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  1. #1
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    Jun 2008
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    AC for dehumidifying

    Some people were telling me that I shouldn't need a dehumidifier, if I have central AC installed, optimized for dehumidifying. Is this true? What they said were things like, you get a DC motor for the AC that can run at quite slow speeds, and you get an AC that's just barely big enough to do the job, so it runs a lot.
    But first, my situation. I live in NY state. It's very humid here. In the spring and fall it's cool and humid, and in the summer it's hot and humid. I don't have any AC or dehumidifier now. I have a 2-story house, about 1000 sq ft to condition, 660 sq ft on the top floor, which gets hot, and half that in the basement/garage, which doesn't get hot except on the hottest days, it's partially in the ground. It was built in 1955. I have allergies so I want to keep the RH at least below 50%, maybe even 40%. Even when it's cool.
    Can I possibly do this with just a central AC? Without having the AC and the furnace going at the same time?
    What I don't get about that claim is the simple fact that sometimes, it's too cold to want to run the AC - but it's damp. And too warm for the heating system to remove enough humidity.
    However, the water-holding capacity of air increases very fast with temperature. I looked at a graph of it. That means, that in theory, if you had air at 50F and 70% RH and you warm it to 70F, it's now at 37% RH.
    Of course, it won't actually be that dry, because it would constantly be pulling humidity in from the environment.
    And if you start with air at 60F and 70% RH and you warm it to 70F, it would now be at 49% RH, again assuming it's not absorbing moisture.
    But what that would mean, is that the temperature window in which it'd be too cold to run the AC, and too warm for the heater to be a good dehumidifier, is a narrow window. And that maybe if you're willing to put up with some discomfort, sometimes running the AC and making the house too cold, and sometimes heating it a bit too warm for comfort, in order to dehumidify, maybe you can use just the AC and not a dehumidifier.
    So what do you think?
    I'm not sure what I think about having an AC that's designed to run nearly all the time.
    I would like to not have to buy both central AC and whole house dehumidifier.
    I have read various things on this forum by the way about AC and dehumidifying, and they still left me wondering.
    Last edited by plarian; 06-26-2008 at 08:57 PM.

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