Air Conditioning- The Best Formula for Temp Management

Discussion in 'Community Discussion' started by Huntn, Aug 19, 2013.

  1. Huntn Suspended

    Huntn

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    The Misty Mountains
    #1
    Here's the deal. At night I run the temp down to 72 degrees F. During the day I let it go up to 76. Is there a formula that shows the most efficient way to allow for temperature variations?

    In other words, am I saving energy by allowing the temp to go up to 76 during the day, just to run it back down at night, or would I save more money overall, by say letting it go up to 74 or higher say 78 during the day instead? Get my drift? I assume that if I'm going to run the temp back down, the best time would be at night, but would it be better just to keep the temp the same all of the time? I'm thinking no. Thoughts? Thanks!
     
  2. Peace macrumors Core

    Peace

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    #2
    It really depends on how good your insulation is.
    If it's really good I'd leave it on 72 during the day and turn it off or up to 78 during the night.

    If it's set to 78 during the night it should stay around 72-75 at night with good insulation.
     
  3. CountryBobs macrumors member

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    Jul 12, 2013
    #3
    Wow. My house is never under 78, and it feels cool. I wonder if my thermostat, or yours, is wrong.
     
  4. Mousse macrumors 68000

    Mousse

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    #4
    Insulation is only part the equation. You need to taking into account how well sealed you're house is as well. When I first moved in to my house, it had laughable amount of insulation. I spent about $2000 in fiberglass bats and installed it myself. I had probably a 5% drop in energy usage.

    Apparently, my house has terrible weather sealing. An energy audit showed my house was leaked A LOT of air; equivalent of a 6x6 foot hole in the wall.:eek: Spent another $1000 in weather sealing products and "Great Stuff" canned foam. After a few weekends of work, my energy bill dropped by 50%.

    New construction is pretty well sealed. If they used spray foam insulation, the home owner will probably need some means to get outside air into the house, it's so well sealed. An old, rickety shack like mine requires a lot of sweat, blood and snot. Well worth it, IMO.

    Anyhow, back on topic. I usually let my AC run at 77 all the time, except when no one is home. I let it run up to 82 when then.
     
  5. chabig macrumors 68040

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    #5
    The most efficient plan is to keep the temperature as warm as possible during the day and night. When you're not home, let it get warmer. When you're home, cool it to a comfortable temperature.
     
  6. samiwas macrumors 65816

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    Atlanta, GA
    #6
    Ours runs at 78 during the day and 72 at night if we are out for the day for our upper floor. The middle floor is the opposite, 73 during the day and 78 at night. If we aren't home, the middle floor stays at 78.

    Definitely depends on the house and insulation. Our house is odd in that it's always hot, no matter if it's hot or cool outside. Once it gets below 45 or so outside, it'll start cooling down in the house. This is why we don't run the heater much in the winter, but do have to run the A/C frequently.

    This is also dependent on people. Some people feel wonderful at 80 while others sweat at 74. I remember someone on this forum saying they never turn the thermostat below 80, because that's what is comfortable to them. If my office isn't set at 71, I will have a wet shirt. I am sitting in a cabin right now that feels fairly cool. Until I go downstairs and come back up, then it feels like a heat wave.
     
  7. G51989 macrumors 68030

    G51989

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    #7
    It depends on your AC system, and how well insulated your house is.

    Getting a smart thermostat helps.

    Mine is programmed to go from 70 degrees to 80 degrees at 7AM when I leave for work.

    At 4PM, about half an hour before I get home it cools the house down to 70 degrees again.

    On weekends it stays at 70F all the time. I program another program into it whenever I'm home more often or gone on vacation

    That way you can always stay cool, while saving money.
     
  8. Zombie Acorn macrumors 65816

    Zombie Acorn

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  9. Huntn thread starter Suspended

    Huntn

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    #9
    Thanks for the responses!

    My insulation is good and I have a new programmable thermostat. I want it cool at night for sleeping. It sounds like you have it backwards unless you think from 72, you can let it coast through the night until 72 the next morning which does not really make any efficiency sense to me. My point is I'm willing to allow the temp to get warm during the day when it's hottest, not keep it the coolest during the day when you'll use the most energy to keep it cool.

    I've got a brand new thermostat, but acknowledge that everyone has a different sense as to what temp is comfortable.

