My air conditioner causes my room to have brown-outs. Can I fix this?

August 19, 2009 12:30 PM

When my window-unit air conditioner revs its motor, my overhead light dims... then seriously brightens up when the AC slows down. It's annoying. Is there anything I can do?

I just moved into an older (1950s-era, I think) house. My room is cooled by a window-unit AC, which has been running close to full blast all the time lately (it's hot as hell right now). Intermittently, the lights in my room will dim as I hear the AC crank up, only to blink back to full brightness when the AC slows down a few minutes later.

I've thought about replacing the incandescent bulbs with CFLs, which would at least reduce the power load, but since I know CFLs aren't supposed to be great at handling irregular voltage, I'm not sure that wouldn't make things worse. (I did read this potentially-relevant question, but I don't think it helps me in my situation.)

Right now, I think of this mainly as an annoyance... BUT, I'm also worried about other issues down the line. I have my laptop plugged into a power strip ("surge protector" of unknown efficacy) but a desk fan I just plugged into the same strip seemed to slow down when the AC cranked up, which makes me think my computer (and, when I hook it up, my year-old TV) is being subjected to the same kind of local brownout... and that can't be good, right?

Worth noting, perhaps, is that the AC looks pretty old, so maybe I could convince the landlord to replace it with a more efficient (?) modern unit. But the landlord had to be essentially forced into doing basic maintenance, so I don't see him doing anything about the AC unless it dies (and AC murder isn't an option since, again, it's hot).

What can I do?

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

The dimming problem is probably not the A/C itself, but the house wiring. The A/C should be on it's own circuit, and the wiring may well be undersized if this is an older building. Your electronics must be seeing the same voltage drop (since the fan is), but the laptop is probably fine with it as long the battery is in it and even slightly charged. I'm afraid the solution does, indeed, involve the landlord.
posted by TruncatedTiller at 12:53 PM on August 19

It's not a big deal for your laptop and other appliances to be subjected to the occasional voltage sag like this. They're all designed to handle these things, because dirty power is a way of life in a lot of places with older wiring. Specifically, your laptop power supply is just converting the 120v of AC coming in to some lesser voltage in DC for your laptop to use, and it's likely able to operate in a range of 100-140 volts input. If you look on it it will probably have a label that explains. Desktop PCs work the same way.

As for the lighting issue, my first thought was to replace your incandescent bulbs with CFL ones, which I think won't fluctuate in brightness based on the input voltage. I'm not sure about your comment that they aren't great at handling irregular voltage. I've read that they don't work well, or at all, on dimmer switches, but that's a bit different. A dimmer switch varies the input voltage from the full 120 all the way down to 0. I think the drop in voltage you're likely seeing is not hugely significant. It's just that electrical products that use the input voltage directly (incandescent lights, motors) instead of through an isolating transformer will react visibly to these changes.
posted by autojack at 12:53 PM on August 19

put your computer and other valuable electronics on a battery backup unit.
posted by HuronBob at 12:54 PM on August 19

You can buy dimmable CFLs.
posted by bfranklin at 1:04 PM on August 19

This has happened at every place Ive lived at with an AC window unit. They draw quite a bit of power. Your hair dryer does something similiar.

I wouldnt sweat it. Youre not introducing massive surges or anything that could damage your electronics.


Worth noting, perhaps, is that the AC looks pretty old, so maybe I could convince the landlord to replace it with a more efficient (?) modern unit.

I doubt thats going to help. A new one might use 5-10% or so less power and thats not going to fix the issue when you have dimming like this. Id try a CFL and see how well it works out.
posted by damn dirty ape at 1:15 PM on August 19

My portable AC (not a window unit) causes the same thing to happen in my old apartment. I bought a UPS for my computers and it's been fine for the last two summers.
posted by utsutsu at 1:40 PM on August 19

a quick fix might be this: having an a/c, a computer, a light, and whatever else you may have in the room is probably drawing way too much current for old wires to handle. a TEMPORARY solution would be to remove all unnecessary appliances from that circuit and redistribute them around your house. if you're in a small apartment, you may be out of luck, but otherwise, this should work for a couple of whiles (maybe even the rest of the summer, depending on where you live).

it sounds like your house, since it is old, was never intended to handle the amount of current modern appliances (a/c, computer, kitchen stuff, hairdryers, etc...). this is pretty common, and is not so easy to fix. talking to your landlord is probably the only long-term solution to the problem.
posted by chicago2penn at 1:43 PM on August 19

You can call an electrician and have them wire up a new outlet near the window that's on its own circuit. I don't think that would be terribly expensive and should fix all the issues.

And as autojack said, computer power supplies are switchmode, which means they are self-regulating, such that the output voltage does not vary when the input voltage sags. Many switchmode power supplies can take just about anything from 100 to 240 volts and still regulate a fixed DC output.
posted by Rhomboid at 2:51 PM on August 19

A new circuit may not solve the "problem". Motors have effectively infinite resistance at the moment they start. They draw a ton of current for that first instant. This drops the voltage everywhere else. Fact of life...I had an a/c unit that was blowing the living room circuit, so I pulled a dedicated line from the 200A service in the basement. The lights still dim a little.
posted by notsnot at 6:07 AM on August 20

Dimmable CFLs are expensive and should not be necessary for your situation. I don't know whether the light_output/Voltage curve of a CFL is more favorable than that of an incandescent bulb, but it might be. I'd say it's worth trying and even if they don't dim less, you'll still save energy.

Your laptop computer should be fine. Most laptop power supplies are rated for input Voltages between 100 and 240, as the standard Voltage in Japan is 100V. Even if the Voltage dips much below 100, which is unlikely, laptop power supplies tend to have quite a bit of capacitance on the DC side, so it's unlikely that the actual power delivered to your laptop will fluctuate much. And, as you say, even if it does, the laptop can switch to battery momentarily to ride out any interruptions.

Motors have effectively infinite resistance at the moment they start.

[Begin pedantic electricity mode]
Motors have close to zero resistance (impedance) at the moment they start. This causes the motor to draw close to infinite current during the moment of starting. After a motor starts, the impedance rapidly increases and the current draw decreases to the motor's steady running current.
posted by Juffo-Wup at 6:48 AM on August 20

Thanks for the help, and the electricity lessons. I think I'm going to try the CFLs and see what happens. I checked the transformer on my laptop, and it does in fact say 100-240 V, so that was good to know.
posted by SuperNova at 3:52 PM on August 24