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  1. #1
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,065

    At what outdoor dew point is a dehumidifier not needed in a basement?

    I have been waiting all summer to answer this question. This is 1,700 sqft. finished basement that I monitor. There are two occupants in the home 12 hours per day. No fresh air ventilation or dehumidification this week. The basement was <53%RH for the week. The outdoor dew point average <40^F for the week. This is a cool basement without much space heating yet. At higher outdoor dew points, I needed dehumidification.
    I posted the data from my Hobo loggers.
    Regards TB

    WI 50RH Bsmnt No dehu.pdf
    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"

  2. #2
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,635
    so coooool! (I just got 4 Ecobee's - put 2 in my rental - there is so much to learn from knowing how stuff actually performs.)

    I see you are targeting a 5f air temp delta but maintaining 15-20f. How did you arrive at those decisions?

    How do you guesstimate the temperature of the coldest surface?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,065
    Quote Originally Posted by tedkidd View Post
    so coooool! (I just got 4 Ecobee's - put 2 in my rental - there is so much to learn from knowing how stuff actually performs.)

    I see you are targeting a 5f air temp delta but maintaining 15-20f. How did you arrive at those decisions?

    How do you guesstimate the temperature of the coldest surface?
    I delayed turning the heat on till this week. The smell of the basement is a good indication of excess dew point. Usually 50%RH in the basement is enough for carpet on the concrete.
    I am attaching a illistration of the balancing effect of air moving through the home with a given amount of moisture load.
    Changing any of the inputs will change the %RH in the home. Controlling the %RH requires adding or removing moisture from the air in the home. Dehumidifier or humififier!
    Regards TB

    Microsoft Word - Moisture balance in the house.pdf
    WI 50RH Bsmnt No dehu.pdf
    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
    Sep 2002
    Location
    I don't know
    Posts
    2,897
    Interesting data - thanks.

    The type of moisture protection around the exterior side of foundation, as well has precipitation has a huge impact from my experience.

    A basement can still be damp even when the outdoor dewpoint is well below 50F due to moist soil and/or precipitation.

    With no damp/waterproofing - typical of older foundations, it might be necessary to occasionally run a dehumidifier or ventilate when it rains well until december or so.

    -----------
    One thing I'm curious about is how much latent heat consumer grade dehumidifiers convert to sensible heat relative to total energy consumption.

    Say a 50 pint unit consumes 500 watts when the compressor is running - about 1700 BTUs/hr.

    How many extra BTUs/hr would be released at 75F/50% humidity?

    What's the effective cop when it comes to heat production? Example: 0.2 units of latent heat converted into sensible heat would be 1.2

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,065
    End of Oct in WI shows low dew points and then switch to +60^F. Interest effect on a ERV home with .4 ach CO2 control. This a foamed walls home.
    The inside %RH tracks the outdoor dew point to the point where the dehu is activated.
    Regards TB
    Data by Hobo Loggers

    10 23 12 erv home 2-789823-16-160.pdf
    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"

  6. #6
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,635
    I've started an album of chart screenshots from my Ecobee portal. Look what can be learned by tracking how a home and it's equipment interact with each other and the environment.

    This thing tracks indoor, outdoor temps, run time, and RH.

    Each vertical red line represents up to 5 minutes of run time.
    Green line is indoor temp
    Black is outdoor temp.
    Dotted red is set temp
    Grey/light blue is RH


    Really interesting to see cycle time and impact. TB - interested in your thoughts about rh line.

    Name:  ecobee%u00252B%2Bweb%2Binterface%2B-%2BGoogle%2BChrome%2B10132012%2B105220%2BPM.bmp.jpg
Views: 61
Size:  32.9 KB
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,065
    I could not blow up the picture enough to see the detail. Any chance you could post a PDF of the graph.
    WI has +60^F outdoor dew point this week, it headed your way.
    I have data on effect on a basement without dehumidifier. The house is getting about 40-70 natural infiltration this week. Basement zooming up.
    You can see the outdoor dew point drive the indoor dew point/%RH. Aggresive air change would cause higher indoor %RH. OK for a day or two. The lag is the material of the house absorbing moisture. Mechanical ventilation and a good dehu is one of the near perfect solutions. This WI's high dew points last gasp.
    Attaching PDF of data.
    Regards TB
    Attached Images Attached Images
    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"

  8. #8
    Join Date
    Aug 2006
    Posts
    2,557
    It's not the Outdoor Dew Point you need to worry about. It's the Indoor Dew Point, you need to find your coldest indoor basement surface Temperature. It could be a window pane or frame. Masonry wall or floor. It could also switch places over the seasons. I.E. Floor in Summer, Window in Winter. My Basement is much dryer since I put deeper sump pits in. Went from 24" to 36" Also put a Watch Dog system in so I don't flood during storm power outages.

  9. #9
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,635
    So even pics at google + are too low? Hmmm, good to know.

    I can get XML. Please send me your email.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

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