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Running A/C all of the time or have thermastat control

Old 05-19-2010, 06:06 AM
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Default Running A/C all of the time or have thermastat control

Is it cheaper to have the A/C run at a set tempature, or program thermastat to raise temp at night then cool back down in the morning.

I have 2 units and was wondering about my down stairs unit. We go to bed at 11:00 and get back up at 6:00 am. Right now the the air is set to 72, I wonder if maintaing all night is no more expensive then having the unit have to cool the downstairs down from say 78 all night to 72 before we get up?
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Old 05-19-2010, 08:27 AM
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The answer is it depends. They are ROUGHLY the same, but different factors will make one win out over the other. The relationship between the amount of time it takes to cool the house back down and the amount of time the AC will be off have a profound effect. The shorter the relative time required to cool down, the more you'll save by turning it down. If it takes 15 minutes to cool my house back down, but it takes 1 hr to cool your house back down, I'd be better off turning mine off at night, where you'll be better off leaving yours at 72.

Also, the better your house is insulated, the more it leans toward leaving it on. If your house is poorly insulated, it will quickly equalize with the outside temperature. this means that maintaining it at 72 would require frequent cycling of the AC.

The only way I found to get an accurate answer was to use a programmable thermostat that will let you see usage. Set it for a week at 72 and record average daily outside temp and actual usage. Then set it to go to 78 at night for a week and record usage and temps. Then repeat the process so you are averaging temps over a month. Look at usage vs outside temps and you'll have your answer.
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Old 05-19-2010, 01:43 PM
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I'm no expert by any means but doesn't an air conditioner draw the most power at the initial start of the compressor/fan and if that's true, it seems to me leaving the AC on the same temp all the time would result in more frequent start ups in order to maintain the temp but if you turned the temp up at night, in the morn the AC start up once and run for a while to get the house the proper temp using less power??
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Old 05-19-2010, 01:56 PM
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The closer you keep the temperature to the outside temperature the less energy you use period. This holds true for heating as well as cooling.

In your case it will ALWAYS be cheaper to run it at a higher temp at night in the summer- the only downside is that it my take a while to cool down during the day.

Likewise, in heating in the winter in the North it is always better to turn the temp down or off at night. Some people think that the system will then work extra hard to bring it back up to temperature thus defeating the savings - this is untrue - systems are basically on or off - they cycle. But again, the downside is depending on the size of the system it may take a while to heat the house up in the morning.

Look at from an energy balance equation. The driving factor in the heat loss or gain from your house is the temperature difference between the inside and outside it. The closer you keep the inside temperature to the outside temperature the less energy you use.

Last edited by LI Sound Grunt; 05-19-2010 at 02:39 PM.
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Old 05-19-2010, 02:07 PM
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Absolutely buy a programmable thermostat!
The temperature you set will take some time to find your right atmosphere but will be well worth it once you get your ideal adjustments!!!
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Old 05-19-2010, 02:35 PM
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Turn it way up at night..

Me, I set it at 80, and won't touch it all summer long. But I really like it hot. I'd be freezing at 72 bRRR!
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Old 05-19-2010, 07:42 PM
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All locations are different but... In north Alabama we advise a 4 to 6 degree temp swing. Anymore and you are going in reverse. This was studied by TVA, and this is what they recomend for energy efficency. But like I said, different power rates, and demand pricing, and heat loss/gain in different areas around the US. Contact your local power distributer for the best information. But do install a prog stat and use it.

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Old 05-19-2010, 09:38 PM
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Ours , here in North Texas runs less if I can keep my wife off the T-Stat , All you need is a AMP Meter and a watch to figure out your KWH.

In AC mode your unit should not run as much at night anyway because it should be cooler outside .

My vote is set it and leave it alone as long as your in the house .
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Old 05-20-2010, 03:47 AM
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I used to use a programmable thermostat in a condo I had that had crappy insulation and windows. I like it cooler at night and the bedrooms were upstairs where it was warmer. So I set it to be higher in the day (while I was at work) and lower at night and the weekends. It saved me money. In my house now I set it and forget it. 3 of the bedrooms are downstairs and the insulation is better. Even with having 7 sliding glass doors! If you get a programmable thermostat, get one that you can set to be different on the weekend. I don't think one answer is good for all homes. Homes that have less insulation will use more power quickly trying to keep the temp set and a programmable makes more sense.
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Old 05-20-2010, 05:02 AM
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The reason I was asking was, I was wondering before I drop $100 on a programable one, was it really worth it.

I like it cold, as do the family. I guess I will just set it and forget it.
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Old 05-20-2010, 07:39 AM
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I have programmable t-stats on my downstairs and upstairs units. I bought both for less than $100 and yes, they are worth it.
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Old 05-20-2010, 07:57 AM
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I keep mine running 24-7 when the boat is at the dock, mainly to keep the cuddy cabin fresh and dry. I set it at 75 and it cycles on and off regularly. Its a 5000 BTU unit and I barely noticed any difference in my electric bill.

When I do an overnighter I really crank it down. Nothing like an ice cold cabin after a long day in the sun. Its gets so cold in my boat that the outside of the hull is wet like a cold can of Coke in the hot sun. LOL

EDIT......I just realized this was in Dockside and this is about home AC. I like it cold and I work from home so I keep it at 72 all day, at night it goes down to 69. I'm not going to sweat to save a few bucks.
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