When is it more efficient to open your house's windows than to use the A/C?
July 6, 2010 6:50 AM   Subscribe

What is the summertime temp/humidity differential that makes it more efficient to open the windows and turn off the A/C in one's house than it is to keep the house closed up?

When we bought our first house, we got quite a lot of advice from inspectors and contractors, including guidelines on using the central air-conditioning system efficiently during the Summer. Our contractor said that you should keep the windows closed at night, even when it's cooler outside, because the A/C works to reduce the indoor humidity as well as the temp. Letting in all that humid air overnight means the compressor will have to work even harder the next day.

While this makes sense, there must also be some tipping point at which it's more efficient to open up every window and flush out the house with cooler outside air. What is that tipping point? What factors need to be measured, and what math needs to be done? I'd like to keep our cooling costs down and also be more comfortable in our house.
posted by fishpatrol to Home & Garden (13 answers total) 4 users marked this as a favorite
I think it would be a very complex calculation. There are two main quantities you'd have to compute: how much more energy it will take to cool humid air the next day, and how much energy you would have otherwise spent with the AC running at night. For the first, you'd need the difference in humidity at the point that the AC comes on the next day, the total volume of air, the total rate of natural heating (e.g. sunlight through windows, appliances, people, etc.), and the efficiency of the unit. For the second, you'd have to know the temperature differential between indoors and outdoors during the course of the night and the total insulation factor of the house -- these two would tell you how fast the house would have cooled off on its own with the windows closed. Then you'd need all of the stuff from the first part but at night instead of day to determine how much energy the air conditioner would have had to spend. All in all that is a TON of data and there's really no way to get it unless you instrumented the whole house and ran a bunch of tests.

You could also do something where you take meter readings after trying it both ways, but for that to be meaningful you'd have to have similar weather conditions for both days and you'd also have to make sure that all other electrical loads for those days were comparable.
posted by Rhomboid at 7:21 AM on July 6, 2010

This is how we do it.

If we are comfortable we leave the windows open. When it stops being comfortable we close the windows and turn on the AC.

The magic formula is "AC run time = 0." I guarantee that will minimize your cooling costs :) Really, there is no magic formula. 78 for person is comfortable and unbearably hot for another. How much southern exposure does your house have? Any shade trees covering the windows? Are the windows aligned with the prevailing breezes? Do you have an attic fan? Do you have ceiling fans? How well insulated is the house? Is the house on a hill to catch the breezes, or in a knoll where you won't get as much of a breeze?

These all will affect the comfort level of the house at any specific outdoor temperature.
posted by COD at 7:24 AM on July 6, 2010 [3 favorites]

It's completely up to your own comfort level. Yes, your AC also acts to dehumidify the air. But, again, what level of humidity you are comfortable with is an individual call. We open the windows as often as possible at night. We far prefer the night air. But, if it's really humid (which it can be here in Indiana) we close things up and run the AC. We have no chart of figures to follow. We step outside, gauge the temp and humidity and act accordingly.

You contractor is talking as if letting in the night air is akin to opening a fire hose through your window. That's crazy. If it's that wildly humid at night, you probably won't be comfortable and will keep the windows closed and the AC on. Frankly, it makes me wonder if the contractor feels your AC system might be undersized for the house and wants to avoid any problems he might have to fix.

There is no magic math that can be done to give you some exact bright line for you to follow. There is a general range of temperature/humidity combinations that define a large generic comfort zone that most people will fall into.

Trust your gut. If the night air feels lovely and comfortable, open the windows. If not, close them and use the AC.
posted by Thorzdad at 7:26 AM on July 6, 2010

I think Consumer Reports says that a fan is unable to wick sweat off your body (ie, cool you) if the humidity is over 70%. So if your windows are open but shaded (light colored panel curtains work nicely for this, and most of the breezes still get through) and the humidity is under 65% or so, fans could keep you happy.

