Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Keep Air Conditioner On All Day, or Turn it Off When Not Home? (2-story apt)

  1. Sep 6, 2011 #1
    I haven't been able to find a satisfactory answer. What would save the most electricity?

    One of my roomates and some of the neighbors below us think that it is better to leave the air conditioner on all day. Normally, for just one house with one air conditioner, I would be very skeptical: I am more convinced by this explanation here: http://michaelbluejay.com/electricity/cooling.html

    My neighbor brings up an interesting point that I didn't consider: that we have two floors. He says, "Like it or not we all share the same house. If we turn off our air when we're not home, your air has to work twice as hard to stop our hot air from displacing it. Since we don't all live on the same schedule, we're going to end up causing each other's floors to work harder than they have to."

    How does this change the dynamic, with the 2 ACs out of sync? Ideally, we would have programmable thermostats, but we don't.

    In addition, there is the issue of moisture, and how long it takes the air conditioner to "warm up." This website (http://www.srpnet.com/energy/ask/air.aspx) says it takes an AC 7-10 min. to reach max efficiency and recommends leaving the AC on due to moisture.

    Now that it's Fall, AC doesn't matter as much, but I am very interested in finding out the truth.

    Sorry! Wrong Forum! I made a new thread in General. Feel free to delete this one.
    Last edited: Sep 6, 2011
  2. jcsd
  3. Sep 6, 2011 #2
    Leave it on. It takes more energy to have to constantly cool down all of the wall, floors, etc. every time you turn it on again after turning it off. My roommates and I found out the hard way one summer when we turned it off when we all went to work. We got smacked for over $100 more on our electric bill that month vs. when we just left it on the whole time.
  4. Sep 6, 2011 #3


    User Avatar
    Gold Member

    Quick question, are the two apartments open to each other (ie. a common entry that air can move between)? Or are they fully separated?
  5. Sep 6, 2011 #4
    You could always get some sort of wattmeter and compare the two cases under similar outside conditions. I don't think there can be any one blanket statement that covers this because there are just too many variables. What is the surrounding temperature, insulation, duration you are gone, do your windows face the sun, are your two homes really connected so that they share some work, or does the floor provide an adequate insulation...
  6. Sep 6, 2011 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I agree, the variables make a definitive answer impossible.

    To the OP: how's the insulation?
  7. Sep 6, 2011 #6


    User Avatar
    Homework Helper

    I have to agree with QuarkCharmer that it depends on your situation. Leaving the AC on all the time could use more or less energy than turning it off when you're out, depending on how well insulated your house is and how long you're typically out for. (The page you linked to says "This means that throughout the day, your house has absorbed way more than one houseful of heat" but that's not automatically true; it depends on the rate of heat flow and the time over which it flows.)
  8. Sep 6, 2011 #7
    Thanks for all the replies!
    Our insulation doesn't seem that great.
    The two apartments are for the most part separate. There is one staircase connecting the two, but there are doors on both ends of the staircase. I think the separation is pretty good.
    Also, we are on the top apartment, so shouldn't hypothetically, we have more trouble cooling our apartment down/AC costs more for us, since heat rises?

    I'm happy to answer any more detail questions so we can get to the bottom of this.
  9. Sep 7, 2011 #8
    Ideally, you'll want the compressor running the most when the air outside is coolest. Problem, is, that's when you typically need your A/C the least.

    I have a programmable, and I set it to follow the daily average temperature. It may be cold at night, but I've a good blanket. I get it cold enough at night, when the air's coldest outside, that it rarely needs to kick in during the day until after the air turns cooler in the evening.

    That alone cut my heating and cooling bills in Florida by about 35%. I had a heat pump there, though. Here I just have an A/C with gas heat. I think the benefits of following the temps in the winter will be much less pronounced because it's not a heat pump, but there's still solar heating and nighttime heat loss, so there will still be some benefit.

