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June 27, 2010 1:03 PM   Subscribe

Running a room air-conditioner just a few hours a day: Smart or wasteful?

I live in coastal south Florida (hot and humid!). The house has central a/c, but the master bedroom is large, south-facing, windowed on two sides and impossible to better insulate. It never gets tolerably cool. There's a window unit in that room, which I've taken to switching on around 2 p.m. and running till about seven, by which time the room has cooled enough to remain livable for the rest of the night. (So far. It's only June.)
Does my plan make sense, or does switching off that room unit require it to work so hard the next day (removing all the built-up heat) that it more than neutralizes the benefit? You know what I mean? Is there a point at which cooling the room from warm to cool every day is less efficient than letting that small unit run to keep the room cool day and night? This may be an engineer's question and I've seen engineers on here. Thanks a lot!
posted by fivesavagepalms to Home & Garden (14 answers total) 3 users marked this as a favorite
 
I can't speak to the heat-transfer efficiency questions. But I do know there are some things you can use to cool the room so it's not as much work to cool it further. Do you keep all the windows covered with shades during the day? Do you have an exhaust fan that you can run to draw cooler air through the room from the shadier parts of the house? If you can use those strategies to keep the room a little cooler than it is now, you can reduce the energy drain from the AC unit.
posted by Miko at 1:13 PM on June 27, 2010


Whenever the question is: Does it take more energy to run device X around the clock, or to run device X on high for a few hours, the answer is almost always the latter.

I don't have hard numbers for air conditioner window units, maybe someone downthread will be able to supply these details, but consider it in the abstract:

Let's say this window unit uses 100 Metasomething-or-others (Msoo) when you use it on low, and four times as much - 400Msoo - when you run it on high. This is probably not realistic, since running an air conditioner at all incurs a certain base energy usage, and the difference between low and high is probably not nearly as great as the difference between off and low, but let's take 4x for arguments sake.

Case 1 - you run the unit round the clock. Because it's keeping the room at a relatively constant temperature, you can leave it on low. 24 hours X 100 Msoo = 2400 Msoo.

Case 2 - you run the unit for five hours from 2pm to 7pm. Because it has to work hard to cool the room, it's on high for give hours. 5 hours X 400Msoo = 2000 Msoo.

So even with these admittedly outrageously loose numbers (there's no way 'high' takes 4x the energy of 'low'), you're still saving energy by not running the thing around the clock.

my apologies to those who can actually talk intelligently about energy in real metrics like BTUs, EERs etc and who are likely to show up shortly and make fun of my Msoos
posted by Two unicycles and some duct tape at 1:18 PM on June 27, 2010 [1 favorite]


the answer is almost always the latter

Err, and by latter I mean former. Running 24-7 on low > running 5 hours on high.

Clearly I need to turn my own air conditioning up higher cause my brain is being poached.
posted by Two unicycles and some duct tape at 1:20 PM on June 27, 2010


Response by poster: To clarify, I only run it on low fan, turned to low-cool. The window shades are shut all day. A big-bladed ceiling fan runs all day.
posted by fivesavagepalms at 1:22 PM on June 27, 2010


You can answer the question with some certainty for $20.
posted by cjemmott at 1:37 PM on June 27, 2010


Response by poster: Sorry this is starting to look like chat, but I think I just figured out my own answer, now that I've given you (and myself) all the relevant data. Running it five hours a day on low seems a lot smarter than running it 24 hours a day on low. Unless, once again, I'm missing something.
Thanks so much for your help!
posted by fivesavagepalms at 1:40 PM on June 27, 2010


Ah, but in Florida, you also have the problem of humidity. If you're not running an A/C full time by this time of year, you're probably setting yourself up for mildew. Replacing all your stuff and fixing nasty home damage is probably more expensive than running the AC. (Mom-in-law just went through repairs in a rental unit where they were trying to save money by not running the A/C. She was furious and the tenants were liable--and the parts she tried to do herself gave her a several-weeks long asthma attack, when she hasn't suffered from asthma for decades.)
posted by galadriel at 1:53 PM on June 27, 2010


I have a nearly identical situation at my house. I have a huge back south facing room. I tried it both ways. Running the Central A/C at a slightly higher temperature to maintain cool all day for one month, the next month I ran only the window unit when I was in the room and in texas in midsummer, it has to be on full blast. The difference in the expense was huge. The central A/C all the time was nearly double what it was to just run the window unit for that room while I was in there. So, according to Texas Electric the window unit running is a better option, as far as using less electricity.
posted by ExitPursuedByBear at 2:08 PM on June 27, 2010 [2 favorites]


south-facing, windowed on two sides and impossible to better insulate

Are you sure?
posted by nicwolff at 2:27 PM on June 27, 2010


On most window units, running it on "low" just turns the fan lower. The compressor still runs the same speed. Try running it on high and see if it doesn't need to be on as long.

And shut the blinds/curtains/shades.
posted by gjc at 3:13 PM on June 27, 2010


As a lifelong resident of the South, I can say I've never seen a window unit that had different power levels for the compressor as opposed to the fan. As gjc mentioned, the settings just control the fan speed.

Window units don't mildew even when run intermittently unless something else is wrong, such as a condensation pan leak, so I wouldn't worry about that.

I've often wondered your exact question but never took the time to gather data, but here's how I do it: If there's a temperature control, set it higher during the day so that the compressor doesn't come on as often, then drop it back later when you are in the room. If there's not a temperature control, just leave the fan on during the day and kick the cooling on later.
posted by Mr. Gunn at 3:33 PM on June 27, 2010


Who said anything about the A/C mildewing? I was talking about the *house*, and everything in it. It's really nasty when mildew starts taking over a house. It gets into clothes, furniture, WALLS need to be torn out and replaced... and if it's your house, or if it happens because a tenant is neglectful of the house (ie, letting humidity and heat build up), it can be seriously expensive.

Basically, in FL, if you don't have an AC running all the time from about June to mid-Sept, you should have heavy-duty dehumidifiers all over the house. At which point...why not just turn on the AC and enjoy it?
posted by galadriel at 4:04 PM on June 27, 2010


Response by poster: I think we're getting off-point here. The central a/c cools the entire house and keeps mildew at bay, it just doesn't cool this one room enough to make it comfortable sometimes.
Thanks for the new knowledge that "low" doesn't turn down or turn off the compressor of this wall unit; I hadn't thought that. I think I was thinking of the "temperature control" that Mr. Gunn was talking about. It's called "thermostat" on the dial--I know what that means--and I've got it set about 4.5 on a scale that goes up to eight. The "master control," which I now take to mean "fan," is set at "low cool."
Damn, I never thought this was so complicated! Just wanted to lie down for a half-hour around 5 p.m. with a hot cat on my chest.
Cats run hot, don't they? She doesn't seem to care what temp the room is.
posted by fivesavagepalms at 4:40 PM on June 27, 2010


"Does my plan make sense, or does switching off that room unit require it to work so hard the next day (removing all the built-up heat) that it more than neutralizes the benefit?"

Everything else being equal a higher evaporator temperature (the inside part of your window A/C) actually makes the unit more efficient.

"s there a point at which cooling the room from warm to cool every day is less efficient than letting that small unit run to keep the room cool day and night?"

If you are using the same equipment to do the cooling in both cases? No.

PS: running a ceiling fan in an enclosed room heats it rather than cooling it. Unless someone is in the room, and would therefor gain the benefits of increased evaporative cooling, you should turn your ceiling fan off.
posted by Mitheral at 5:14 PM on June 27, 2010


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