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Most people that live in cold climates know that you should not turn your heat off when you go away for an extended period of time. Just turn it down so the pipes don't freeze. What about the opposite? If you are going away in the summer should you leave the AC turned on, just turn it up a little bit?

I am sure the best answers depend in your climate, if you live in Arizona and temps reach 115 degrees F (46C) every day, you should probably leave it on. So for the sake of a good question and answer, where is the cutoff? Outside temp averages in the 90's, 80's 70's, what should you do?

If you do turn it off and the temp reaches say 100 degrees F inside for an extended period of time, what things could happens to your home and its contents?

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    I think AC is only for the comfort of the occupants, I'm not sure the house would ever get to a temperature that would be damaging. Possibly the refrigerator would have to work a bit harder, but likely not to the point of failure (unless it's on it's last leg already). If you think about it I'm sure there are folks in other areas of the world where temperatures are similar or higher, and they don't have AC at all in their homes.
    – Tester101
    Jul 21, 2011 at 11:57
  • As you said it does depend on climate. In the humid areas (where I live) the mold would start growing real fast in a sealed up home with no AC.
    – 3264
    Jul 21, 2011 at 12:44
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    @TNT: That's an argument for a dehumidifier, not for AC.
    – keshlam
    Jun 3, 2014 at 13:02

6 Answers 6

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A timely question since there is presently a heat wave where I am :-)

I have followed a couple of strategies on this over the years.

  • As a general rule, and lacking any better reason, I turn the AC off if I am going to be out of the house for more than a quick trip. It won't take that long to cool off when I get home.

  • For the bathroom and some other areas where built up humidity can become a problem, I either run an exhaust fan or a small dehumidifier. In my case, the dehumidifier will consume less energy than the AC (local specialized solution vs generalized solution).

  • For larger areas, I run a fan to keep the air circulating, again rather than run the AC. One of my newer AC units has a very energy efficient fan mode, so on that unit, I use it.

To give you an idea of my environment, this week the temperature has been pretty consistently at or above 30 degrees C with humidity adding 10 to that.

In the case of central air (which I don't have), I believe some have programmable thermostats which can keep the environment "cooler", ie 25 deg C, when you're out ... this seems reasonable to me.

As was mentioned here if there are pets to be taken into consideration, or any other reason (such as it gets absurdly hot where you are and food in the cupboards may melt) then running it at a low rate is probably the best answer.

I also make liberal use of drawing shades on my windows and doors to reduce the effect of the sun beating in and warming. This really has a huge impact here.

Another thing I didn't see mentioned was that if you are going away and have high daytime temperatures but moderate relative nighttime temps, you can combine turning your AC down with an electrical timer, so that the AC is turned off overnight. Since you're not going to be home, comfort isn't the main issue. A temperature controlled thermostat like I mentioned on the central air or modern AC units is the best bet IMHO, and I'd set it on a higher-than-normal-but-still-cool range (here that would be 25-27 C, when I'm home I prefer being around 22 C).

Hope this opinion helps.

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  • I should also mention that we have time of day billing here, which makes it much more expensive to operate the AC during a weekday daytime. So the difference is actually quite noticable
    – Stephen
    Jul 21, 2011 at 20:21
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I see a few problems with leaving the AC off for a while, depending on climate.

  1. The AC provides dehumidification. So if you live in a humid climate, leaving the AC off can cause issues in the house (swelling doors and window frames, cracking plaster, mold, etc). You generally want the humidity in your house to be 50% or less.

  2. Stress on your fridge. If the room is too hot, the fridge will struggle to keep your food cool. This may shorten the life of your fridge, or cause food spoilage. (Not sure what the upper operating temperature limit is for a fridge.)

  3. Any pets that you leave behind on vacation. Even if you take your dog or cat to the kennel, what about your fish tank? Will your fish be happy in >100F water? Don't let the temp in your house get higher than what your pets can tolerate!