    Basic programmables are cheap these days, as little as $15. In your situation, you should be them for all residents. You don't all have to agree on the same temp, but if you followed the same cycle, the bills would be more even.
  10. Sep 7, 2011 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    Hrmm. I just responded to this identical post elsewhere....
  11. Sep 7, 2011 #10
    One thing to consider is that perhaps you are over working your AC if you leave it on all day. At the apartment complex I work at they have AC in several of the rooms of the clubhouse. During the summer the residents always turn on the AC and never turn it off when they leave. And nearly every summer one of those AC units winds up dying. They just slowly over time lose their "coolness' until they are blowing nothing but warm air. These are wall/window units.
  12. Sep 7, 2011 #11


    User Avatar

    Staff: Mentor

    Sorry guys, this question really does have an easy answer: turn it off or raise the thermostat. The first link is correct about why.

    For the shared walls thing, you lose some of the benefit if you and your neighbor have different schedules, but not a lot of it.

    Regarding efficiency: yes, it is more efficient when run longer: which is when you get home and turn it on. Bonus: it is also more efficient when the room is warmer.
    Last edited: Sep 7, 2011
  13. Sep 7, 2011 #12


    User Avatar

    Staff: Mentor

    That's not even wrong. Leaving it on means the house and walls are kept cooler, which takes more power because they absorb more heat.
  14. Sep 7, 2011 #13


    User Avatar
    Science Advisor
    Homework Helper

    Assuming the apartments aren't insulated from each other internally, the neighbors are right that one person turning off their air conditioner increases everyone else's air conditioning load.

    Warm air will rise, so the top floor apartments always have a larger air conditioning load. But, the key is the relative temperature difference, not the actual temperature of the air. If the lower apartments are cooling all of the air in their apartments, then the 'warmer' air is still cooler than the outside air and the upstairs air conditioner has less work to do.

    Likewise, the cooler air will settle downward. If the upstairs air is being cooled, the cooled air will settle downward into the lower apartments, reducing the amount of cooling the downstairs air conditioner has to do, since the air is already cooled.

    If you're the only selfish person and you have some kind of timer that turns your air conditioner on shortly before you get home, then you save money while the downstairs apartments' air conditioning load increases. If the people in the lower apartments are selfish, they turn their air conditioners off when they're not home and your air conditioning load increases.

    It still costs you (and the other residents) money if all are fair, since the most efficient use for everyone would be to have synchronized schedules in which everyone arrives home at the same time, everyone leaves at the same time, and everyone has their air conditioners programmed to turn shortly before they arrive home (since who wants sit around a hot house waiting for the air conditioner to cool the air).

    Even with unsynchronized schedules, it's still probably more efficient overall for everyone to be selfish and turn off their air conditioner when they're not home, since there's no reason to cool unoccupied rooms and the air flow from one apartment to the other probably just isn't great enough to justify the extra cost.

    I'm not sure the most efficient solution overall actually benefits you, though, since you're always working at a disadvantage. The hot air from their refrigerator, stove, oven, etc, is always coming up to your apartment to be cooled, while most of your heat stays upstairs in your apartment. While everyone leaving their air conditioners on costs more total money in the long run, that practice could also change the heat balance, saving you money while costing money for the downstairs apartments.

    That would be tough to figure out for sure, but one thing is a pretty good bet. Running your air conditioner as you please (on when you're home; off when you're gone) will be more likely to draw complaints than an actual change in the neighbor's air conditioner behavior. Almost all will still run their air conditioner 24 hours a day regardless of what you do and the few that turn their air conditioner off while they're gone would do so regardless of how you operated your air conditioner.
    Last edited: Sep 7, 2011
  15. Sep 7, 2011 #14


    User Avatar

    Staff: Mentor

    To put it another way:

    Baseline: Neither of you turn off/back your AC during the day.
    Case 1: 1 person turns it back. The other pays for more AC - but not much more. Nowhere close to the double cited earlier.
    Case 2: Both people turn it back. Both save.

    So there is no scenario under which it would be beneficial for you to not turn your AC off/back during the day.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook