A friend and I have been having a long-running, friendly argument over what constitutes a better electricity savings: turning the thermostat up to 78-80 while you're gone for the day, and lowering it when you get back, or turning it off completely and turning it on when you return.

My friend thinks that turning it off wastes electricity, because the A/C simply overcompensates for the higher temperature in the house. My op is that the A/C coming on every 20-30 minutes over the course of the day has got to use more electricity, even if the temperature difference it has to overcome is less.

Thanks in advance!

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

uhhh, I'm going to say that it depends on various factors such as outside temp, insulation and direct/indirect sunlight. Also, the type of A/C unit.

The only way to find out would be to run the A/C both ways over the course of a 24hr day or better still over the course of a longer period of time and check the average wattage usage with a meter. But as mentioned there is a lot of factors to consider, the temperature over the course of the day can vary so much that the results of usage would vary too. IMO, unless the house has excellent insulation properties then running it when the house is empty would be a total waste of time. On the other hand every time the unit starts up it uses slightly more electricity and puts more strain on the motors than continuous running.

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Its better to burn out! than to fade away....

We are not alone, but we may as well be!

We have a programmable thermostat. In the summer, we set the temperature to 82 degrees during the day and about 78-80 when we're due home. It rarely runs during the day (Colorado). We're also fortunate that the outside temperature typically drops 30+ degrees after sundown. We open the windows at night and chill the house as much as possible then close them in the morning. The insulation keeps the house cool enough through most of the day so the AC rarely runs before 5 PM.

In a really hot climate, you probably wouldn't want to let your house get too hot. Not only the air gets hot but all of the furniture, walls, etc gets hot also. You can get to the point where the AC runs full time and is unable to remove all of the heat. An AC repairman told me that it doesn't make much sense to let the temperature get more than 6 degrees (F) or so above your normal temperature (when you're home) because you'll spend too much energy cooling the house later. He said the same applies to setting the thermostat to a lower temperature during the winter.

Unless your insulation is 100% effective, that is magic, turning the air conditioner off when you leave the house will definitely save energy because without magic insulation some heat will enter your home while you're out. Of course you'll have to put up with a hot house when you get home, but I think some air conditioners have timers so they can turn themselves on a short while before you get home.

If your air conditioner was somehow less efficient when operating in high temperatures there might possibly be an exception, but I don't see why it would be less efficient, and I can think of a reason why it might be slightly more efficient.

You can always just not use the A/C at all and save 100% of the electricity used for the A/C.

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Fields of Space

LOGIC, n.
The art of thinking and reasoning in strict accordance with the limitations and incapacities of the human misunderstanding.

In the Year 2525.

"One small step for (a) man. One giant leap for mankind".

If an astronaut doesn't need good grammar, niether does you.

Host of Seraphim

Or you can set it for 68F, have it run almost all the time, do chemical photographic processing, and generally enjoy a nice, cool climate.

Heck, you're not only an American, you're a Texan! Consume, pardner!

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You ain't never been in San Antonio in the summer have you?

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"An armed man is a citizen
An unarmed man is a subject"

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Good one, Frantic Freddie!

You can track folks who do the San Antonio Stroll outside in the summer by just following their sweat tracks.

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Nope. And I don't intend to.

A/Cs been around since, what the 50s?

If your grandparents can stomach the heat, so can you.

If been in Arizona in 120F weather with rain. It's not that big of a deal.

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Fields of Space

LOGIC, n.
The art of thinking and reasoning in strict accordance with the limitations and incapacities of the human misunderstanding.

In the Year 2525.

"One small step for (a) man. One giant leap for mankind".

If an astronaut doesn't need good grammar, niether does you.

Host of Seraphim

What does that mean, anyway (the bolded part)?

Thermodynamically, it is more efficient to leave it off during the day. As the air temperature in the house rises, the delta T between in and out lessens and heat transfer diminishes. However, Larry Jacks makes a valid point: not only the air, but the entire mass of stuff inside the house will heat up and will act as heat sources after the A/C comes on. It will take a while to cool down.

Looking at problems from the point of view of extremes is often useful. Say you leave the house for a week in 100 degF temperature (no nighttime cooldown in this hypo). Without A/C, the temperature inside the house will approach outside ambient and heat flow will approach zero. With zero heat flow attained, the work the A/C must do upon your return levels out and goes no higher. The work required to lower the inside to normal - say 78 degF - upon your return can be calculated.

Contrast this with the hypo of leaving your A/C at the same normal comfortable temperature of 78 degF. During the entire week there would be a 22 degF delta T that the A/C would be working against. That work can be calculated.

Now double the time to two weeks. The work in the first hypo remains as is, that of the second doubles. Increase the time to infinity. Clearly, leaving the A/C off requires proportionally less work as time increases.

That's looking at one extreme. The second is easier to describe: leave the house for an infinitesimal amount of time. Clearly the work loads between the two situations (A/C on and off) are equal.

