have a small half-ton 9.7-EER window A/C downstairs & one half-ton 7-EER unit upstairs
    ...with the one running downstairs with 50 to 55% RH & 75-F; the one in the small bedroom upstairs is OFF with an open stairwell & around 82-F with 65% RH upstairs
    ...as you walk up the stairs, all at once, you run into the hot high humidity air which seems to migrate only a very little bit down toward the first-floor level.

    The upstairs air being hotter would, I'd think, be the main contributing factor for it not affecting the first floor's humidity conditions.

    If it had much of an effect I doubt the little first floor A/C could get the humidity to 50 or 55%.

    When I turn the older 7-EER little less than half-ton window unit on upstairs it usually pulls both the temp & humidity down to around 75-F & 50 or 55%RH - relatively fast; the reading is taken in the bedroom.
    Upstairs, I only cool one small bedroom, the hallway & the bathroom.

    I don't usually condition the upstairs until evening...my electric bills are extremely low the year-round; the worst contributor to my bill is a large dehumidifier in the basement.

  7. #7
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    Shophound, Kdean1, second opinion, motoguy128------ A +++++++++++. Thanks. Helps alot.

    I'm going to update the humdifier downstairs, with one of the aprilaire automatic controls that senses outdoor temp. I get too lazy with the manual one, and when it gets cold i forget to turn it down and get moisture on the windows (I know this is bad for them). I'll make sure the unit is also big enough.

    (udarrell, interesting set up, thanks for posting)

    Steve

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