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by HungryGuy on June 1st, 2005

HungryGuy

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Is it cheaper to turn the air conditioning off during the day and let it run full blast for a few hours to cool the house when I get home from work, or to let it run during the day at a slightly warmer setting?

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Answers. 11 helpful answers below.

  • by Tapaculo on August 10th, 2007

    Tapaculo

    I've seen this question on a couple of forums, and I have no confidence any of the answers are more than opinions. Most people here say it's better to leave it on, maybe lower. Here's the opposite opinion, the most convincing I've seen, from a "Saving Electricity" site.

    http://michaelbluejay.com/electricity/cooling.html

    "It's a myth that leaving the AC on while you're away at work uses less energy than turning it on when you get home. Here's why:

    Heat goes to where it's not. With the AC off, your house will absorb heat from outside, but at some point it will be so hot it can't absorb any more heat. When you come home and turn the AC on, the AC has to remove the accumulated heat only once.

    But if the AC is on when you're gone, then your house is constantly absorbing heat because your AC is constantly cooling down the house. The AC has basically turned your house into a heat magnet. So your AC is removing absorbed heat over and over and over again.

    Let's say you leave the AC off, and your house absorbs 20k BTU's of heat and then stops, because that's all it can absorb.

    Now let's say that you have the AC running instead. The house absorbs 5k BTU's of heat, so the AC kicks in and removes it. Then it absorbs another 5k BTU's, and your AC kicks in and removes that. Repeat that process several times during the day.

    This is not a gray area, and there's no question about it: running the AC when you're not home wastes energy, period."

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  • by PRODIGY on August 20th, 2009

    PRODIGY

    i live in an apt and i did a little experiment for 4 months. during month 1 & 3 i left the air on a constant setting all day long and my bill average around $75. during month 2 & 4 i turned the air off when i left the house @ 6am and didnt turn it back on until 6pm my bill was about $52 avg. take it for what it is its cheaper, at least in my experiment. dont know if it works the same for houses but if you have an apt turn off the air during business hours that is when power is more expensive.

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  • by jenniferprofitt on June 9th, 2005

    jenniferprofitt

    It is cheaper to let it run during the day at a slightly warmer setting rather than turning it on full blast when you get home. Keeping your house around the same temperature all the time instead of letting it go from one extreme to the other will help minimize your bill.

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  • by Slightly less anonymous on November 2nd, 2009

    Slightly less anonymous

    There's one problem with the first answer. There's a large possibility your house will be hot.

    Pretend it's a really hot day. And your house gets really hot. It's 90 degrees. You want your house to be 70 degrees.

    Air conditioners are supposed to take a break when they are being overworked to keep the coils from freezing up. So you come home, your house is hot, you starts the air conditioner, and it gets the temp down a little, then takes a break. Works, takes a break. Depending how big your house is, how powerful the air conditioner, you may not get the house to your desired temperature until probably, well, morning, when both the air conditioner and the colder night air have helped. So now you've spent money cooling your house, you've worked your air conditioner and you still spent the night in a hot house. Your bill may be less money, but what's the point?


    I'd recommended changing your habits based on the temperature, the season. If you have a programmable air conditioner, that's the best solution. You can program it to start to cool the house an hour before you get home. You can program it to not turn the air on unless the temperature is above a certain number. That way, on really hot days, eventually, the air conditioning will kick it in, keep the house from getting to hot, so that when you get home you can get it to the temp you want in a reasonable amount of time.

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  • by gawain on March 3rd, 2008

    gawain

    I have to agree with the first answer. The lower the temp is inside your house, the faster the heat leaks in. If the heat is being absorbed faster over the same time period, then more heat will be absorbed overall. Thus, the air conditioning has to run more to remove it.
    This can be very deceptive because running it full blast for 2 hours seems like a lot compared to letting it cycle on and off throughout the day. However, if you actually sat there with a stopwatch and timed it, you would find that the unit runs much less if you turn it off during the day.

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  • by Anonymous on October 24th, 2007

    Anonymous

    It is cheaper to turn it off. I like the example given above about a heat magnet. It is easy enough for any non-engineer to understand, and is a pretty good model. Let's give one extreme of the example. Say you have your AC on all night, and wake up at 6:00 AM to go to work. The outside temp was 80F during the night. During your sleep the inside temp has bounced between 79F and 77F (you set your thermostat at 78F and there is a 1F deadband with the sensing unit). Now you leave your house and turn off the AC. Now the heat monster kicks in. From the outside air temp, heat will transfer to your cooler inside home. Depending on the thickness of your insulation, this might take a short or long while, but heat will still transfer. Also, your pets are causing the temp to rise since their bodies are around 100F. And, if your windows are not shaded, the sun will heat your house radiatively. At 8:00 AM, the temp is now at 85F, and your house is heating up faster because you still have the same k constant with insulation, but the delta T(emperature) is greater so your house will heat up faster. Say the outside temp peaks at 90F at 1:00 PM. Depending on how well your house is insulated, your inside temp might be slightly above what it started at, 78F, or it could equal 90F, or it could even be HIGHER if your windows are not shaded. If it hit 90F, and you want it at 78F, then your AC needs to cool. But, if you don't come home until 8:00 PM, and the temp outside is back to 80F, and your house has cooled to 80F (same as outside), then you only have to cool 2F to get 78F. But, if you had your AC on the whole day, your AC would cycle, trying to maintain your thermostat temp (which is probably around 81 since many people turn their thermostats up when they leave). Your AC would have to take out the heat from 90F during the day, where, if you left your AC off, your house would cool naturally to your surroundings without the need for AC. This is what your AC does anyhow. It sends heat to your surroundings past a "semi permeable membrane", only waiting for the same heat to diffuse back into your house.

    By the way, your AC only works at 2 speeds, off and on. It does not work any harder when it is hotter outside, it only works longer! As far as wear and tear on your compressor, it hates to start and stop. It likes to either run for a long time or stop for a long time.

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  • by dawgfan on December 4th, 2006

    dawgfan

    It is easier to maintain a comfortable temp than it is to overcome a huge deficit. Also, if you wait to turn it on until you come home, you are actually asking your air conditioner to overcome much more of a heat load than just simply maintaining: It has to overcome heat in your air, but also in your furniture, walls, etc and it is trying to battle the hottest part of the day outside.

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  • by Gary666 on December 12th, 2006

    Gary666

    Much cheaper to let it run.

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  • by Mirage V2.0 AWOL on December 12th, 2006

    Mirage V2.0 AWOL

    The same logic applies to heat as well.
    The folks who constantly adjust the thermostat throughout the day, have the higher bills.

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  • by crazykentuckyguy on October 11th, 2009

    crazykentuckyguy

    If you live in a trailer or have a tin roof then you want to leave it on around 78 all day at work and turn it down when you get home. This helped my bills back in the day and kept it from being 90+ inside when I got off work.

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  • by Aaron_S7653 on June 6th, 2011

    Aaron_S7653

    I work on various commercial a/c and refer units everyday, and in most cases it is much cheaper to have them run less frequently and for longer periods of time to avoid the very high amp draw of start-up several times throughout the day. Heat moves or "radiates" to cold just as the first answer stated, in the long run you will save money on both energy cost and wear & tear on your a/c unit.

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