HVAC Summer Efficency Question

My friend and I are having a debate on an age old summer question. Is it more efficient to leave the A/C on all day and keep my apartment cold, or turn it up and cool my hot apartment when I come home?
My belief is that the apartment gets hot due to the greenhouse effect and the A/C should be on all day, so it cools intermittendly.
His belief is that it's wasting energy to cool all day.
What is the answer to this question?
Thank you.
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Vito Cavallo wrote:

Frankly it is not all that simple. In reality it can be cheaper to leave it on all day. It may be cheaper to turn it down during the day. (turning it off and opening the windows is a killer unless you have very low humidity.)
Ignoring the fact that it is more expensive to cool (remove the same units of heat) when the temperature difference is high (Hot out side) than when it is cooler outside, then it would always be cheaper turn it off or down anytime possible as that would reduce the total number of heat units (BTUs) that it would need to move.
However most people would want cool it down in the afternoon when it is hot outside, it may be a wash or it may cost more or less depending on the temperature difference and the efficiency of the unit.
The other factor is a practical one. If your unit is properly sized, it will not have enough extra capacity to cool you home down quickly after being off most of the day. If it were larger, then it would be oversized and it would not be able to control the humidity no matter when you turned it on or off. (Note: in some parts of the country where it is very hot and dry, this would not be true so a larger unit might be in order IF it did not loose all the gains from the temperature/efficiency difference.
In short there is no simple answer. You are both right. ;-)
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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Do you have access to your electric meter? If so why not do a test on similar days? One day do it one way the next the other go out read the meter at say 8 pm and compare results, then you have the proof you need to win the debate or the proof you need to stop debating!!
Post the results here too...
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Rich wrote:

You may want to adjust those results based on the local cooling degree days for those days.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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Vito Cavallo wrote:

Any setback saves energy.
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Moderation. Increase the temp 5-10 degrees during the day, the reverse of a setback thermostat in the winter.
You only need a few degrees difference, not all or nothing.
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Yes.
Yer welcome....
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My supplier, Reliant Energy, in my city does not vary the rates depending on the time of day. Therefore it is always a tradeoff of comfort versus cost.
So in my case and probably yours, it is ALWAYS cheaper to set back or turn off the AC. The argument that it takes longer to bring the house back down to set temperature and thus it is cheaper just doesn't wash.
But a properly designed AC will take several hours to bring the temperature back down to set level. That's the comfort factor mentioned in the tradeoff mentioned in the first paragraph. Mine takes about 3 hours to bring inside temp down to 78 from 85 on a 95 degree day.
Pj
On Wed, 04 Aug 2004 16:22:12 -0400, Vito Cavallo

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Dear Vito, An AC moves heat out of the house. Well, what are the sources of heat? You guess a few, I'll fill in the rest, and tell you where I'm going with this.
--

Christopher A. Young
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We all know you're going to hell in a handbasket......
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