In sci.physics, Travis Jordan
wrote
on Thu, 22 Jul 2004 23:11:19 GMT
:
J Jensen wrote:
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.
False.
http://www.ontario-sea.org/34kyoto/ac.html
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).
False.
http://www.ontario-sea.org/34kyoto/ac.html
There are admittedly some interesting issues here. I'll admit,
I'm wondering how much heat the contents and the surrounding
walls of a room actually have, compared to the air.
A 3m x 3m x 3m room doesn't have that much air; the mass of
air in the room is readily computable by Guy-Lussac:
n = PV/(RT) = 101350 pascals * 27 m^3 / (8.314472 J/(mol K) * 300K)
= 1097 moles = 31.8 kg (assuming air is about 29 grams/mole).
This is one reason why liquid oxygen is so darned effective
at vaporizing barbeques. :-) (Cf George Goble's experiments
therein, which are perambulating around the Net; a Google
search points to
http://web.archive.org/web/200210032...rdue.edu/~ghg/
Kids, don't try this at home unless one knows what one is doing. :-) )
Of course, lucky me: I'm on a second floor condo and the only
slab is the floor. As it is, I suspect the slab doesn't get
all that hot anyway; it is, after all, sitting on the ground,
which after one goes a certain depth is a nice, constant
temperature. (However, there are a number of issues here, too,
such as crawl space.)
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.
False.
Many, many variables in this equation.
3. The a/c uses less current at night ( you measure it with an
ammeter as it is running ).
Depends. Compressor current consumption can vary with the load.
Well, a theoretical Carnot heat engine would of course be more
efficient if the delta-T is lower; in the night it's generally
cooler. (Of course running A/C when it's cold outside is
mostly pointless, unless it's reversible and one wants to
*heat* the house [a heat pump].)
Real A/C, of course, depends on many things, such as the
compressor's ability to compress, the amount of refrigerant
in the system (one A/C unit where I used to work we had to
overcharge in the winter to keep it from freezing up, then
remove some of the coolant in the summer so that it would
actually cool more efficiently; unfortunately around here
it can get cold in the summer which means it occasionally
had problems anyway), and of course whether things are
blocking up the ductwork, keeping the cooling coils from
warming up or the hot coils from cooling down.
4. The a/c uses less current if you spray the outside unit with the
garden hose and then measure it with the ammeter.
See above.
I'll admit I'm not sure about this, but perhaps it's because
my unit is getting on the old side; spraying the hot
coils may affect the compressor pressure, to the point
where the unit simply can't do its job since the pressure's
too low.
5. Shading the outside unit (compressor and condenser) does not reduce
electricity costs [Assume shade does not block air flow].
It depends.
http://homeenergy.org/archive/hem.di...95/951102.html
There are probably a lot of issues here. If one shades the
entire wall, for instance, the cooling load is lighter,
presumably. (Raw insolation = 1,350 W/m^2, approximately.)
Of course, shading the A/C might work until the unit shading
the A/C warms up from the sun and the A/C heating it. Ideally
one would install a highly reflective mirror system to reflect
the sunlight back at the Sun, but that would probably annoy the
neighbors... :-) (Not to mention low-flying planes.) And then
there's the "greenhouse effect", where gasses (water vapor?)
in the atmosphere simply reflect the infrared; reflecting the
infrared using a mirror simply distributes the problem, making
everyone else's domicile in the area a very small amount hotter.
So if everyone installed a mirror over their A/C units (or the
entire house!), would we be better off? An interesting question.
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.
Fans don't cool rooms, they cool people (due to the evaporative cooling
effect on skin). To save energy, turn fans off when you leave the room.
Unless they are attic fans, pulling air *through* the attic, or
pulling air through the house during the evening or morning
hours, before the outside air gets warmer than the inside.
As for cooling people -- that works until the humidity gets so
high that the sweat doesn't evaporate. Shower time, or maybe
time for some swimming.
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.
False.
There are many factors that influene the measured temperatures besides
the refrigerant charge..
The target I've heard is 20 degrees cooler than outside.
--
#191,
It's still legal to go .sigless.