The result of this exercise: the amount of work required to maintain any intermediate temperature at any intermediate time would be between those two hypotheticals. In all cases, the amount of work the A/C must do to lower the inside temperature is less than if the A/C had maintained that temperature all along.

I just made this up as I type this morning with half a cup of coffee, so I've probably made a dozen logical errors and maybe one or two thermodynamic errors. For sure, I'm ignoring the efficiency of the A/C unit at various temperatures and run times.

Thanks much for your responses!

I'll try keeping it off for a few weeks, and if there's no appreciable difference I'll just turn it up.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

I have a situation where half my house has cathedral cielings and they are not that well insulated. When the sun is out, even on a cool day, this will heat up the house quite a bit.

So; the heating of the roof is causing more of the heat than the outside temperature. Not running the air will always be more efficient since the outside air is providing cooling relative to the inside of the house.

For me, it works most efficiently if I keep the thermostat high during the day, and lower after the sun goes down.

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Numbers are not case sensitive. (me)

Our house is newer and we live in Michigan. We turn it off all the time. Even when it's constantly on at 75° F - it doesn't run that often this time of year. I find that even on really hot days with the A/C off and the house all closed up with the blinds closed, it doesn't get too much over 80° F in the house. Opening up the windows lets fresh air in, but it also lets the heat in with no insulation protecting the house. Windows closed and blinds closed helps to keep the heat outside for longer. Our house also has very tall trees behind it (60'+ maples to the east and a little to the south) and one huge tree (70' oak - estimate) on the southwest corner, so we don't get much in the way of direct sunlight until the afternoon. We also have fewer windows to the west where we get the most direct sunlight.

Winter is our biggest energy battle. Can't leave the heat off for very long and natural gas costs a lot more to use than running our A/C unit.

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Spock Jenkins of the Vulcan Jenkins'.

Unless your insulation is 100% effective, that is magic, turning the air conditioner off when you leave the house will definitely save energy because without magic insulation some heat will enter your home while you're out.

It isn't magic, it's just that our house is well insulated. Colorado, remember? The same insulation that helps hold in the heat in the winter helps keep out the heat in the summer. Nothing magic about it. By opening the windows overnight, we can chill the house to 68-72 degrees. Closing the windows (and the blinds) helps maintain the temperature. Sure, the house gets warmer during the day - typically to the upper 70s. However, our thermostat is set so the AC doesn't run unless the temp rises above 82. That almost never happens. The automatic thermostat will lower itself to 78 degrees at about 4 PM so on hot days, the house will be more comfortable. With our low humidity, 78-80 degrees inside is no big deal. I doubt our AC runs 100 hours a year but when you need it, it's nice to have.

In the winter, we do the opposite. We open the blinds on the south and west sides of the house to take advantage of passive solar heating (300+ days of sunshine a year). The thermostat is set to about 62 degrees at night and during the day when we're at work. If it's a sunny day, the furnace seldom runs. The house can get warm enough from passive solar so that the furnace may not run until close to bedtime. I sleep better when the house is cool, so the thermostat is set to 62 at night. It jumps to 65 shortly before we wake up but only for 45 minutes. We save a lot of money that way.

I have heard similar things. It also depends a lot on the relative humidity and if you open the windows when the AC is off (something my wife loves to do for "a little fresh air". If it is very humid and you open the windows, not only does your house get warm, but the inside humid gets high. So when the AC comes back on, it first has to remove all that moisture, then it can start dropping the temperatures.

I also use a programmable thermostat, and have the daytime setting around 82F. With the windows close and a well-shaded house, it hardly turns on during the day.

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At night the stars put on a show for free (Carole King)

Mhmm... God Bless living in the basement.

Too often we assign human traits to our machines. A/C units do not experience strain. They simply lower the INPUT air temperature by 25 degrees (or so). If the house is 80 the A/C will be putting out 55. If the air rises to 90 by the time you get home from work the output air will be 65.
The closest thing to strain would be the power surge experienced by the two motors when it turns on (cycle).

As your house heats up during the day, the relative humidity actually drops. So when you get home the 90 inside won’t feel as hot as the 90 outside. Turning the unit on will have one power surge instead of the many experienced by the unit set to 82.

The drop from 90 to 82 will be very quick since the difference between the in/out is almost nill. The total ON time will be less than the summed on time from the house set at 82.

It is also a myth that the A/C has to remove the humidity before it can cool the house. Humidity is simple a by product of cooling the air. When the evaporator inside the house gets cold the water in the air condenses on it like a glass of iced tea. It then dribbles down to a tube and into your drain. If cold air is coming out of your vents then humidity is being removed, no extra work.


Therefore leaving the A/C off makes the most sense, from a logical point. To some people comfort overrides logic.

If your AC can lower the temp from 90 to 82 very quickly, it is far more powerful than ours is. We recently had our AC replaced, and during the work, the house rose to 88 degrees. Once the work was done, the AC was then on full blast for almost 8 hours to get the house down from 88 to 78.

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