any studies on which is more efficient... A/C or dehumidifier?

Wondering which would be more efficient in this situation...
A/C is set at 76 degrees. Dehumidifier is set at 55% RH. it's 67 degrees outside, approximately, but the house is right at 76 because it was hot yesterday. Outside RH is 80+ percent.
Is it more energy efficient to open all the windows and turn on a fan, and let the outside air cool the house off (thus forcing the dehumidifier to run,) or just leave the windows shut at let the A/C do its thing?
no, this isn't actually a hypothetical... this is exactly what I woke up to this AM. I chose to open the windows because I like fresh air, but on reflection didn't know which was really more efficient.
nate
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On Sun, 24 May 2009 09:29:41 -0400, against all advice, something

I don't have an answer for you, but neener neener neener.

I haven't run anything in my house for days.
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Steve Daniels wrote:

yeah, yeah. I would far rather be on top of a mountain somewhere in PA, or quite frankly where you're at (SWMBO occasionally thinks she wants to move to the PNW as well) but Our Nation's First Swamp does offer the benefit of allowing me to be gainfully employed...
nate
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Nate Nagel wrote:

LOL
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If the windows are open, the energy to run the dehumidifier is about 98% wasted. Otherwise, fans are cheaper to run than AC
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The AC removes moisture from the air. I'd choose C, none of the above.
I'd suggest to turn off the dehum. Cause the dehum pumps heat into the house. It's equivilant to leaving a 750 watt filament heater running.
Use the AC to cool the house and dry it out. That way, your electricity is both cooling and drying. At least you won't be paying for the dehum to pump out heat, and then the AC to move the heat. The AC will have a lower electric draw when it's cold out, cause the compressor won't be working as hard.
At my place, I try to run the AC as little as possible during the day when it's hot. And then turn the AC on in the evening when it's cooling down, outside. Have the AC professionaly serviced and the outdoor unit washed out with chemicals and a garden hose. Will restore the efficiency lost to dirt and crud.
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Nate Nagel wrote:

http://einstein.atmos.colostate.edu/~mcnoldy/Humidity.html
At 67F and 80% RH, the dew point is 61F. At 76F and 55% RH, the dew point indoors is 59F. So the absolute humidity is about the same.
As long as it's at least 5F cooler outside than in, I'll probably open the windows even if the dew point is several degrees higher outside.
I keep a wet/dry bulb thermometer beside my thermostat and turn on my AC as needed, according to the wet bulb. If the house is humid in the morning, I can take care of that by running the AC two or three times for ten minutes or so. The amount I save by letting the house cool at night is more than I use to reduce the humidity.
I don't try to keep my humidity as low as 55%.
A dehumidifier is an AC that dumps the heat right back in the house. If it were winter and I wanted to reduce humidity without losing heat, I'd invest in a heat exchanger, instead.
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E Z Peaces wrote:

I don't need to dehumidify in the *winter* - exactly the opposite.
Summer (well, let's be honest, spring and fall too) here, humidity is silly high though.
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Nate Nagel wrote:

I would depend on the AC to control summer humidity. A dehumidifier warms the house.
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E Z Peaces wrote:

I do for the most part. Still need a dehumidifier in the basement to take the edge off when it's not silly hot outside.
I can't imagine how people lived near DC before air conditioning.
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Nate Nagel wrote:

They wore funny looking triangular shaped hats and powdered wigs.
TDD
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DC has more hot air than most places.
I lived in Philly with no AC as a kid. People did what they could to keep cool, even slept in parks in some city areas I'm told. After we got married, I bought an AC for the bedroom the next year, but we manage one summer with just a fan. A few years later we did the kids bedroom and downstairs. I've not been without it since. We use it a lot less where we now live, but it still gets used and makes sleeping so much better.
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wrote:

    There is no simple answer to the question because you are missing so many parts of the calculation.
You don't know:
* The delta (the change) of the outside air temperature or the outside humidity, it will be changing * How quickly your home will absorb the moisture out of the air, or release it into the air * How quickly your home will absorb the heat out of the air, or release it into the air. * Exactly how long you are going to be in the home and for what times. * How tolerant you are to changes in humidity and temperature and since this will change with how active you are, we will also need to factor that in * Other sources of heat or moisture into your home (this would include heat gain via insulation etc. * Other people who might visit and how often you might open a door or window.
    I'm sure I could find a few more issues like sun light clean or cloudy skies (day and night) etc.
    It really is a difficult task to properly measure it which is why so many sellers of this or that gadget can make all kinds of claims that don't seem to make much sense.
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The enthalpy of the inside air is about 29.8 btu/lb. So, in theory, you're better off opening the windows as long as the enthalpy of the outside air is less than that.
For 67 degree outside air, the psychrometric calculator says that as long as the outside RH is less than 89%, the outside air is "better."
In reality, there are a number of factors which will affect your actual comfort, many of which have been pointed out. So, you'll just have to try it and see.
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What ever makes you feel better, is more efficient. So have a beer.
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ransley wrote:

Oddly enough, there's one sitting in front of me as I type this :)
nate
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