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  1. #1
    Join Date
    Jul 2003
    Location
    Philadelphia Pa.
    Posts
    461

    Programmable T stat return temperature

    I’ve been trying to save on cooling costs this summer and have been running my house warmer. I have a programmable Tstat and have it set as follows:

    Wake 77F
    Leave 80F
    Return 80F
    Sleep 76F

    I am home through the day so there is really no “leaving” but I let the stat step through all of the periods anyway. On another thread
    (http://hvac-talk.com/vbb/showthread.php?t=344682) Pro user 2old2rock stated: “A/C systems are better at maintaining temps than dropping the temp.”

    So am I using more electricity by not having a lower return temperature, maybe one that is between 80F and 76F?

  2. #2
    Join Date
    Nov 2004
    Location
    Naples, Fla.
    Posts
    1,403
    There is no rule or guidline as to energy used to recover vs energy used to maintain - it's a varible of YOUR insulation.

    At the end of the day ----- if it runs less, it uses less power. Most ultilities aggree every degree you raise the temp it saves 10% of the cost of opperation.

    Spend your money on better insulation, keep your cooling in & the heat out.

  3. #3
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    The higher the temperature you set, the more energy you save. The following paragraph applies to scenarios where you do actually leave and turn the unit off. But the principle works for your situation too.

    Think of the house as a bucket full of cold. The bucket has a lot of holes. The cold leaks out. The AC fills her back up. Keep the bucket topped off all day long and the AC has to keep pouring the cold in – perhaps several or even a half dozen buckets worth. But turn the thing off while you’re gone and it empties out just once. When you return it fills it up just once. It’ll take a while to fill the empty bucket back up. That long run time is why some people erroneously think it’s costing them too much. But it’s still cheaper to fill it up from empty when you get back than to perpetually top it off all day while you’re gone.

    The analogy above is scientifically flawed. You’re not actually filling with cold. You’re removing heat. Whatever. The point is higher settings are more energy efficient.

    AC systems are better at maintaining a temperature than dropping one. The “better” in that case is a question of comfort. In terms of BTU removal per dollar spent, they’re better at dropping the temperature. An AC engaged in recovery is more fully loaded than one maintaining: You get more heat removed for every rotation of the compressor during recovery. Cyclic losses are reduced as well: Longer run times and fewer of them are better than more and shorter run times. And, per the analogy above, set the temperature as high as you can as often as you can and you’ll have to remove less heat overall.

  4. #4
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    The higher the temperature you set, the more energy you save.
    is this in heat or cool mode


    .

  5. #5
    Join Date
    Jul 2003
    Location
    Philadelphia Pa.
    Posts
    461
    Guys thanks for your suggestions guys

    I have my leave temp of 80 set to start at 8:30am, return temp of 80 is scheduled for 8:30pm, then sleep at 76F scheduled for 11:30pm. When the outside temp is in the 90’s the unit will have it’s first on/off cycles at about 4:30pm and runs for about 8 to 10 minutes an hour , but as the hours pass and more folks are in the house, cooking and such, the cycles start lasting for up to 20 minutes. Recovery usually starts around 10PM and seems to take at least 1 ˝ hours, so I thought having a lower temp setting before recovery might save some ….Guess not

    I’ve been watching the electric consumption each day and found that we use about 22kwh in 24 hours when there is no Ac, oven, or washing done. When AC only is added at a temp in the low 90’s, consumption goes up to about 44kwh per day so maybe this is not so bad.

  6. #6
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    Your best bet is to keep your doors and windows closed, blinds pulled, shades pulled on the east, south and west side of the home. R-50 in the attic, all the windows & Doors sealed properly along with all the other mouse holes you might have will help to maintain your set point alot easier using less engergy.

    Your also better off leaving it set to a higher set-point during the day time along with the shades pulled should be pretty comfortable and come early evening your AC will kick in a couple of times to bring the home back to set point but your RH should remain pretty stable within 1%-2% of what it was at the morning set point time.

    IMO if will run more if you raise the state that 1,2,or 3 degrees during the day because like they say what goes up must come down and when you have to pull all that extra heat from the home you have to remember all the objects within the home also get heated not just the air. Find a happy medium for 90% of the day and what you like to have durning the evening hours when everone is home for those couple of hours and set it and forget it.

  7. #7
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    Air conditioners remove heat. The less heat you remove (setting temperatures higher for the daytime cooling "setback") the less energy they need. So you save money with a programmable thermostat doing what you're doing. You may not save a lot, but you save money.

    The opposite (heating with a heatpump) is a little more complicated as, if your heating setback is more than 2-3 degrees, the backup strips come on when the thermostat raises the temperature. Heating strips cost about 3x as much as a heatpump to deliver heat, so you may be negating any potential savings by lowering the temperature at night.

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