Central A/C - leave running or use set-back thermometer?

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On 18 Jul 2006 06:52:30 -0700, snipped-for-privacy@sme-online.com wrote:

True, but not sure how much effect this would have on the equation generating monthly electric bill. Probably not over $5.00 on the worst month of the year.
Here is some info from the manufacturer of a small AC unit showing increased wattage due to temp differences:
80 outside air 420 watts 95 outside air 510 watts 110 outside air 540 watts
That shows a significant increase in wattage used when the ambient temp is higher. Certaintly not intuitive for the general population, but good hvac guys know this and use an amp meter to troubleshoot performance.
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JimL wrote:

Thanks for everyone sharing their ideas.
I'm no refrigeration guy, but have had a tinge of thermodynamics in my background. With that, it makes no sense to me that the outside temperature wouldn't be an issue. The compressor is doing work, effectively moving BTUs from inside to outside The gas in the outside condenser is being force to condense. This will be more difficult if the outside ambient--and therefore condenser--temperature is higher. The higher the outside temp, the higher the condenser coil temp and therefore its pressure (I THINK this to be true; there may be something about the system that says otherwise). Some people put evaporative cooler nets (wet) around the outside condenser to lower the temps at the condenser coil and thus improve efficiency. (That's not an option in humid climates or places where water is at a premium.)

obvious that if the heat sink is at a higher temperature, the spread between inside and outside temps is greater and therefore the system has to put in more energy (work harder) to keep the BTUs flowing at the same rate.
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wrote:

I forgot one thing I meant to mention. I used to feel that it must be hard to get an electronic oscillator to start oscillating, and that eventually it would get tired doing so. Yet I've had tube and transistor radios and tv that run for hours (or in one case, weeks, on end, and they never seemed to get tired.
I think I thought it was hard to start them, because I wouldn't want to oscilate. And that when they started it was sort of like a fluke (even though, if the device wasn't broken, it always happened. Later I learned how the circuit works and it didnt' seem amazing anymore.
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When you leave the AC on, it has to work harder to keep the house cool (versus keep the house warm). This uses more electricity.
However, if the recovery time is very long, it might be more important to keep cool, compared to the energy saving. Since you describe it "struggling", I'd leave it on. I'd also call a HVAC company to clean and service the unit. Might be running very inefficently.
--

Christopher A. Young
You can\'t shout down a troll.
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Stormin Mormon wrote:

I had a unit that ran almost constantly even with a setback thermo. It got so bad that if more than two people were in the house (i.e. grandkids came over), we had to go escape to the mall to get cool. AC bill more than doubled. Time for a new unit.
Now in spite of at least 50% increase in electricity rates, house stays at a reasonable temp and AC bill is staying close to the peak before the old unit got REALLY bad.
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I saved about 25 percent of my summer electric bill when I used set back during the day. That's a huge savings when you are talking about $400 electric bills.
It started when I bought a digital thermostat that allowed me to easily control my AC.
We like 74 for sleeping, 78 up until bedtime and 86 setback when we are not home.
Huge savings. Don't listen to those that say otherwise. They are lying thru their teeth.
The only other advice I can offer is that it takes about 45 minutes to make the house livable, so use your digital thermostat to start cooling 45 minutes before you get home from work.
On Mon, 17 Jul 2006 20:04:18 -0400, "Walter Cohen"

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JimL wrote:

Much the same here. My thermostat is set for 83 on the second floor with a comfort setting of 80. About a half hour before I get home from work the thermostat switches to the comfort setting. Seeing as how heat rises that means the ground floor is quite cool yet the up stairs is quite comfy at bedtime.
The scroll compressor I had installed years ago has paid for itself many times over. Runs steady but doesn't have the high draw of standard compressors.
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I am so jealous.
I have an expensive Trane compressor and wish I could trade it for a goodman scroll.
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Sorry, but 45 minutes would be a joke for me. That might bring the temp down only 2 degrees. As I mentioned, it took 3 hours to bring the temperature down to 'livable' conditions.
Walter
wrote:

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On Tue, 18 Jul 2006 19:20:17 -0400, "Walter Cohen"

You would be surprised how much reduction of humidity can be coupled with that 2 degrees. The house would feel much cooler due to the reduction in humidity.

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JimL wrote:

Yes. And wouldn't it be nice if the builders these days would put in a vapor barrier that was worth more than the ink to print the words, so that the moisture in the house wouldn't rise so much even when no one is home.....
That piece of kraft paper on the face of the horribly outdated R13 insulation batt is about as good a vapor barrier as is a kitchen colander.
It's 110 outside, and 90+% of the new homes hereabouts still have only the R13 in the walls, improperly installed. Homebuyers don't understand, so builders don't care, so we are stuck..
Rant off.
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With myself, if it is going to be a very hot day, I'll leave my A/C on all day. If a mild day, then I'll leave it off.
On mild days, I can come home and turn on the A/C and it will cool down the house in not too much time. On really hot days, the A/C takes forever to cool down the house.
Or depending on if I will be away all day and not return until 10:00 PM, then I will leave it off. Then when I get home, I can open the doors/windows and cool the house down quickly.
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Bill wrote:

Coincidentally, I saw this article in teh paper on Thursday. The information indirrectly from SRP, an Arizona power producer:
"Turning your unit off in the morning when you leave the house and then waiting until you return at night to turn it back on will save you the most money. The problem is that not only will the air in your house have a chance to heat up, but the furniture, carpet, walls, etc. will aso heat up. With all this heat, it will take your air conditioner longer to bring the temperature back down to a comfortable level. "The continuous operation of the units is actually very efficient. Without all the starts and stops of a typical cycling unit, you're able to run the system at its peak efficiency. However, setting the thermostat at 85 will aso save you money (perhaps not as much as turning the unit off), while not taking as long to recool your house. Both options work; it's just a matter of how much discomfort you're willing to put up with."
Sounds good to me!....
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the
home
The question omitted whether better = feeling more comfortable or better = cheaper. If you can read your electricity meter, you can try both methods, record meters for similar sample days, and see how elec. consumption compares.
--
Don Phillipson
Carlsbad Springs
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Very good!
Although you could probably argue that for any duration greater than a few hours, either criteria could be met by using a setback thermostat and adjust the restart time to allow for sufficient cooldown.
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