    I understand. My focus was managing the planned variations. As an example only lets say I keep the temp at 80 during the day and 70 at night. When the night time cooling kicks in, it will have to cool the house off 10 degrees. I'm wondering if there is a point where it would be break even to keep the temp cooler during the day to reduce the cooling kick for night time? If the day temp was kept at say 76 or 78 all day long and then have only a 6-8 degree temp difference to overcome at night or will you always save more energy keeping as warm as you can stand during the day, and then cooling it down to whatever you want at night?

    I program mine. :)
     
  10. hallux macrumors 68020

    hallux

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    Apr 25, 2012
    #10
    I leave mine on the same temp all the time, 75. However, I don't have a programmable t-stat so if I set it up to 80 when I leave for work in the morning it'll run ALL night to get me comfortable again. The windows are also so drafty I need to put plastic on them in the winter, the drafts make curtains made of denim move quite a bit.

    In the summer it's set to 75, in the winter it's set to 70
     
  11. vjarnot macrumors newbie

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    Aug 19, 2013
    #11
    Might be worth looking at a Nest thermostat, if you want to maximize geek factor.
     
  12. ValSalva macrumors 68040

    ValSalva

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    #12
    Cool! Literally ;) I like it frigid too.
     
  13. TheAppleFairy macrumors 68020

    TheAppleFairy

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    #13
    Is that a window AC and you set the temp to 64? Or are you actually getting the house temp down to 64?

    Personally I like the house temp to be anywhere from 68 to 70, but doing that made my electric bills go through the roof. So I generally keep it around 75, and wear shorts and a light shirt. Keeping it 5 degrees warmer saves me at least $100 a month if I compare it to last years electric bill.
     
  14. rdowns macrumors Penryn

    rdowns

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    Jul 11, 2003
    #14
    Always had window ACs until I moved into my current place 5 years ago. It has central air. Originally kept it at 69º until my first few electric bills. Now I keep it at 75º with a fan running.
     
  15. prostuff1 macrumors 65816

    prostuff1

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    #15
    Holy crap, I could not get my house down to that temp via my AC if I tried.


    I generally do this:
    1. Set the thermostat to 80F for when I am not home
    2. open all windows at night and turn on my whole house fan.


    With the ways temps have been in the Columbus Ohio area these past couple weeks I have been able to get the temp in my house down to 64F at night. I then leave a couple windows cracked when I leave for work to keep air circulation moving. When i get home it is usually about 75F in the house. I usually shut the windows, turn the AC on for about 30 minutes to pull the humidity out of the air, and the AC stays off the rest of the night.

    I don't mind it being 80 in my house so long as the humidity is around 40%. Once it gets above 50% things get uncomfortable.
     
  16. Huntn, Aug 20, 2013
    Last edited: Aug 20, 2013

    Huntn thread starter Suspended

    Huntn

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    #16
    As you guys probably know, ceiling fans make a huge difference making the room feel several degrees cooler than it is.

    To get a better idea of what I'm asking in the OP, I think you'd have to know how at a specific outside temp, how often the AC has to kick in to hold a temp and how long it has to run to drop the temp 1 degree. This is when my AC kicks in, when the house temp gets 1 degree above what I've set. So if the temp is set to 76, I'd have to know how many times during the day this happens. Now compare this to the cooling difference for the evening set at 72 degrees, a 4 degree cooling difference. This info then could be used to calculate if there is a break even point, where you could keep the house at say 75 during the day and not have to cool it an extra 1 degrees at night. But the more I think about this, my guess is that the warmest you can keep it during the day and night will always save more energy. The temp diff is not really a factor.

    BTW, if you are in the market for a new AC, the top of the line Trane includes 2 compressors. A small one that initially runs and a big one that runs when the temp difference is more than 2 degrees between actual and desired. This unit also monitors and regulates humidity in the house.
     
  17. GoCubsGo macrumors Nehalem

    GoCubsGo

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    #17
    Upstairs, when the a/c actually worked, I would keep it at 76º during the day (I was upstairs all of the time) with a few hours where I let it get to 78º until bedtime. Then back down to 76º. Very rarely did I push it down past 76º.

    Downstairs, I have two units, I would keep it programmed to go on when it reached 80º until 5 pm, then it was 74º-76º (if I was cooking it would be turned down) until bedtime, where it was programmed to kick on if it reached 80º.

