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  1. #1
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    Mar 2007
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    Default Programmable thermostats and condensation problems

    Some interesting considerations with regard to programmable thermostats and structures with condensation problems - new to me, anyway:

    Setback Thermostats | CMHC

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    Michael Thomas
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  2. #2
    Join Date
    Mar 2007
    Location
    Colorado Springs, CO
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    Default Re: Programmable thermostats and condensation problems

    second problem is the possibility of high humidity in the winter.

    Cool air can hold less moisture than warm air, so the relative humidity (RH) rises as the air cools. For instance, house air at a reasonable 35 per cent RH at 22°C will see an increase to 50 per cent RH when the same air is allowed to cool to 16°C (6°F) {sic - 16°C = 61°F}. This can lead to condensation on windows and walls (for instance, in closets or behind furniture).

    Basically, you are creating a more humid environment, all things considered, when you allow the house temperature to drop significantly.
    Hmmm. I'm not sure I agree with this line of reasoning. Sure, the cooler air will be more humid (i.e., a higher RH) but the amount of moisture in the air remains the same.

    Water will condense on a cold surface if the air drops to or below the dew point of the air. Assume the article writer is correct - air at 71°F and RH of 35% will have RH of 50% at 61°F. There is the same amount of moisture present in the air at both temperatures. The difference is that the cooler air is able to hold less moisture than the warmer air.

    I don't see how letting the air cool down a few degrees inside a house will cause moisture to condense on windows and walls. If a window or wall is cold enough to condense moisture out of 61°F, 50% RH air it should be cold enough to condense moisture out of 71°F, 35% air. (One exception is if the surface is exactly at the dew point when the inside air is at 61°F but is ever so slightly above the dew point at 71°F because the surface is warmed a fraction of a degree by the slightly warmer room air.)

    Am I missing something here?

    Last edited by Bruce Breedlove; 12-17-2007 at 09:15 PM. Reason: clarification
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