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    Typical max. ambient temperatures for domestic refrigerators are around 95–100 °F (~35 °C).
    – Vebjorn Ljosa
    Jul 21, 2011 at 15:58
  • True; a lot of people have a second fridge (often an older one they replaced in the kitchen, but it's still working) out in their garage for additional food storage, even down here in Texas. However, the other points are still very good, and even the fridge thing is still relevant; the fridge will work harder in a hotter setting.
    – KeithS
    Jul 21, 2011 at 17:49
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    Another thing to be aware of: corks in wine bottles can and do come out if the wine gets hot enough. If the bottle is stored on its side, that makes for an awful mess.
    – tnorthcutt
    Jul 21, 2011 at 21:37
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There is a proven efficiency in turning up the thermostat for times you won't be home. However, if your home is reasonably well-insulated, your AC isn't running all the time; it just comes on every hour or so for about 15 minutes to maintain the set temperature. Turning the AC completely off and letting the house warm up will then require your AC to run continuously for several hours just to return the house to a comfortable temperature. If there are areas of your house that warm up quickly, you're even worse off.

Generally, you get the most efficiency overall by keeping your AC on, but setting it to 78-80.

Think of it like your car. If you drive smoothly, accelerating and braking slowly, and maintain between 55-65mph, your car is usually getting maximum fuel efficiency. By contrast, hard acceleration and braking and driving 70-80 will drastically reduce your gas mileage. The same applies to your AC; the harder you make it work, the more energy you use.

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    Unlike the car, the AC works with the same effort when it's on, the only difference is how long it's on. Your home doesn't heat constantly to infinity, so the closer the inside temp approaches the outdoor temp, the more the AC usage approaches 0. The longer you can do that (I'm talking about days or weeks) the more you save.
    – BMitch ♦
    Jul 21, 2011 at 20:44
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I live in Phoenix and have turned up the thermostat a few times when on vacation in the summer. I also own a guest house that we leave at 99 degrees (the max on the thermostat) for most of the summer. Here are my experiences.

Leaving the guest house at 99 does not bother it at all. The house feels hot when you enter it but nothing seems to be wearing out/molding/falling apart.The guest house does not contain a lot of stuff, just furniture/bedding/kitchen stuff. It does have a fridge that runs all summer. The house is 1200 sq ft, has old windows and not enough insulation, and uses $50 dollars a month to keep the lights on. Last summer someone came to stay in the guest house and I had to cool it down to 78 (typical temp in Phoenix). It took almost 24 hours to cool the house and it used over $25 dollars in electricity that day (I watched the meter because I was curious).

When I went on vacation last year I turned the A/C in my house to 95 for over a week. I definately saved a few dollars each day in electricity. The worst thing that happened is that a few candles melted and some chocolate in a cabinet bloomed. The ruined chocolate and candles cost more than the dollars I saved.

This summer I turned the A/C up to 90 for a week while on vacation. I saved less and some different chocolate candies that were on a high shelf in the kitched bloomed a little bit. Other chocolates on a low shelf in a pantry were unaffected.

I know people who leave Phoenix for the summer. All of them leave the A/C on. Some turn it up a little, some turn it up a little more.

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    Chocolate blooms either way... sounds like you should pack up the candy and chocolate to the fridge before travelling :)
    – Stephen
    Jul 21, 2011 at 20:20
  • I agree, It was the only thing that seemed to be affected so that is what I wrote about. I don't think the houses are any worse off for the heat.
    – Scott Bruns
    Jul 21, 2011 at 23:37
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Heat can totally RUIN books over time. It will turn them into toast. My mother lost a huge book collection because she turned off the AC when away. Heat can ruin old papers like genealogy, photographs, also any vitamins and medicines that aren't in the fridge will be ruined. Pet kibble or canned food ? Yes of course, heat ruins the food value and I'm sure it turns kibble into rotten useless stuff if it gets hot. Canned food ? You can't turn off the AC or let it get real hot without ruining that too I would think. It only makes sense that just about anything that is left in extreme temperatures will suffer for it, but you might not see it immediately. Like antiques. And as my mother found out, like I said, books.

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We live in So California and My husband and I debate all the time about whether to leave the windows closed in the 90-100 degree F range or leave the windows open, during the day,when we go away. We have a Dog and a cat, and if we go away for a weekend, we have a friend come in a few times each day to check on the animals. We don't leave the AC on for the weekend. My husband thinks we should leave the windows open to allow fresh air in, during the day. My feeling is that the house is heating up more by allowing all that hot air in to circulate. I believe keeping the windows closed is better as our house is nicely insulated and our friend turns the AC on for a little bit when she comes to check in on the animals and then shuts it off and leaves windows closed. Leaving the windows open at night is not an option due to security issues

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