    My bills are higher than they should be and I know, without any professional setting foot into the house, that it is not the most energy efficient house. I'm working on it though. Luckily for me, each time I get close, something major breaks. :mad:
     
  18. G51989, Aug 20, 2013
    Last edited: Aug 20, 2013

    G51989 macrumors 68030

    G51989

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    #18
    One of my best freinds actually runs an HVAC business, and deals in multiple brands. He has said that while the Tranes are pretty good. If you want the best efficiency, and longest product life you should go with a Carrier Infinity system, they are very quiet and actually convert to DC power in the unit, which means they use DC fan motors and compressors, which run cooler, last longer and use less power than AC motors.

    I recently had an infinity heat pump system installed, Heatpumps are a great choice, because they have a reversing valve in them, so the AC works in reverse, and on chilly fall days, it can pull heat from the outside air and assist the furnace, which saves lots of money.

    Anyway, my systems outdoor unit is so quiet all you can here is the fan noise, you can only hear compressor noise if your really close to it. The inside unit is the same way.

    After I had the system installed, my cooling bills dropped nearly 60%
     
  19. Huntn thread starter Suspended

    Huntn

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    #19
    Sounds like a great selling point. I'll have to ask the next time I need to buy one... :)
     
  20. G51989 macrumors 68030

    G51989

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    NYC NY/Pittsburgh PA
    #20
    Yeah, I know lots of guys who do that kinda work for a living.

    They all mostly tell me that Carrier might be the most expensive, but they're very well made, reliable, and long lasting, and they'll save you in the long run on your power Bills.

    Lennox is also a close second.
     
  21. prostuff1 macrumors 65816

    prostuff1

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    #21
    Bryant is also a good brand. They are owned by the Carrier group if I remember correctly.
     
  22. Tomorrow macrumors 604

    Tomorrow

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    #22
    Allowing the temperature to set back while you're not home is most definitely an energy saver.

    I have programmable stats on both my AC units. The downstairs unit is set to 76 while we're home during the day, 78 overnight, and 85 when we aren't home. The upstairs unit is the same, except it sets back to 80 overnight.
     
  23. Huntn thread starter Suspended

    Huntn

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    #23
    Once again with a string of 100° days, I'm thinking about this... again. Any engineers about? :)

    Let's say you return to your house from vacation and the inside temp is 85°F. You choice is to turn the air down to either 75° or 72°. The temp differential from 85 to 75 is 10°. I might assume the cost to lower the temp in the house is the same cost/per degree, but I think due to inefficiency, it's not proportional. That the lower you go, the incrementally more the cost will be, but still I wonder because if something like insulation and leakage is being factored, why wouldn't the inefficiency measured be at the same rate per degree? In other words, what would be the cost per degree over a 10° range vs a 13° range?

    The other aspect, let's say temp setting is 75° day, to 73° night vs 73° day, vs 71° night. Both examples have a 2° temp swing, but again I assume the lower temps would cost more even with the same differential.

    The answer to this maybe somewhere back in the replies, but I've not spotted it, yet. :)
     
  24. C DM macrumors Westmere

    Joined:
    Oct 17, 2011
    #24
    I believe it's not just the temperature differential that would play a role but basically how hot it is outside and how hot the house itself is. On a hot day going from 75 to 73 would require less than going from 73 to 71 (on top of maintaining the house at 73 prior to that).
     
  25. Beerstalker macrumors 6502

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    Peoria, IL
    #25
    Its all about the temperature difference between the inside and outside. The bigger the difference in temperature the more heat transfer takes place, no matter how much insulation you have. Obviously a house with better insulation loses less heat/gains heat slower than one with worse insulation, but a house with good insulation loses less heat/gains heat faster if the temperature differential is higher.

    So if the temperature outside is 100°, the house is going to gain heat faster with the thermostat set at 70° then if it is set at 75°. That means that the AC will have to run more often with it set at 70°.

    So the higher you can set the temperature during the day, the lower the temperature difference between inside and outside, and the less heat you will gain. The trick is making sure that you keep it cool enough that your AC can still get the house cooled back down to the temperature you desire before you get home. So if you set it to 90° when at work you might find that it is still 80° when you get home because your AC hasn't been able to cool the house enough yet. You might have to turn it back down to 83° so it can be 73° when you get home.

    The opposite is true in winter, the lower you can set the temperature during the day when gone, the less heat you will lose, and the less energy your furnace will use during the day.
     

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