Personally, I am in the camp of keeping the A/C on all the time, because any humidity drives me nuts, and it does feel as if when humidity gets into the furniture, etc it takes 2 days of running the a/c to dry out again.
posted by MeiraV at 7:48 AM on July 6, 2010

The reduction in humidity is a byproduct of cooling, the AC doesn't switch on just to reduce the humidity, but rather in response to temperature.
posted by electroboy at 8:33 AM on July 6, 2010

That is true, however humid air takes more energy to heat or cool than dry air, and when the AC does switch on because the temperature has risen above its set-point, it will consume more electricity cooling humid air than dry air.
posted by Rhomboid at 8:43 AM on July 6, 2010

Without hauling the thermo textbook out, I'd guess that the difference between the amount of energy required to heat/cool air between say 50% RH and 90% RH is probably pretty negligible.

You could definitely make the argument that a more humid house might make you set the thermostat back, but I'm not sure the energy difference would be noticeable.
posted by electroboy at 9:30 AM on July 6, 2010

Response by poster: Yeah, it sounds like I'd have to turn the house into Biosphere 3 to be most efficient (and to know how efficiently the system was working). I'm hoping for a simpler method. The message I got from Mr HVAC was that the temperature gauge may say it's cooler outside, but it's not actually cooler once humidity is taken into account. As far as HVAC systems are concerned, what is a more complete definition of "cooler"? If I could easily calculate when it was cooler or warmer outside, using that broader definition, I'd open and close windows accordingly and call it a day.

I've considered using Heat Index, but it can only be calculated when air temp is greater than 80 degrees. Here in Southern Ohio temps often dip into the 70s at night, but the humidity stays high. Because it's a very rare day that feels cool outside, I'm thinking more about opening windows at night, which would eliminate elements of sun exposure.

If the outdoor temp and humidity is <>
This likely means buying a device to measure temp/humidity indoors and outdoors. I can imagine going as far as setting up a small server running WSDL to log these measurements and sending me a tweet when the right conditions are met. That's several steps down the line, though, without knowing how to use the numbers.
posted by fishpatrol at 9:48 AM on July 6, 2010

I read somewhere that the airs feels humid when the dew point goes up to around 65. As far as I can tell, they were right. It's almost as if someone turns on the humidity switch.
posted by bentley at 10:55 AM on July 6, 2010

My tipping point is if it doesn't feel clammy out, and there is a good chance I won't be turning on the AC the next day.

Don't think so much about the temperature as much as the comfort. And the ACs job is to make it feel nicer indoors than it does outdoors. So if it's nice out, open the windows.
posted by gjc at 5:44 PM on July 6, 2010

It's most certainly not negligible. Condensing out all that water is not a free side-effect of cooling, it has its own energy cost. You don't see many battery powered dehumidifiers.
posted by Rhomboid at 11:39 PM on July 6, 2010

A dehumidifier is basically an air conditioner, except the cool air and the hot air exhaust to the same place.
posted by electroboy at 1:44 PM on July 7, 2010

Right now I hate this topic, because I lack an A/C in my new apartment, and as long as I find myself elsewhere during the hottest daytime hours, leaving the windows closed has worked relatively well.

However, yesterday I found myself sitting around and the heat started to get to me. It was probably 87 or 88 in the living room. I glanced at the outdoor thermometer, which read 95.

I finally ended up turning on the coldest part of the shower, jumping in for long enough that a non-Oregonian would be ashamed of my water usage (this water just got down here from the mountains, if I don't use it some asshole will use electricity to heat it up anyway), and then going out and riding my bike for miles to feel the hot air blowing the water out of my hair.

It was fucking glorious, but once I got back I was glad I never opened any windows. It was 97 outside and about 85 in the living room.

At night, though, my only complaint is that I can't take the roof off this building like I do with my Jeep. :)

With a non air-conditioned space, this whole calculation becomes vastly more important, if not more complex. It's quite simple, in fact. All you have to do is hire a gigantic helicopter to take your residence from parallel to parallel as you wish.
posted by Clamwacker at 5:54 AM on July 10, 2010

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