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  1. #1
    Join Date
    Jul 2007
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    23

    Can heaters really keep a loosely enclosed space dry? humidity theory

    Hi all,

    There seems to be a common practice of using simple heat sources for dehumidification. (for tropical climates at least, I guess excess moisture is less of a problem in temperate climates)

    Eg. fixing an always-on lamp inside wardrobes, and various cabinets which are non-airtight. For protecting clothing or electrical equipment from moisture damage.



    I can imagine that when the lamp is first turned on, the interior temperature increases until the heat loss rate equals the lamp power. Relative humidity drops accordingly until the temperature stabilizes. At this stage, this should not be controversial.
    But I do not expect the low-RH to be sustainable, given that the cabinet is not air-tight or moisture tight. My intuition is that moisture would migrate into the heated cabinet until the RH is equalized inside and out. Is this correct?

    Another possibility is that water vapor tends to spread so that absolute humidity is equalized...

    So what is the relevant theory of humidity?

    TIA

    (I am not talking about electrical "dry-boxes" marketed for storing cameras and lenses, those I'm sure have a way of pumping out moisture, and those are practically air-tight. I am talking about a low-tech heater-in-cabinet arrangement.)
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    Last edited by kaon; 02-13-2008 at 10:16 AM. Reason: add diagram

  2. #2
    Join Date
    Apr 2002
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    11,808
    leave the closet door open or undercut it and draw some return from it

  3. #3
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    Apr 2002
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    heat lowers RH but does not change the amount of moisture in the air.

    heat makes mositure evaporate out of materials

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    The answer to the question is that the relative humidity in the box is 45%RH. Provided there is no additional liquid moisture in the box. With a liquid water source in the box, the %RH of the air in the box will rise. If the box has a perfect vapor barrier, the %RH will rise to 100%RH or until the liquid is evaporated. Air leakage or marginal vapor retarder allows moisture to move to the outside which will reduce the %rh of the air in the box. 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"

  5. #5
    Join Date
    Jan 2008
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    Hells Kitchen, Phoenix Arizona
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    warmer air can hold more moisture...So if you have 95% rh at 25c if you increase the temp to 40c the airs ability to hold moisture goes up...The %rh will be less with the same amount of moisture in the air...
    "Overkill is an often underrated achievement", Will Hayden -- Red Jacket Firearms

  6. #6
    Join Date
    Dec 2007
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    Soudern Kal-e-phone-e-ya
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    kinda like the light bulb in the dead refrigerator we use to store welding rods?



    however, the refer is mostly airtight, so that may not be apples to apples.

  7. #7
    Join Date
    Jul 2007
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    23
    Hrdworkingacguy and teddy bear have roughly the same idea.
    The RH inside drops initially that's for sure.
    But suppose the cabinet is only loosely covered, as suggested in the diagram, and we leave it undisturbed with the heater on, for a few days/months until we reach steady state. We assume there is no moisture source inside.

    In case the diagram doesn't display for you, the example figures I used are:
    OUTSIDE:
    temp: 25 degrees C
    RH: 95&#37;


    INSIDE:
    temp: 40 degrees C
    RH: ???


    I think that IF water vapor tends to diffuse across temperature differentials so as to equalize RH, then inside RH eventually increases to 95% as well.

    Any thoughts?

  8. #8
    Join Date
    Apr 2002
    Posts
    11,808
    vpaour pressure is what makes water vapour diffuse. The higher the temperature, the higher the vapour pressure

    moisture migrates away from heat and towards the cold

    Rain gets a brick wall wet. Then the sun comes out, beats on the wall and drives the mositure through the brickl

  9. #9
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,261
    Quote Originally Posted by kaon View Post
    Hrdworkingacguy and teddy bear have roughly the same idea.
    The RH inside drops initially that's for sure.
    But suppose the cabinet is only loosely covered, as suggested in the diagram, and we leave it undisturbed with the heater on, for a few days/months until we reach steady state. We assume there is no moisture source inside.

    In case the diagram doesn't display for you, the example figures I used are:
    OUTSIDE:
    temp: 25 degrees C
    RH: 95%


    INSIDE:
    temp: 40 degrees C
    RH: ???


    I think that IF water vapor tends to diffuse across temperature differentials so as to equalize RH, then inside RH eventually increases to 95% as well.

    Any thoughts?
    The grains of moisture per pound of air are the same on inside and outside of the enclosure.
    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"

  10. #10
    Join Date
    Jul 2007
    Posts
    23
    Quote Originally Posted by teddy bear View Post
    The grains of moisture per pound of air are the same on inside and outside of the enclosure.
    TB
    Thanks, that answers my question.

    If it is true that water vapor tends to spread so as to equalize absolute humidity (by mass) across connected air masses at different temperatures, then the loosely-closed and heated cabinet would maintain a stable state of lower RH inside.

  11. #11
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    Different situation, but when I rebuilt my harley, frame up, I had the frame sandblasted. I have a heated/insulated standalone garage and the frame stayed flash-rust free for the 2 weeks it took for my powdercoater guy to be ready to coat it.
    I also rebuilt the Evo engine the same way, with no rust.
    I kept the garage at 70 degrees the whole winter.
    I did tighten up the garage as much as possible.
    But my Modine heater was the real dehumidifier when it came to rust prevention.
    jogas

  12. #12
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    Apr 2002
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    11,808
    dry winter air infiltrating was your dehumidifier

  13. #13
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,261
    Quote Originally Posted by jogas View Post
    Different situation, but when I rebuilt my harley, frame up, I had the frame sandblasted. I have a heated/insulated standalone garage and the frame stayed flash-rust free for the 2 weeks it took for my powdercoater guy to be ready to coat it.
    I also rebuilt the Evo engine the same way, with no rust.
    I kept the garage at 70 degrees the whole winter.
    I did tighten up the garage as much as possible.
    But my Modine heater was the real dehumidifier when it came to rust prevention.
    jogas
    Add a good dehumidifier for the same rust-free storage for the summer. Maintain <50%RH to eliminate potiential summer rusting or musty odors of quality items. To test the humidity, get a good %RH meter. Accurite at wal-mart or Therma-Stor <$15. RegardsTB
    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"

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