How do I get my window AC to work when it's cool outside?

I have this frustration every spring and fall. My AC units work well all summer (so I know they are capable) and then as soon as we get some cool nights, they decide (apparently) that it must be cool enough inside as well, and stop doing the job.

Right now it’s in the 40s outside, but in here it is 86 degrees. :frowning: I have the AC cranked all the way up, but it’s doing no good. I know from the past couple days what will happen if I leave it cranked: later in the day, if I’m not in the room to monitor it, it will get a little warmer outside and then the AC will start blasting and the temp will go down to like 65 (whereas I am just looking for something in the low to mid-70s).

Couldn’t they come with some kind of little thermostat connected to a wire or something that you could hang on your wall a couple feet away?

Err, open a window? How in the hell is it 86 inside if it’s 40 outside? Do you have the heat on?

I’ve never opened a window in this house since I moved here over a decade ago (I think they are probably painted shut anyway), and I don’t plan to start. (I have serious hay fever and lots of air purifiers that give me relief indoors.)

I suppose it’s 86 inside because of cooking, the fridge and dehumidifiers running, etc.

Probably more because the building slowly absorbs heat all summer and re-radiates it into the interior. It’s common for a house to stay cool for the first month of summer weather (and warm for the first month of winter) until the structure gains or loses enough heat to affect the interior even when nothing else is going on.

If you have to live in a sealed box, either more efficient central air or at least filtered outside air intakes would be a huge cost and comfort savings. Living in a hotbox when it’s 40s outside is… avoidable. Running AC when it’s too cold outside for them to function properly is a complicated waste.

It ought to work very well-it should be very easy to dump heat outside.

The problem seems to be that if if it comes on when the outside is very cool, it will overcool the room/house when the outside temp goes up. I’ve had such quirky setups that defied simple operating logic.

But arranging to pull cool air from outside, filtered as necessary, would be far more cost-effective and beneficial to the interior environment - fresh air for ventilation and cooking odors and humidity and all those things. I’d look into that. Leave the AC set for normal cooling when it’s warm outside and in.

Makes sense.

I suppose it’s possible that it would be a cost savings over the long run, but when you live paycheck to paycheck, you can’t come up with money for things like that. We barely afforded the window AC units (and BTW, that does remind me that it isn’t quite true that we have never opened the windows: we did so to install the air conditioners, but that’s really it).

Understood. But a window fan with a HEPA filter over it need not cost too much, and the cost would probably be offset by lowered electrical costs.

Well, that’s intriguing. I thought HEPA filters (like those inside my air purifiers) were complex, boxy things. You’re saying they are thin enough they can just be laid over a window fan?

AC compressors are not designed to operate in cold ambient temperatures. It has to do with the compressor lubrication failing in cold temperatures - if the compressor runs in this condition it can seize up or be otherwise damaged. AC units that need to run when it is cold outside (like one we had for a server room in one of my previous offices) have special cold-weather compressors (I think it was just a small electric heating coil added to the compressor to increase its ambient temperature).

Interesting. So that I guess would be an advantage to those portable units that have a hose that connects to the window.

Well… if you can go spend $3-400 for one of those, it seems like a fitted window fan might be within your means and have more benefits. Your existing AC can run in warm weather, and fresh-air ventilation in cool.

If you absolutely have to AC the interior of your house, no matter the conditions, then that’s a different issue.

HEPA defines the filtration, not the size and shape. Pick one that suits you.

There are all-in-one filtering fans, but I was suggesting a cheapish DIY solution of fitting a HEPA filter over a suitable window fan. Both items are Home Depot/Lowe’s standards. You can spend more for a packaged solution, too.

It depends on how serious your exposure issues are. If you just want to limit dust and pollen in the house to reasonable, low-reaction levels, a simple solution will work. If you need something closer to boy-in-a-bubble air purity, a nickel and dime solution (any of them) isn’t going to help. You’ll really need a sealed house with central air that has an above-standard filtration system.

I mean, I do go outside*, so it doesn’t have to be boy-in-the-bubble.

*Although if I’m suffering pretty bad by the time I get home, I immediately strip off my clothes, put them in one of the plastic bags I keep near the door, and jump in the shower.

Thanks, Marvin. I had thought that the compressor would keep itself warm by running.

AC is a carefully balanced system, unlike heating. We had the attic blower go out on our central air, and only noticed when the outside unit started growling and grumbling, trying hard to re-compress coolant that had not gained any heat or expansion. (Then we noticed it was kind of hot and sticky.) If the condenser goes below certain limits, the cycle will be out of balance and the best thing that can happen is that the compressor pops a thermal limiter.

Does the window AC unit have a “blow fresh air in from the outside” setting? And is there some way to rig up a filter (if the OP’s allergies are so bad that he doesn’t want to be blowing unfiltered air indoors)?

As others said, the window unit may be shutting itself off if its too cold outside. However, if that’s the case, it should come back on as soon as it gets out of the 40’s. IIRC 40 degrees F is usually the cut off for a regular AC.

I also think your best bet (and by far cheapest) is to use a box fan in the window. However, you’ll have to get two windows open for it to work so you’ll have to find a way to get another one open, preferably somewhere at the other end of the house so there’s a cross breeze.
I’ve found the best thing to do is to have a box fan in the window pointed out. I do that for two reasons, first, I don’t end up with my screen being filthy or bugs being sucked into the house and secondly, I can crank the fan all the way up and not have it blowing stuff all over the room. Also, you can put this fan in a back bedroom and not have to listen to it.

As far as your allergies, you could grab one of those HEPA furnace filters from Home Depot and find a way to wedge it into the open window that air is being drawn into. The whole project, fan and filter, will probably be $50. Maybe more if you end up needing a second ‘intake’ window and HEPA filter because of the restriction the filter creates.

But, honestly, after the startup costs, the fan is will be about a 10th of the cost to run than the window unit. if the difference from outside to inside is 80 to 40, it should cool the area down pretty quickly.

You could probably rig up a heat exchanger for times like this. Take the window AC out and put a piece of sheet metal in it’s place. The metal will get very cold from the outside air. Blow a fan against the metal and it will cool the air in your room.

Do you have single-pane windows? If so, the glass would probably be cold enough to do the same thing. A few fans on low arranged so they blow air across the surface of the glass would also cool the air.

If your window AC is shutting itself off because of the cold, the blower fan which stays on is actually heating up your room. The heat from the blower motor is warming the air when the AC isn’t on.