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re: question about air conditioning!
27 feb 1999
 wrote:
>>daniel woodard wrote:
>>> 
>>>         hi!  i have a basic question about air conditioning.

it's a bad idea, energy-wise. i'd say don't do it.

>>> does anyone have an idea how much it actually costs to cool
>>> a typical bedroom in a summer month?  i'm talking about a
>>> window air conditioner, electric, and the room has blow in
>>> cellulose insulation above and on the two sides facing outside.

the average outdoor temperature in july where i live is about 77 f,
with average daytime lows and highs of 67 and 86. we might want to
keep the room at 75 for 6 hours a day when it's 82 f outdoors...

if the room were say, 8'x12', with 2 r13 walls and an r20 ceiling and
a couple of 2'x4' r2 shaded windows and 1 air change per hour of air
leakage, and the 2 inside walls were uninsulated (r1) and the rest of
the house were not air conditioned, we'd have a thermal conductance of
8'x12'/r20 = 4.8 btu/h-f for the ceiling, 11.7 for the outside walls, 
4 for the windows, 160 for the inside walls(!), and 1achx8'x8'x12'/55
= 14 for air infiltration, a total of about 195 btu/h-f. so keeping the
room cool takes 6h(82f-75f)195btu/h-f = 8,190 btu/day, not counting
dehumidification. this might come from a 6000 btu/h ac running 1.4
hours per day. 

>a typical 6000 btu room a/c uses around 8 amps(never more than 8000
>btu for a larger room).  at 120 vac, that's 960 watts while it's
>running...

or 0.96kwx6h =  5.8 kwh/day, ie 46 cents at 8c/kwh or $13.80/mo.

>watt-hrs = volts time amps(typically 120 x amps)...

            x power factor, which might be close to 1.

putting 1.5" of styrofoam board on the inside walls and another layer
of sheetrock over that might reduce the room's thermal conductance to
about 54 btu/h-f, and caulking the windows and baseboards and putting
a sweep under the door to the rest of the house might reduce the air
infiltration to 0.5 ach, making the room's conductance 47 btu/h-f,
and reducing the ac bill to $3.30 in july. 

other alternatives include filling the room with thermal mass and blowing
outdoor air through it at night with a fan, then closing the windows
during the day, spending lots of time in an waterbed with a thin pad,
a bean bag chair or belt or collar filled with water or copper bbs,
cool sox (tm), knee-high synthetic socks kept damp with a squeeze bottle
on the waist, camel-kool (tm), a thin water-filled back pack worn under
a shirt with no insulation between you and the water, attached to a
radiator built into a pith helmet, with a battery/photovoltaic fan, and
head sink (tm), a close-fitting metalized heat sink hat holding lots of
interesting long pointy punk metal fins for personal heat dissipation.

perspiring helps too, as do basement apartments, swimming pools,
cold tubs, drinking more liquids, exterior window shading, wearing
less clothing, desktop fans, moving to maine, and having a higher
surface to volume ratio, ie thinth.

nick




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