more fun with air conditioning

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In regards to the recent posting I saw about running the a/c or opening the windows, I would like to list several statements that people have made to me about air conditioning. The location is Texas, where the temperature is about 75 F at night and 100 F at the hottest part of the day.
1. Keeping the a/c cooling the house all day uses less electricity than turning it off and then back on in the evening or when you return from a vacation.
2. Running the a/c a few degrees colder at night cools the big cement slab that the house is built on, and thus saves electricity during the day (the a/c is set back to normal living temperature during the day).
2b. If the temperature inside the house reaches 78 F at 10 AM on both days with the a/c set colder the previous night, and also when it was just set normally the previous night, then that proves setting it colder made no difference.
3. The a/c uses less current at night ( you measure it with an ammeter as it is running ).
4. The a/c uses less current if you spray the outside unit with the garden hose and then measure it with the ammeter.
5. Shading the outside unit (compressor and condenser) does not reduce electricity costs [Assume shade does not block air flow].
6. If you have high ceilings and ceiling fans, it is more energy efficient to leave the fans running at low speed all the time to pull down hot air and get it to circulate through the a/c system.
7. It isn't worthwhile to check on the amount of Freon (or whatever) that is in the system -- all that matters is measuring the temperature of the cold air coming out (say 62 F) and the outside temperature or maybe the attic temperature.
--Jeff
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<snip>
Hmmm, if the air going into the heat exchanger is 25 degrees cooler (100-75), what happens to compressor efficiency and compressor hp requirements?

Essentially ditto for #3.
There are companies selling swamp cooler precoolers for AC heat exchangers.
Easy enough to prove or disprove. Just watch your meter spin and spray down the AC.
<snip>
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Jim Pennino

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Would the current go down, or the cold being delivered go up?
Bob
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wrote:

mkay !? Understand that and you will understand W H Y *_you_* never apply ANY condensing medium change to ANY working system...EVER !
BTZ
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On Thu, 22 Jul 2004 19:38:04 +0000 (UTC), snipped-for-privacy@specsol-spam-sux.com wrote:

