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re: simple cooling system
28 aug 2004
james baber   wrote:

>...i live in fresno california, where the temperatures in summer are high...

nrel says fresno in august is 81.9 f, with an average daily min of 63.8 and
an average humidity ratio w = 0.0092, which makes pa = 29.921/(1+0.62198/w)
= 0.3461 "hg, which makes the dew point about 9621/(17.863-ln(pa)) = 514.7 r
or 54.7 f. evaporative cooling might help. 

>the average hourly temperature for each hour for the past 74 days has been
>80.5 f, with an average of 96.8 f at 17:00 and an average of 71 f at 08:00...

>i concur with gary about the beneficial effects of a wholehouse fan, but i 
>cannot run mine 24 hours a day as you could in england.

i doubt that's a good idea in england either. imo, an attic should be well-
vented, and a whf should only run when the outdoor temp is cooler, or maybe
a little bit warmer than indoors, with internal electrical usage.

>by running mine only when the outside air is cooler than the inside air,
>and only when the inside air is less than 82 f, i have been able to
>typically reduce the time i run my air conditioning by 75%.

you might run it when outdoor air is cooler and indoor air is > 56% rh,
and turn on an indoor mister when the indoor air is warmer than 82 f.

the wholehouse fan uses 450 watts / hour...

just watts. "watts / hour" is meaningless. i like grainger's $73 4tm66
3-speed 16" reversible window fan with thermostat, which moves 5850 cfm
with 90 watts on the highest speed.
 
>one major benefit of banking the cooler thermal mass of the house and its 
>contents until needed in the afternoon is the obvious savings in electrical 
>energy in kwh.

doing that well may require 2 fans, one to move outdoor air through the
house and another ceiling fan to mix it around in the house. the outdoor
air won't cool much mass if it just passes through the house in a compact
stream. it's also nice to have moving vs stillish air near the mass
in order to raise the surface airfilm conductance.
 
gary might run a whf in billings in july, when the daily min temp is 58 f
and the daily max is 87 and w = 0.0080. with a f cfm fan and constant day
and night temps, he might have something like this: 

               1/f
58/87-----/ ---www----- t   rc = 6k/400 = 15 hours, with the fan off. 
       |            |
       |    1/400   |       70 = 87+(t-87)e^-(12/15) = 47.91+0.449t
        -----www----|            makes t = 49.2 f, so it looks like 
                    |                  he needs a longer time constant
                   --- 6,000 btu/f     to keep the house 70 f max when
                   ---                 it's 87 f outdoors for 12 hours.
                    |
                    -

if 2" of concrete floor over hollow blocks adds 32'x32'(5+2/12x25) = 9387
btu/f and raises rc to 38 hours with the fan off, 70 = 87+(t-87)e^-(12/38)
= 23.3+0.732t, and t = 63.8 f at dawn. if 63.8 = 58+(70-58)e^-(12/rcn), 
rcn = -12/ln((63.8-58)/(70-58)) = 16.5 hours = 15387/(400+f), f = 532 cfm.
not much. 

but why waste coolth by keeping the house close to 63.8 f just after dawn?
if the slab is t (f) at dawn and we keep the house air 70 f all day with
a cooling thermostat and a slow ceiling fan (without which cool air will
stay near the floor) and an occupancy sensor, the house will only need
12h(87-70)400 = 81,600 btu of coolth, max, and 81600 = (70-t)x15387 makes
t = 64.7, so rcn = 20.6 hours and f = 347 cfm, or less, with indoor mist.

>...the second major reason is because i am on an time of use electrical rate 
>structure the cost of electricity is much higher (@$0.32524/kwh) in the peak 
>demand period (12:00 - 18:00) vs all the rest of the time (@$0.09015/kwh)...
>
>i also use it to warm the house on warm afternoons in the winter instead of 
>burning gas to heat the house...

nice, if the outdoor dew point is above the indoor house wall temp,
to avoid condensation...

nick




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