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re: more fun with air conditioning
2 aug 2004
turtle  wrote:
>"j jensen"  wrote in message

>> 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.

about 88 average over 24 hours, and about 82 at night...

>> 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.

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.

>> 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).

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.

>> 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.

that would say it's a small difference.

>> 3. the a/c uses less current at night ( you measure it with an ammeter as
>>    it is running ).

i imagine so. how much less? how does the cop depend on the temp diff?

>> 4. the a/c uses less current if you spray the outside unit with the garden
>>    hose and then measure it with the ammeter.

definitely. but i'd use rainwater, with no minerals. 

>> 5. shading the outside unit (compressor and condenser) does not reduce
>>    electricity costs [assume shade does not block air flow].

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

>> 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.

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

>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.

why on earth would you say that? do you work for turtle power and light? :-)

>...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.

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. 

>also do not run it in ambiants of below 40f.

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.


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