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  1. #1
    Join Date
    Jun 2008
    Posts
    100

    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.
    Laura
    Last edited by plarian; 06-26-2008 at 08:57 PM.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,074
    Unfortunately. No one can guarrantee that an A/C will keep your humidity at 50% or less under all conditions.

    You will need a dehumidifier during the milder days to keep the humidity at 50% or less.
    I run my A/C when its 68°F outside.
    If it were 65°F outside and humid inside, I would put up with the higher humidity for that day.
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  3. #3
    Join Date
    Jun 2008
    Posts
    100
    Quote Originally Posted by beenthere View Post
    Unfortunately. No one can guarrantee that an A/C will keep your humidity at 50% or less under all conditions.

    You will need a dehumidifier during the milder days to keep the humidity at 50% or less.
    I run my A/C when its 68F outside.
    If it were 65F outside and humid inside, I would put up with the higher humidity for that day.
    So do you find that the days when it's too cold to run the AC, and too warm for the heater to dehumidify, are not very common? If it were only a few days here and there, it wouldn't matter.
    Laura

  4. #4
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
    Posts
    2,013
    Sounds like you've done some homework. The thing that you might not have had the "lightbulb" moment about yet is that there is usually not a magic system out there that does everything for every house and every homeowner.

    Best thing to do when starting on any path to improve your comfort in your home is to improve the home itself. Anything that you do to reduce the infiltration of outdoor humidity, air and heat in and out of your home will also reduce heating and cooling costs, a win win situation. If you put off doing these things until after you've installed HVAC equipment the chance is almost 100% that the equipment will be to big and won't provide the comfort you were looking for and paid for.

    The two stage air conditioning outdoor units on the market matched with a variable speed air handler/furnace being controled by a good control/thermostat will give you the best there is available out there to do what you want with your central HVAC system.

    I understand that you don't want to spend money on both a dehumidifier and a new HVAC system. Can it be guaranteed that you will not have to? I'd have to answer that No. Again it comes down to the home, how you live it, the day to day actual conditions outside and inside your home and many other variables.

    Your top floor will always cool less than the lower levels unless the ducting system was installed properly in the beginning. Sometimes this can be solved with modifications that are to numerous to list and are best left to someone that actually visits your home and sees first hand what you have and can best figure out a solution that you can agree to.

    Good luck with your quest to resolve your issues. I'm sure others here will also have plenty to say on the subject.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,074
    The days its too cool for A/C/ in my area, the humidity is not that high that its uncomfortable.
    After it drops below 65, the humidity drops and no problem with high humidity.
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  6. #6
    Join Date
    Jun 2008
    Posts
    100
    If I wanted to keep the humidity below 50% fairly reliably - it would be ok for it to be over 50% uncommonly - could I probably do it with a central AC and a small portable dehumidifier? Is it an accurate guess that the window of temperatures where it's too cold to run the AC to dehumidify and too warm to dehumidify with the heater, is a narrow window and doesn't happen that often?

    Quote Originally Posted by firecontrol View Post
    Best thing to do when starting on any path to improve your comfort in your home is to improve the home itself. Anything that you do to reduce the infiltration of outdoor humidity, air and heat in and out of your home will also reduce heating and cooling costs, a win win situation. If you put off doing these things until after you've installed HVAC equipment the chance is almost 100% that the equipment will be to big and won't provide the comfort you were looking for and paid for.
    Unfortunately, the energy efficiency improvements are slow, incremental things, done over many years. Reducing the humidity is important, maybe crucial for my health. I'm sick a lot with allergies, I was healthy in dry Southern CA, and here in NY I've been so sick (too sick to work on home improvements, a lot of the time!) So it just can't wait. Dehumidifying is also important for buildings around here, your house can get eaten by carpenter ants if you don't. I could try to get AC that's at the low end of what I need, anticipating that I'll do energy improvements in the future. (better insulated door, plexiglass soundproofing panels for windows, more thoroughly insulated attic I'm likely to do; maybe, white metal roof to keep it cool, or attic ventilation fan to replace passive roof ventilation)
    Laura

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