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Turning off the air conditioner.

  1. Jul 14, 2008 #1
    I can't seem to find a straight answer so I thought asking here might come up with a credible response.

    Everyone I know tells me that it wastes more electricity to turn off your air conditioner because of the amount of energy it takes to cool the house those extra degrees when you turn it on. Everyone I've asked/spoken to recommends to set the thermostat up a few degrees so it lets it stays cooler in the house overall (than no air conditioning) and the heat difference isn't as drastic when you turn it to the regular setting and the air conditioner doesn't have to work so hard. I am sceptical as this doesn't make intuitive sense to me at all, and I can only find anecdotal evidence in web searches so I was hoping someone could explain it to me.

    In my mind, if you turn off the air conditioner there is a limit to how hot your house can get. When you do eventually turn on the air conditioner, you only need to remove a single house full of heat. Conversely (by my way of thinking), if you are constantly cooling the house you are effectively turning it in to a heat magnet so there is no end to the number of times you can remove the house full of heat, depending on how hot it is outside.

    Does anyone know which thinking is right and why? Does it make more sense to turn off the air conditioner when you're not home, or to just set it down a few degrees so there isn't such a drastic temperature change when you do put it to the regular setting?

    Thanks!
     
  2. jcsd
  3. Jul 14, 2008 #2
    Naturally it depends on how often you are home. A house can be kept pretty cool without A/C if the shades are kept down and there are trees for shade outside. Also, windows open at night help alot. Naturally insulation helps to keep it cool in the summer as much as it helps to keep it warm in the winter.
    -
    I wouldn't necessarily buy into this if I were you:
    I think that line of thinking is someone trying to justify leaving the A/C on all the time because they are too lazy to schedule it. Try it for yourself. I don't think any one plan is the right one. It will vary from house to house.
     
  4. Jul 14, 2008 #3

    russ_watters

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    Your instinct is correct. Several things to note:

    1. The worst you could do by turning off your AC during the day would be to equal the energy use of leaving it on. But the reality is that you will save at least something because:
    2. Heat transfer rate is proportional to temperature difference so if you let the house warm up, it will absorb less heat from the environment than when it is cool.
    3. A heavily-loaded air conditioner is more efficient than a lightly-loaded one.

    During a single afternoon, such as when you are at work, the amount of savings is not anything spectacular (but worth doing for 10 or 20%), but if you go away for a weekend, the savings is huge.

    I'm an HVAC engineer, btw...
     
  5. Jul 14, 2008 #4
    Could you expand on what you mean by heavily-loaded vs. lightly loaded? If it's what I think you mean, what's your opinion on using freer flowing filters such as these...

    http://thinkspaceblog.files.wordpress.com/2008/03/thinkspace-buildout-071.jpg

    vs. say, a typical paper-type filter such as one of these...

    http://wood.bigelowsite.com/air_filter/Filter_4.jpg
     
  6. Jul 14, 2008 #5
    Thanks very much russ, I appreciate your input. I didn't know that about air conditioners, but that makes a lot of sense to me as there are many other things that work in a similar fashion.
     
  7. Jul 15, 2008 #6

    russ_watters

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    Heat transfer rate is proportional to delta-T, so if you send hotter air through the cooling coil, the heat transfer rate between the air and cooling coil improves. By quite a bit.
    It is a stretch to call that a filter. It is almost completely useless.
     
  8. Jul 15, 2008 #7
    Basically it is easier for the air conditioner to remove heat the closer the inside temperature is to the outside temperature. Even ignoring the fact that your house will absorb less heat when the house is warmer, it is still more economical to turn the air conditioner off when you aren't home.

    There are two reasons for turning it off.

    1. The house absorbs less total heat energy once it warms up to the outside temp. So when you get home, the air conditioner has less total heat to move outside than had it been running all day.

    2. The air conditioner is more efficient at moving heat from a hotter house to a hot outside than from a cooler house to a hot outside. Even if we ignore point 1, and assume that the total heat the A/C will have to move is the same if you leave it on or not, it requires less energy input to do this for a warm house than for a cooler house.

    The only advantage to leaving it one when you're not home is that you don't have to come home to a hot house (good if you have pets in the house, though if this is the case, you'd be better off to just A/C a single room for them).
     
  9. Jul 15, 2008 #8
    Excellent explanations. Thanks guys.

    I never even put two and two together, but it's the exact same with automotive A/C systems.
     
  10. Jul 15, 2008 #9

    Gokul43201

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    I've heard this a lot too, and ot's the most utter nonsense, in my opinion.

    There is one good reason to leave the AC on though - that's if you can't handle the temperature when you get back home during the time it takes to cool down to a bearable point. The argument is purely on the basis of comfort, not cost.
     
  11. Jul 15, 2008 #10

    Vanadium 50

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    There is one factor that's been neglected - and that's that you have other appliances. Your refrigerator uses less energy if it's in cool and dry air than if it's in hot and humid air. Personally, I doubt that there is a net gain by turning the A/C on, although I suppose it's possible if one was in a small apartment, it was a very hot day, and the A/C was much more efficient than the refrigerator.
     
