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Thread: Air Conditioner myth

  1. #1
    DIY Junior Member
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    Default Air Conditioner myth

    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. That's why heat from outside goes into your cooler home. With the AC off, at some point your house will be so hot it can't absorb any more heat. When you come home and turn the AC on, the AC removes all that heat.

    But if the AC is on when you're gone, then you've turned your house into a heat magnet. But keeping it artificially cool, there's no limit to the amount of heat it can absorb. It can always absorb more heat. And your AC has to remove that heat constantly. Your AC kicks in and removes some of that heat, then the house is cooler so it sucks in more heat from outside, so your AC kicks in again and removes that heat, and so on.

    This means that throughout the day, your house has absorbed way more than one houseful of heat. And your AC had to remove it all. By contrast, with the AC off all day, then it has to remove just one houseful of heat when you come home and turn it on.

    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.

    The actual numbers will vary, and I haven't tested this to see exactly how much the penalty for leaving the AC on during the day is, but there is zero question that running the AC all the time uses more energy than turning it on when you get home. This is not a gray area, it's simple physics, and no person with any knowledge of this subject disputes it. Running the AC when you're not home wastes energy, period.

  2. #2

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    If AC was just a temperature issue ....... it is not.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    Humidity also moves from high concentrations to low. Leaving the a/c off can mean that all of the structure and contents can absorb some of that heat, and even if the air is returned to 'normal', the walls and furniture can radiate that heat to you, making you uncomfortable. The better solution is to seal any air leaks, provide a good moisture barrier, improve insulation, and limit how cool the house actually needs to be to feel comfortable (which is as much a result of moisture content as it is to temperature). So, depending on your structure and location, you may save money, but may not maintain comfort levels.

    Your furniture and structure like wood floors and even walls will like it better with a consistent temp and humidity.

    As with most things, you pay your money and make your choices as to what's best for you. One study showed that a well constructed ICF structure had enough thermal mass to buffer daily temperature changes to a period of about 3-days. In such a structure, you might get away with setting the a/c (or heat) back, since the place is so well insulated and with so much mass, that you may not notice. But, let it exceed that buffer, and it would take days to recover, too.

    There's no disagreement that less energy use can be had, but there will always be a disagreement over what decrease in comfort will ensue.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Senior Robin Hood Guy Ian Gills's Avatar
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    I do not use AC.

    I just keep the refrigerator door open to keep my house cool in those hot summer months.

    And then I just sit back and listen to my gas-powered radio.

  5. #5
    Plumber Cass's Avatar
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    I also have a gas powered radio...I start my car...turn on the radio...and watch DVDs about travling to far off countrys...I save wear and tear on the car and have tunes I like while on a virtual vacation...

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Default ac

    The difference is how long it takes to cool down the house when you finally get home to turn it on. but as far as;
    I just keep the refrigerator door open to keep my house cool in those hot summer months
    That works for about 5 minutes and after that the heat being extracted to cool the air is just radiated back into the room by the coils, unless you have a split refrigerator with the compressor and its coils outside.

  7. #7
    Senior Robin Hood Guy Ian Gills's Avatar
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    I like watching the heat go round and round HJ.

  8. #8
    Customer Service Manager Plumbing Wholesale Peanut9199's Avatar
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    What prompted this post, did someone say something different?

    I was waiting for some advertising at the end of it for some wierd product.

  9. #9
    Plumber Cass's Avatar
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    There was spam attatched to the signature line that I removed....
    Last edited by Cass; 09-17-2009 at 02:19 PM.

  10. #10
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    We don't need AC enough up here to leave it on
    See you are making a mistake leaving the fridge door open
    Everyone knows the freezer cools you off faster

    Just stick some frozen peas down your pants
    DIY Handyman (not 4 hire)
    I have enough to do to my own house

  11. #11
    Senior Robin Hood Guy Ian Gills's Avatar
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    Thanks Dave.

    In English English "pants" mean something quite different i.e. underpants.

    So I'll say no to that one thank you!

  12. #12
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    I thought I was speaking Engrish
    So THAT is what Charles meant
    DIY Handyman (not 4 hire)
    I have enough to do to my own house

  13. #13
    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Peanut9199 View Post
    What prompted this post, did someone say something different?

    I was waiting for some advertising at the end of it for some wierd product.
    I wondered the same. The OP is in Southeast Asia so I'm not sure if they are referring to some sort of local myth, something a buddy said, or what.

  14. #14

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    I took it to be a bait posting. Not deserving a serious detailed answer.

  15. #15
    Plumbing Designer FloridaOrange's Avatar
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    Default

    Quote Originally Posted by Peanut9199 View Post
    What prompted this post, did someone say something different?

    I was waiting for some advertising at the end of it for some wierd product.
    Quote Originally Posted by Runs with bison View Post
    I wondered the same. The OP is in Southeast Asia so I'm not sure if they are referring to some sort of local myth, something a buddy said, or what.
    Quote Originally Posted by PEW View Post
    I took it to be a bait posting. Not deserving a serious detailed answer.
    I saw it as spam....without the advertising.
    Matt
    Semi-professional plumbing designer
    Enjoying life in SW Florida

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