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re: advice needed for cooling residential hydronic system...
29 jun 2001
fbmcgalliard  wrote:

>...this will work ok if you go barefoot a lot, or have a fan to
>circulate the warm air at the top of the romm aginst the cool floor.

the fan might help a little, but the room air should be well stirred
by convection currents caused by air rising up the warm outside walls,
in to the center of the ceiling, back down to the floor, and back up
the walls. 

>your outside air temp was, i thought, doing a 30f delta most of the time (a
>clear air, high desert kind of temp range). and the solar heat on a flat
>black surface, even moderately protected from direct wind, is going to be at
>least another 10-20f...

the idea was to lose heat from a wet surface at night.

>so when the air is around 100, you will have a lot of 70f water to cool
>the house. when the air is around 20-30 you get a lot of 70f water to
>warm the house...

let's be more specific with numbers, fred.
do you agree with the calc below?
if not, why not?

10 t=63'24 hour indoor temp (f)
20 ta=74!'average outdoor temp (f)
30 tamin=57.7'average daily min temp (f)
40 tamink=(tamin+460)/1.8'average daily min temp (k)
50 w=.0065'average outdoor humidity ratio
60 pa=27.07/(1+.62198/w)'ambient vapor pressure ("hg)
70 td=9621/(17.863-log(pa))-460'dew point temp (f)
80 tdc=(td-32)/1.8'dew point temp (c)
90 tm=3'hours since midnight
100 tskyk=tamink*(.711+.0056*tdc+.000073*tdc^2+.013*cos(15*tm/180*3.14))^.25
110 tsky=1.8*tskyk-460'night sky temp (f)
120 print ta,td,tsky
130 l=32'cube dimension (feet)
140 wa=.08*4*l^2'shaded window area (ft^2)
150 uw=.25'us u-value of windows (btu/h-f-ft^2)
160 gg=wa*uw'window thermal conductance (btu/h-f)
170 rw=24'us r-value of 6" sip walls (ft^2-f-h/btu)
180 gw=(5*l^2-wa)/rw'wall thermal conductance (btu/h-f)
190 ach=.5'house air changes per hour
200 cfm=ach*l^3/60'air infiltration rate (ft^3/h)
210 print gg,gw,cfm
220 g=gg+gw+cfm
230 cgain=24*(88.2-t)*g'conductive heat gain (btu/day)
240 euse=300'electrical energy use (kwh/mo)
250 egain=3412*euse/30'electrical heat gain (btu/day)
260 gain=cgain+egain'daily heat gain (btu/day)
270 print cgain,egain,gain
280 pw=27.07/29.92*exp(17.863-9621/(t+460))'vapor pressure at temp t ("hg)
290 evaploss=100*(pw-pa)'evap loss (btu/h-ft^2)
300 radloss=1.714e-09*((t+460)^4-(tsky+460)^4)'rad loss (btu/h-ft^2)
310 loss=6*l^2*(evaploss+radloss)'6-hour nightly loss from wet wall (btu/day)
320 print evaploss,radloss,loss

    74            42.76764      22.78391
    81.92         199.68        273.0667
    335462.4      34120         369582.4
    25.10836      35.12275      370060

>...the cistern can range from 500 gallons, probably enough to support
>your needs for a few days...

let's see... 3daysx24hx35kbtu/h/(69f-60f) = 280k pounds or 35,000 gallons.

>to 10-20,000 gallons, enough to support your needs for a month or so...

that works best for short months :-)
>...i personally prefer the heat powered motors for this kind of task, but
>they involve maintenance at a moderate level.

what heat powered motors?


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