# Leaving the AC on when I'm away **URL:** \ **Category:** In My Humble Opinion\ **Created:** [July 24, 2011, 9:15pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210 "2011-07-24T21:15:36Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [July 24, 2011, 9:15pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/1 "2011-07-24T21:15:36Z")
I’ll be out of the country for three weeks in August. My house has central air, and if the weather were more moderate I’d simply turn off the AC. But considering the weather we’ve been having lately, I’m re-thinking my policy of leaving it off with all the windows closed. And I’m really not sure what my fear is . . . spontaneous combustion? Burning of houseplants? So if the outside temperature is in the upper 90s, how hot could my house get, assuming it has a normal number of windows, facing all directions? And what possible damage could be done? The last time I left in this kind of heat, I set the thermostat at 90, just in case. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [July 24, 2011, 9:34pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/2 "2011-07-24T21:34:06Z")
> [@](#): > > The last time I left in this kind of heat, I set the thermostat at 90, just in case. That’s what I do when I’m on vacation. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [July 24, 2011, 9:35pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/3 "2011-07-24T21:35:48Z")
If you do that out here, all your candles will melt. I’d set it at 90 - that’s probably safe. ---
**Author:** ![chiroptera](https://avatars.discourse-cdn.com/v4/letter/c/df705f/32.png) [@chiroptera](https://boards.straightdope.com/u/chiroptera)\ **Post date:** [July 24, 2011, 9:53pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/4 "2011-07-24T21:53:33Z")
Same as winter absences…I leave the heat set at about b50 in winter so the pipes don’t freeze and the houseplants stay alive; I’d leave the thermostat set at 80-90 in summer so things don’t melt. I’d have someone come check on my house every few days, just in case of whatever. A neighbor of mine has a key to my house, peace of mind for me. ---
**Author:** ![Al\_Bundy](https://avatars.discourse-cdn.com/v4/letter/a/e79b87/32.png) [@Al\_Bundy](https://boards.straightdope.com/u/Al_Bundy)\ **Post date:** [July 24, 2011, 10:26pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/5 "2011-07-24T22:26:28Z")
I’d turn everything off including the water. Keep all the drapes and shades fully closed. It won’t get too hot and you will save money. ---
**Author:** ![thelabdude](https://avatars.discourse-cdn.com/v4/letter/t/dc4da7/32.png) [@thelabdude](https://boards.straightdope.com/u/thelabdude)\ **Post date:** [July 24, 2011, 11:05pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/6 "2011-07-24T23:05:54Z")
I shut the air off at 1 PM Friday. We got back at 10 PM Saturday. The house was sweltering and sticky. It seemed to take a long time to cool down. It hasn’t been serviced in years, perhjaps due. Setting the thermostat at 90 shouldn’t take much power and might keep the humidity in the house down. I really think the system had to dry things out before it could cool much. I do kill the pump and put the water heater on pilot. ---
**Author:** ![Hedda\_Rosa](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hedda_rosa/32/1204_2.png) [@Hedda\_Rosa](https://boards.straightdope.com/u/Hedda_Rosa)\ **Post date:** [July 25, 2011, 3:48am UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/7 "2011-07-25T03:48:13Z")
> [@thelabdude](#): > > I do kill the pump and put the water heater on pilot. We used to do that too, but last year we replaced our heater and the guy said it is better to keep the heat on; the way he explained it was -assuming your heater isn’t a sterile environment, more gross stuff is going to grow at cooler “pilot only” temps than if you keep the water well heated. Sounded plausible to me. Whe we go away in the summer we leave blinds closed and all air off, but then we live in CA where humidity isn’t a worry. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 25, 2011, 12:27pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/8 "2011-07-25T12:27:34Z")
> [@Hedda\_Rosa](#): > > We used to do that too, but last year we replaced our heater and the guy said it is better to keep the heat on; the way he explained it was -assuming your heater isn’t a sterile environment, more gross stuff is going to grow at cooler “pilot only” temps than if you keep the water well heated. Sounded plausible to me. If you’re on a well that provides untreated water to your house, then I suppose this is possible. But municipal water supplies are treated with chlorine to inhibit bacterial growth, so having water remain resident in the heater’s tank at “warm” temps for an extended period shouldn’t be problem. > [@thelabdude](#): > > Setting the thermostat at 90 shouldn’t take much power and might keep the humidity in the house down. If nobody’s home, why would humidity go up in the first place? We leave our A/C off during the day, turning it on only when we come home from work around 5 PM. During recent weeks when daytime highs were in the 90’s with full sun, the ground floor temps inside the house reached about 84 degrees; upstairs temps were more like 88 degrees. This is after having cooled ground floor (via A/C) to 76 degrees during each night. If you _don’t_ have that nightly cool-down (because you’re away on vacation), then it’s likely your house would get a bit warmer during the day - possibly mid-90’s - but since nighttime outdoor temps typically cool off into the 70’s, I wouldn’t expect the temp inside your house to keep building and building over the course of three weeks. Years ago my family used to take 3-week summer vacations, and we left the A/C off. No issues. ---
**Author:** ![MandaJo](https://avatars.discourse-cdn.com/v4/letter/m/ac91a4/32.png) [@MandaJo](https://boards.straightdope.com/u/MandaJo)\ **Post date:** [July 25, 2011, 12:52pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/9 "2011-07-25T12:52:13Z")
> [@Machine\_Elf](#): > > This is after having cooled ground floor (via A/C) to 76 degrees during each night. If you _don’t_ have that nightly cool-down (because you’re away on vacation), then it’s likely your house would get a bit warmer during the day - possibly mid-90’s - but since nighttime outdoor temps typically cool off into the 70’s, I wouldn’t expect the temp inside your house to keep building and building over the course of three weeks. Typical where you are, perhaps. In Texas right now the lows are in the mid-80s, and not for long: it takes a long, long time to cool off and dawn comes before you know it. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [July 25, 2011, 1:20pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/10 "2011-07-25T13:20:20Z")
> [@MandaJo](#): > > Typical where you are, perhaps. In Texas right now the lows are in the mid-80s, and not for long: it takes a long, long time to cool off and dawn comes before you know it. Yeah, I definitely wouldn’t turn the A/C off, and probably wouldn’t warm it up above say… 80-85 degrees only because A/C systems don’t do temperature swings very fast, and I don’t want to sit around in a 85 degree house for hours on end waiting on my A/C to cool it down. If you have a programmable thermostat, you can game it a little bit- get it good and cool starting in the early mornings (like 4 am) when it’s not too hot (and the A/C’s at its most efficient), and then set the temp higher during the day, so that hopefully as the day warms up, your house will warm up a little bit slower, and then you can just keep it from going too high inside, until the following morning when you’ll cool it back down. ---
**Author:** ![Tastes\_of\_Chocolate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tastes_of_chocolate/32/3232_2.png) [@Tastes\_of\_Chocolate](https://boards.straightdope.com/u/Tastes_of_Chocolate)\ **Post date:** [July 25, 2011, 9:32pm UTC](https://boards.straightdope.com/t/leaving-the-ac-on-when-im-away/590210/11 "2011-07-25T21:32:07Z")
> [@Machine\_Elf](#): > > If nobody’s home, why would humidity go up in the first place? Around here, the humidity comes in with the outside air. We had days last week where the temp was about 95° and the dew point got to over 90°. If I left the house closed up and uncooled for long in that kind of weather, I’d expect to come home to find mold growing, my wallpaper peeling and water puddling at the bottom of the windows, where it condensed at night, on the inside, when the outside temp dropped.