We know that windows should be closed while running the AC, but since the central AC and thermostat of my house is on the first floor, I was wondering whether leaving the window open decreases the efficiency of the AC, thus increasing my electric bill. It's usually the case that the second floor of my house is warmer than the first floor since I have only one zone, so opening the window brings in fresh air.

  • I mentioned above "whether leaving the window open decreases the efficiency of the AC, thus increasing my electric bill." Is this specific enough to reopen this question? Jun 20, 2023 at 22:20

4 Answers 4


So long as you're paying the electric bill you can waste energy however suits you.

Or you could rebalance your system for the summer so it cooled the second floor correctly, and waste less money. And energy.


Physics hates that idea

Doing that would be foolhardy in the sense of unaware of how the technology works... roughly akin to using white-out on a computer monitor :)

Here's the thing. Everybody thinks A/C is about temperature. Not even by half. A/C is also about humidity because of the way humans work. To be more precise, how we cool ourselves. And also how evaporation works.

Our skin is always slightly wet. That's why a stiff breeze feels colder than standing air - that's why wind chill is a thing, and why wearing thin disposable rubber gloves in cold weather feels much warmer than you'd expect. You are deleting the wind chill factor. What is causing that is water evaporation from your skin - for 1 pound of water to evaporate it must absorb 1000 BTU of energy, or 300 watts for an hour. That's a lot!

Even if you are not actively sweating, you are still "wet" enough for wind chill/humidity to affect you. That's why we care about it. Like I say, put on a thin rubber glove and it changes everything.

But evaporation does not work if humidity is high. The drier the air is, the better evaporation works, the better skin cooling works, and the cooler the air feels — get it?

What we feel is what matters.

So removing humidity is a big part of what A/C systems do. Really. Air is very light and easy to cool (0.24 BTU/lb/degF vs water's 1.00), so suppose there is 1600 pounds of air in a 2000 sf house, lowering that by 30 degrees will only take about 12,000 BTU. (i'm moving very fast through some complicated physics here). But if it has 40 pounds of water in it because it came through a window as 90F air at 80% RH, removing that water will cost you 40,000 BTU — that's 3/4 your total air conditioning load!

This. This is why tight houses are efficient. At least in summer.

So I'm not horrified at letting hot air in. Air is cheap to cool. I'm horrified at letting humid air in!

And since humidity and the perception of coolness are so closely tied together, you're defeating your purpose. The humid outside air will feel much warmer than it actually is.

Solve the problem at the source

You've already figured out that the source of the heat isn't the air itself, it's the 300 BTU per square foot of heat from direct sun. Anything you can do to shade or reflect that is going to help. Talk with your paint supplier about the reflectivity or albedo of your house paint. The nearer to white, the better.

Also talk with your roofer and HOA about lighter colored roof options or coatings. The lighter the roof is, the better.

Also, it would be nice if there were some sort of, I don't know, like a panel of some sort on standoffs about 6” above your roof, so air can freely blow under those panels. That would act as a thermal break, so the sun heats up the panels instead of your roof, and the air under the panels carries away the heat before it can soak into your roof. Not sure how you would find someone even willing to do that, it's not like there's some sort of huge sales effort trying to install panels like that on every entertainment channel lol.

  • Ok, the issue is about bringing humid air in and that causing the inefficiency. But I've heard that humid air rises to the top. If that's the case, why would opening a window on the second floor impact the efficiency of the central air that's on the first floor? Jun 15, 2023 at 13:34

If you're not getting enough cool air upstairs, things I find helped:

  1. basics: room vents upstairs wide open, Room vents downstairs minimal.
  2. Likewise with dampers in ductwork in basement. I change my dampers for heating and cooling seasons.
  3. Is the system providing all the flow it can? Is HVAC Filter clean? Are there big leaks in ductwork? I had a remodeler cut off a duct when he put in pass-through and leave it open, pouring conditioned air into the wall cavity.
  4. Is the system providing all the cooling it can? can you check the refrigerant pressures? Is the coil clean?
  5. Could you put a booster fan in one or more upstairs ducts? I did that for my longest upstairs run and it helped.
  6. is there an upstairs return near ceiling? If yes, is it open? If open, try blocking off some portion of the downstairs floor return. if no - you've got a challenge. consider adding one - ductwork through closet or possibly an abandoned chimney

So, the A/C cools the whole house but it is set up so poorly that the second floor remains so hot that opening windows cools it down?

In that case the thing to do is either:

  • Balance the system -- if the A/C has the overall capacity to cool the whole house -- Close all the downstairs vents so the A/C cools only the upstairs. Leave all the windows closed. Leave internal doors open and downstairs will be cooled by convection. If this seems to work you can then fine tune it by opening some downstairs vents and making adjustments until the rooms are all similar in temperature.
  • Make it downstairs-only -- if the A/C is not powerful enough to cool the whole house -- close all the upstairs vents, cover any upstairs return vent with cardboard, and open all the upstairs windows. The A/C will cool downstairs only.

Note that by closing too many of the vents, you could reduce air flow through the system enough to cause new problems especially freezing the evaporator. Pay close attention and consider these steps experimental, aiming to guide you to the right solution.

  • I once read that vents should not be closed more than 3/4 for the system to work correctly. I don't know if it's factual, but I thought I'd throw it out there for an HVAC expert to weigh in.
    – RetiredATC
    Jun 14, 2023 at 2:07
  • @RetiredATC good point. In a properly designed and balanced system that is true. And to be fair, either of these solutions could make things worse by freezing the evaporator. Glad you mentioned it, I'll note it in the answer.
    – jay613
    Jun 14, 2023 at 2:16
  • @RetiredATC, that advice seems related to the potential for excessive back pressure on the system (and therefore increased motor load and reduced air over the exchanger). It's probably worth following.
    – isherwood
    Jun 15, 2023 at 13:27

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