Spin?
Unless you live in the sticks, KWh meters are rapidly going digital. ;-)
-- -john wide-open at throttle dot info
~~~~~~~~ "The first step in intelligent tinkering is to save all the parts." - Aldo Leopold ~~~~~~~~
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Rapidly doesn't mean completely
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In sci.physics, Robert Morein
wrote on Sat, 31 Jul 2004 14:32:50 -0700

I would be surprised if meters are replaced just because the readers want digital. Most likely the meters are replaced when the building is rewired, and that's fairly rare. New buildings, of course, will get the new meters.
I see the same problem here as with highway signs: how many metric highway signs have you seen lately? :-) Probably not many. I'm not sure if new highway signs are even required to be metric -- certainly the roadwork projects here aren't using km yet.
In any event, kWh already *is* generally metric, although one might make a (weak) case that they should be reading either joules, kilojoules, or megajoules. (1 kWh = 3.6 MJ. The nearest Imperial unit might be a BTU.)
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All the ones I've seen spin? is there some other way of measuring actual usage?
~^Johnny^~ wrote:

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1 no How dumb, you are saying cooling it when you are on vacation is cheaper than not cooling it 2 no Dumb again 2b exact exterior temp , solar load, wind, humidity have been maintained in a lab or your dumb again 3 no , unless voltage is higher at night, which it often is. 4 Hard water , mineral buildup 6 Question is openended 7 Hacks do a lot of stupid things, just ask Daves Heating or I can listen to you.
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J Jensen wrote:

Maybe, it really depends on many many factors. It would be best to measure the specific application.

See above, it might.

Big if. If it does it really means your measurements/test is not valid. College Physics 101 will tell you that if your test is accurate and all factors are considered, there must be a difference, even if it is small. Soft of the butterfly effect.

I believe that is generally true, assuming it is cooler at night.

Same as above, but you may end up damaging your condenser unit.

This is an example of inadequate measurement. There is a difference but it is so small that it is difficult to measure. See 2b.

Only if it is you goal to cool the ceiling.

Worthwhile for what?

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Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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It seems to me that it would clearly be best to let it get hot in the house if no one is there during the day and just run the a/c hard to cool it down in the early evening. The only issue is that the a/c is then doing a lot of work when the outside temperature is still near its hottest. I am told by someone that this caused the unit to run and run for hours continuously, but my suspicion is that that was due to a lack of freon or some other malfunction (it was a small house!).
[snip]

I am glad that everyone seems to agree that running all the fans and cooling all the air in the house is a bad idea, but, believe it or not, it was proposed by a home efficiency expert to came out to the house several years ago. (I wasn't home at the time so I didn't question him on why he said that).
--JEff
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J Jensen wrote:

If designed properly, an AC system WILL run continuously on the hottest (outdoor design temperature) days in order to maintain the desired indoor (design temperature). Obviously, if the home has heated up beyond the indoor design temperature then it will take the system a while to cool it back down.
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J Jensen wrote:

When the equipment is in good condition and properly sized it may run for long times under this situation. That is normal. A larger system would run for less time, but not be as efficient under most situations. I used a lot of conditional statements because there are so many variables that just measuring the results are usually the easiest way to answer the question.

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Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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J Jensen wrote:

http://www.ontario-sea.org/34kyoto/ac.html
http://www.ontario-sea.org/34kyoto/ac.html
Many, many variables in this equation.

http://homeenergy.org/archive/hem.dis.anl.gov/eehem/95/951102.html
effect on skin). To save energy, turn fans off when you leave the room.

There are many factors that influene the measured temperatures besides the refrigerant charge..
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[snip]

That is about what I thought -- the slab really doesn't change temerature appreciably unless you really made the house really cold and held it that way for a long time, which would cetainly not be saving electricity! And if the a/c kicks on at the same time in the morning with or without "cooling" the slab all night, then the rest of the day it is going to run just the same as always.

[snip] --Jeff
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This is Turtle.
You got too many Question here and I will answer just the first 2 .
Answer to 1 ) If you turn a hvac system off less than 8 hours. It will cost you more money to recool the house from a very high temperature to the lower temperature than just moving up to a higher temperature on the thermostat. If your going to be gone more than 8 hours move the thermostat to about 85F or so and turn it back down when you get back home. If your going to be gone for 24 hours or more like on vacation. move the thermostat to the highest setting of about 95F and keep the house below 95F because refrigerator , Freezers, and Wine Coolers are not designed to run in temperatures above 95F. Most or a lot of Refrigerator & Freezers will stop working at 100F or above. If you read the installation instruction when you bought the refrigerator or freezer it tells you to not run the appliance in ambiant above 95F. Also do not run it in Ambiants of below 40F.
Answer to 2) and 2B) .
2) Hog wash.
2B) You found the Hog wash answer was the answer.
TURTLE
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[snip]

I never do turn it entirely off, but that is probably a good point about the effect on other applicances...
[snip]
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About 88 average over 24 hours, and about 82 at night...

Newton said the rate of heatflow into a building is proportional to the indoor-outdoor temperature difference. IMO, turning the AC off will save energy, even if only for a few minutes.

This wouldn't help, with a constant COP that doesn't fall with a higher indoor-outdoor temp diff. But it might, if the AC can move more heat energy with the same electrical energy at night, given a smaller night temp diff, eg if the COP were 3.3 at night and 3.0 during the day, or with lower off-peak electric rates.
A 4" floorslab can store about 8 Btu/F-ft^2 with a 4-hour time constant. It might cool from 75 to 70+(75-70)e^(-16/4) = 70.1 F after 16 hours in 70 F air. With R20 insulation outside, RC = 20F-h/Btux8Btu/F = 160 hours, so it might only warm from 70.1 to 94+(70.1-94)e^(-8/160) = 71.3 in 8 hours when it's 94 F outdoors. Or less, with little air movement in the house. A slab or a basement might be a efficient place to store coolth during a daytime setback, since cool air falls. We might only bring coolth up into the living space with a ceiling fan and a thermostat and an occupancy sensor as needed.

That would say it's a small difference.

I imagine so. How much less? How does the COP depend on the temp diff?

Definitely. But I'd use rainwater, with no minerals.

Shading should help, but as others say, the improvement may be small.

Maybe not, if you are seated :-) You might look up "displacement ventilation."

Why on earth would you say that? Do you work for Turtle Power and Light? :-)

I wonder what goes wrong. It can't keep up with the cooling load? At any rate, just putting the fridge inside a house with some thermal mass and shaded windows and little internal heat gain may be enough. Very few places in the US have a 24-hour daily average temp over 95, and fewer still have average night temps above 95. A house on vacation might keep itself cool with night air, using an exhaust fan and a differential thermostat that turns the fan off when outdoor air is a few degrees warmer than indoor air (to account for internal heat gain.) Brand Electronics may soon be selling a controller like this.

In wintertime, I unplug the barn fridge and keep the apples and carrots from freezing with a 100 W bulb in a trouble light in a lower bin, using an EH38 "Easy Heat thermostatically controlled device" ($10.99 at Lowe's) that turns the light on at 38 F.
Nick
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This is Turtle.
Well first I see you don't work on hvac system and know what the run times are for a properly sized hvac system verses a cool down time for a indoor temp. of about 105f down to 70F to 75F . 8 hours you may save a little but at 4 to 6 hours of down time will cost you 4 to 6 hours of run time at 105F to get it back to the regular temp. inside. Also your going to waiting about 1 to 2 hour before you can stay in there when you come home.
Now if you have oversized hvac system like 5 tons on 1,500 sq. ft. house. Your answer would be ok, but a properly sized system would cost you big time on a 4 hour down time.
TURTLE
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This is basic 300 year-old physics, Turtle. Turning an AC off for even 1 minute saves cooling energy :-)

But the AC setback still saves energy.
Nick
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