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Can you save money by lowering the thermostat?

One of the factors that affects the cost of cooling a building is the outside temperature. It's cheaper to cool a building when it's 75°F outside than when it's 105°F. This got me thinking: would it save money if I lowered the thermostat when it's cooler outside than inside?

For example, I keep my A/C at 75°. A strong thunderstorm just came through and lowered the outside temp from 85 to 70. It's going back up now. I thought that if I lowered the thermostat to say 72° for a while and then put it back to 75°F, I'd save some money while the house returned to 75°. Of course if this really works, the thing to do would be to hook up an outside thermometer, or connect the cooling system to the internet. Before I do any of that I thought I'd see if this actually might save on energy. Thoughts?
1929 Ideal Heating vapor system.


  • TechmanTechman Member Posts: 2,144
    on time

    Any running of the comp is money.Granted it is cheaper to run the comp when the ambient is 70* vs.85*.But when the outdoor temp drops to 70* from 85* then the indoor temp should also drop,everything being equal.The thermal swing cycle thing may take some time but its there.So I say you do not save any money.
  • Brian_74Brian_74 Member Posts: 237
    Followed you until the end

    Thanks, Techman. I followed everything you said until the last sentence. If I run the comp more when it's cheaper and less when it's more expensive, wouldn't that save money?
    1929 Ideal Heating vapor system.
  • TechmanTechman Member Posts: 2,144
    edited June 2010

    If the only two factors were the two you mentioned,then sure ,you'd save money.But the on time of the compressor ,any time is costing you.If the energy rates were lower after a storm ,or when the OA temp dropped then you'd save $ comparitivly.With the stat at 75* $ and the OA dropped to 70* then the overall load on the system also drops.Less heat infiltration is less internal heat =less run time .Now you want to lower the stat and bring on the money consuming machine.If the OA stayed at 70* & the stat @ 75* then only internal heat sources would raise the indoor temp ,which would be rarely.Better I hope?Leaving the stat @ 75* after the storm,the comp would kinda not run,lowering the stat and your house meter is now spinning away .
  • Brian_74Brian_74 Member Posts: 237
    compressor on time

    I think I can restate the question in terms of compressor on time only. During the storm there's a significant drop in OA. I lower the thermostat to 70° during this time. This results in more on time for the compressor than if I had left it at 75°. Eventually, the OA returns to 85° and I return the thermostat to 75°. If the extra on time is less than the off time (while the house returns to 75 from 70), that would save money. If the extra on time is more than the extra off time, I'd lose money. It's this extra off time that I didn't see in your answer.

    You're really helping me to think about this, by the way. I appreciate it.
    1929 Ideal Heating vapor system.
  • TechmanTechman Member Posts: 2,144
    on/off time

    I guess we will have to have an engineer give us an actual on/off time senario to compare the real on/off times to see minute by minute.So what you are saying is ; you run the ac for 2 hrs to drop the temp from 75* down to 70*  then it takes 3hrs to warm back up to 75*,and being cooler OA it cost you less in time on of the comp,as compared to leaving the stat at 75* and having the comp run for whatever time that it would run. Heaven , send us the PROFESSOR!!!!  LOL
  • Brian_74Brian_74 Member Posts: 237

    Sorry that it took me so long to explain, but that's exactly what I meant.

    I'm beginning to think that there's no way to answer this question unless I want to turn my house into an EPA study. :-)
    1929 Ideal Heating vapor system.
This discussion has been closed.


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