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re: more fun with air conditioning
5 aug 2004
turtle  wrote:

> wrote:

>> >> 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...
>>
>> 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...
>>
>> 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.
>>
>> >this is turtle.
>> >
>> >...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? :-)

this is basic 300 year-old physics, turtle. turning
an ac off for even 1 minute saves cooling energy :-) 

>...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.

but the ac setback still saves energy.

nick




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