Air Conditioner myth

Discussion in 'HVAC Heating & Cooling' started by MayClaire01, Sep 17, 2009.

  1. MayClaire01

    MayClaire01 New Member

    Messages:
    1
    Location:
    Southeast Asia
    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. PEW

    PEW DIY Senior Member

    Messages:
    487
    If AC was just a temperature issue ....... it is not.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,802
    Location:
    New England
    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.
  4. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,780
    Location:
    USA
    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. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    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. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,240
    Location:
    Cave Creek, Arizona
    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. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,780
    Location:
    USA
    I like watching the heat go round and round HJ.
  8. Peanut9199

    Peanut9199 Customer Service Manager Plumbing Wholesale

    Messages:
    875
    Location:
    Ontario, Canada
    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. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    There was spam attatched to the signature line that I removed....:D
    Last edited: Sep 17, 2009
  10. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    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
  11. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,780
    Location:
    USA
    Thanks Dave.

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

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

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    I thought I was speaking Engrish ;)
    So THAT is what Charles meant :eek: :D
  13. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    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. PEW

    PEW DIY Senior Member

    Messages:
    487
    I took it to be a bait posting. Not deserving a serious detailed answer.
  15. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    I saw it as spam....without the advertising. :confused:
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