  12. Jul 15, 2008 #11

    BobG

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    If your refrigerator were really old with the weatherstripping in disrepair, etc. and the rest of the apartment were extremely well insulated, then possibly. Generally, I think it's safe to say your refrigerator has a lot better insulation than your house or apartment. Besides, lowering the temperature means your water heater also has to work harder to keep the water hot (although that's normally better insulated than your house or apartment as well).

    If I were turning the air conditioner off, I would open the windows, though. Even with the windows open, I think it's safe to say the house will be hotter than the outside air. You have the same amount of heat to dispel regardless of whether you leave the air conditioner on or off, but if you turn the A/C off and leave the windows open, some of that heat will be dispelled already before you close the windows and turn on the A/C.

    If it's an apartment, then the air conditioners in the other apartments will remove some of the heat from your apartment unless you have incredibly well insulated individual apartments. The free heat dissipation, either through the windows or by other folks' air conditioning, makes turning the A/C off definitely the best deal. In fact, leaving the A/C on probably means you're paying to dissipate heat from the freeloader living next door. In fact, it was probably your next door neighbor that told you it was a good idea to leave your A/C on all the time.
     
    Last edited: Jul 15, 2008
  13. Jul 15, 2008 #12

    russ_watters

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    The temperature of the coils is much hotter than the air, so it is less sensitive to an increase in air temperature. It is also a tiny fraction of the size of the house a/c.
     
  14. Jul 15, 2008 #13

    russ_watters

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    I wouldn't - with insulation values so high these days, it can take days for a house to reach equilibrium with the outside air, and when it does, it ends up somewhere betwen the day and night temperatures (perhasp 2/3 or 3/4 of the way toward the day temp).

    I've been away for long weekends and never seen my downstairs above 85 or upstairs above 90 even in days when the outside temp goes well above 90.
    Not sure where you would get that - there really is an equilibrium temperature where there is no heat going in or out of your house, and you seem to understand. As the temperature inside rises, the amount heat being absorbed from outside decreases.
    That's true.
     
  15. Jul 15, 2008 #14

    LowlyPion

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    By slowing or speeding heat flow you can effectively create a bias to the low side and limit the maximum daytime rise. At the point at which the outside temperature drops below inside, blowing fans and doing air replacement, swapping out the warm air until morning and then closing up when the reverse is true, isn't bad for a lot of days a year.

    For me in climates where 80+ is the lowest nighttime temp (that usually means humidity too) - well I'd say turn the air conditioner on. Don't be stupid about it.
     
  16. Jul 15, 2008 #15

    Evo

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    Interesting, I would have thought with the walls and furnishings getting so hot and damp that it would be better to leave the A/C on at a slightly warmer temperature if you're only going to be at work.

    I definitely leave it on for the same reason Gokul cited, by the time the A/C manages to pull the heat and humidity out to a comfortable level, it's almost time to go back to work.

    Especially like my old house with so much poorly insulated glass on the west side of the house, it felt cooler stepping outside to get what tiny bit of breeze there was, and the night time temperature would definitely drop faster outside then inside. But that's just a comfort issue.
     
    Last edited: Jul 15, 2008
  17. Jul 15, 2008 #16

    Moonbear

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    Huh? How does the inside get hotter than the outside with windows open? Unless you're living in a giant glass house, this doesn't make sense. With the windows open, my house stays as much as 10 degrees cooler than outside as long as I keep the shades down.

    Unfortunately, when I got the "free" cat, I forgot to factor in these hidden costs, such as not being able to just turn off the A/C for a weekend away. So, I'm a bit odder than most...I keep the air off when I'm home, and turn it on when I'm away, so I can rely on a thermostat to keep my cat from overheating if the weather gets very warm.

    Though, my boyfriend discovered a different problem of leaving heat or air off during the day and only turning it on when home at night...the furniture started cracking from the temperature changes (he considered it more of a problem than I did, since the piece that cracked is one I thought was hideously ugly, so was happy to see need to be replaced :biggrin:).
     
  18. Jul 15, 2008 #17

    Evo

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    That would be due I guess to the drop in outside temperatures once the sun goes down. The outside air would be cooler than inside the house.

    It also depends a lot on how much humidity there is. In Houston, the air is so thick that you can't breathe. They have air stagnation warnings. Even here they have health warnings about the elderly and sick needing to stay indoors with the A/C on and they even have free airconditioners that they install for poor people every summer. Heat hasn't been an issue this year though.
     
  19. Jul 15, 2008 #18
    your idea can draw people's attention. By turning it off, you also mean to teach some adaptive lessons, energy savings.... Only a few degrees dropped, it means nothing, right ?
     
  20. Jul 15, 2008 #19

    russ_watters

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    In the 9-10 hours most people spend away, the house doesn't get hot or humid enough for either one to make much of a difference unless it is reaaaaly hot outside.

    Winter, however, is a different story.
    A programmable thermostat will fix that...
     
  21. Jul 15, 2008 #20

    russ_watters

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    Please. Your cat can't handle an 85 degree house? Who owns who here?
    Just needed to quote that one for the record. :biggrin:
    You said heat or air - do you remember when, exactly, that happened? It is pretty unlikely that temperature would be the problem - usually, it is low humidity in the winter, and turning the heat on blasts nearby furniture with hot, dry air.
     
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