my home, the oven

July 14, 2008 4:38 AM

which uses less energy: (a) running one window air conditioner at higher power, or (b) running two window units at lower power?

my apartment gets insanely hot due to the vacant apartment below, which obviously doesn't get air conditioned. the other day i got home and my apartment was 95 degrees and it took a good 2-3 hours to cool down to a comfortable temperature. (the downstairs apartment seems to collect heat--my floor is always really warm and the underside of a throw rug was actually hot.)

folks are moving in downstairs in about 4 weeks, but in the meantime i'm guessing i need to leave an air conditioner on while i'm away at work, just to keep the climate reasonable enough to make the apartment habitable when i get home at night. so what's the most energy-efficient way to do that? run one window unit and use fans to circulate the air, or run both at lower power?

posted by thinkingwoman to home & garden (11 comments total)

That's hard to say without knowing what the energy draw (the wattage) of each of your a/c units is. Likely, running one of them on high will have less draw than two at low power. Keeping low wattage fans on low (especially ceiling fans if you have them) is a good way to circulate the cooler air.

But, as the question stands, I don't think anyone can say without more information. Are both a/c units the same size. Do they have automatic thermostats?
posted by beelzbubba at 5:33 AM on July 14, 2008

Can you get the landlord to put a window fan into the downstairs apartment to vent out the hot air? That might be the most efficient way to handle this.
posted by chengjih at 5:58 AM on July 14, 2008

Let's say both units have the same electric motor. The peak flow is A f³/m for some quantity B W/m. Meaning that you'd want to know if A/2 f³/m draws B/2 W/m. The answer is: por el corazon de maria, no. The amount of energy wasted by pushing electrons through twice as much copper wire, plus more surface contact, plus a marked change in the potential difference between inside and outside air presssure ... that's probably measurable for a 600W window fan running for 2-3 hours. We're not even talking about the shocking amount of electricity required to run a heat exchanger coil for that long. Yikes.

My solution: instead of leaving your fan (or a potential fan underneath your apartment) on all day, set it on a timer. I'm quite sure that Texas heat is oppressive, given your posting history, so good luck!
posted by electronslave at 6:12 AM on July 14, 2008

I can't definitively answer your question about power draw, but I've always assumed that the difference between "High" and "Low" was just one of fan speed.

Every AC I've used has an "Energy Saver" mode, which will bring the room down to a decent temperature and then turn off until it's hot again.

Why not set your AC to something like 80 during the day on that, so you come home to a slightly-warm home and can easily cool it down the rest of the way, without keeping your house nice and chilled while you're gone?
posted by fogster at 7:04 AM on July 14, 2008

There is very little difference in energy use between high and low on a window A/C. The fan turns slower and this reduces the amp draw of the compressor slightly (because of lower suction pressures) but no where near half.

Best thing would be to do as electron slave indicates and put both A/Cs on timers to start a couple hours before you get home. The timers will have to be able to handle a 15 Amp inductive load (IE: a run of the mill light timer won't work). They can be commonly found here for controlling block heaters but that's probably not a source in Texas.
posted by Mitheral at 7:20 AM on July 14, 2008

If you have a second AC unit, why not ask the landlord if you can put it in the apt below you until the new tenants arrive? Would be similar to, but more effective than that suggestion of a fan. Of course, this assumes there's electricity available in the apartment (could you temporarily run an outdoor extension cord?)
posted by timepiece at 8:53 AM on July 14, 2008

Use two at low power.

The power draw isn't the issue. My experience with window AC units (in Portland Oregon) is that leaving a unit on high for an extended period of time results in a freeze-up. This happens when the moisture in the air finally collects in a significant manner on the coils, freezes, and then blocks air flow. This problem doesn't solve itself, the AC unit needs to be turned off until it thaws.

So, I suggest running two units on low, since one unit running on high while frozen and not cooling is definitely not efficient.

Of course, YMMV with the humidity in your area.
posted by terpia at 9:40 AM on July 14, 2008

Could you put window shades up in the vacant apartment? A white shade that blocked out all the direct sunlight would greatly reduce the amount of heat that builds up during the day (look for blackout fabric, it's fairly inexpensive). Are you using shades in your own apartment already?

Also, what is the outside temperature during the day? If the outside temperature is lower than the inside temperature you experience upon returning home, you could open two windows and use a fan to circulate the air. Then when you got home, it would only be as hot as outside (whether that's below 95 degrees, I don't know).
posted by reeddavid at 10:14 AM on July 14, 2008

I live in an upstairs apartment with window units in Texas. I also have a cat. In order to keep her cool during the hottest parts of the day I have a heavy duty appliance timer which I use to turn the living room unit on. We get home about 1-2 hours after it has been turned on and it feels divine (especially after biking home in 95+ degree weather).

If you get a timer make sure that the timer can handle the wattage that your window unit uses.
posted by collocation at 12:14 PM on July 14, 2008

There is very little difference in energy use between high and low on a window A/C. The fan turns slower and this reduces the amp draw of the compressor slightly (because of lower suction pressures) but no where near half.

True, but maybe misleading. I think that modern window A/C units run on a thermostat located at the unit, and running a fan on high will draw cold from the coils (not acceptable terminology for physics class, forgive me) far faster than running the fan on low. This will also result in a colder room, of course.

Not an answer to your question, but a necessary clarification, I think.
posted by Kwantsar at 1:13 PM on July 14, 2008

ok, more information:

both air conditioners are approximately the same size (i believe 6,000 btu, but i'm not sure). they both have variable settings--i can set the temperature between 1 (warmest) and 5 (coldest) and 3 fan speeds. there's no "energy saver" option and no thermostat. they came with the apartment, and buying new ones is not in the budget right now.

i like the timer idea, but i am concerned about the state of my electronics and other things at such high temperatures. to give you an idea of how hot my place gets, the other day i accidentally left a cup of hot coffee on the kitchen counter that i didn't have time to drink. when i got home 9 hours later, it was still very warm.
posted by thinkingwoman at 2:44 PM on July 14, 2008