wrote in message
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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? :-)
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 105ºf down to 70ºF to 75ºF . 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 105ºF 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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