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re: advice needed for cooling residential hydronic system...
27 jun 2001
al youngwerth wrote:
>i want to put cool water in my hydronic radiant floor system to cool
>my house. here's my story... i've got myself in a bit of a bind...
>i'm building a new home in boise, idaho. i had originally planned on
>heating and cooling the house with a 4-ton open-loop ground source
>heat pump. after $2500 in well drilling costs, no water from my well!
bummer.
>so now i am rethinking my entire heating and cooling system.
good...
nrel's says the average july temp in boise (el. 2868', 13.3 psi, or 27.07
"hg) is 74 f, with an average daily min and max of 57.7 and 90.2, a 111 f
record high, and a humidity ratio w = 0.0065.
>the house is about 4700sqft including a full daylight basement, very
>well insulated (r-50 ceilings, r-30+ walls (walls are up to 12" thick)
>good windows, well sealed).
kinda like a 32' cube...
>i installed a hydronic radiant system on both floors and have 40 cu yards
>of concrete in the basement and 22cu yards of concrete on the main floor
>(3 inches thick). this house has a lot of thermal mass.
about 42k btu/f for the concrete, like 5,200 gallons of water...
>...the worst case cooling load is about 45k btu/hr but we only need
>to cool for about 8-10 hours during the day (if we hit 102 in late
>afternoon, we're typically 65 by morning).
sounds like the thermal conductance is about 45k/(102-70) = 1400 btu/h-f,
(sounds highish) with a 42k/1400 = 32 hour time constant...
>i planned on heating and cooling my house radiantly through the
>floors. our typical summertime humidity is < 15%. that should give me
>30+ degrees difference between inside air temperature and floor
>temperature before condensation is a problem.
the dew point is about 43 f. with a floor-to-air thermal conductance
of 4700x1.5 = 7050 btu/h-f, you only need a 45k/7050 = 6.4 f temp
difference for 45k btu/h of cooling.
>...i know someone here in boise that has a similar setup but he
>is just using a whole house fan to cool his house in the mornings.
>he's happy with it but his house will get to 82 degrees after it's
>been 100+ degrees for several days. i want to stay cooler than that.
looks like you might keep your house 63 f on an average july day.
>the cost of a closed-loop system seems very prohibitive to me: 4-ton
>unit with all the necessary accessories is about $5k but i figure the
>2000' ground loop will cost another $3k...
plus about 45k/3412/3x0.1 = 44 cents per hour for the electricity.
>i can get a gas boiler for about $1.5k.
how about a reflective concentrating solar trough attic for heating?
you can collect about 400 btu of 180 f water per square foot of solar
aperture on an average december day and store the heat in a large
insulated tank in the house.
>...what i was hoping to find is some sort of relatively inexpensive
>solution for cooling the water. an evaporator that cooled water
>instead of air.
looks like trickling water over the roof or a wall at night would work.
nick
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
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