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an oklahoma city house
7 jan 2004

mr. okie writes:

>...we are getting a little cramped in our 1700 sq. ft. house.

i know the feeling. sip (structural insulated panel) houses are excellent
insulators, and they can go up fast. is there a manufacturer near you?
you might look at the national sip association web site. premier has an office in mead, ne (800) 228-4412. draw your house
on the back of an envelope, send it to them, get back a cad drawing, look
it over, send it back, get another back, and after a few iterations, it
arrives in the form of big (up to 8'x24') flat panels on a flatbed truck
with precut holes for doors and windows, and they screw them together
in a week or so. 

>       we are looking at building a 2000 to 2200 sq. ft. home with 
>several pasive solar designs but find your idea on the solar closet very 
>interesting as it continues to produce the warmth even on cloudy days. 

a solar closet can keep a house warm for a few cloudy days... a "shelfbox"
improvement can also make hot water for showers. 

>we live near okla. city...

...a fine climate for solar house heating. nrel says 870 btu/ft^2 of sun
falls on the ground and 1340 falls on a south wall on an average 35.9 f
january day with a daily max of 46.7. east, west and north walls get 570,
590, and 230 btu/ft^2. 

>we have mostly sunny days...

if sunny days are like coin flips, a house that stores overnight heat can
be at most 1-1/2 = 50% solar-heated. storing heat for two cloudy days can
be 1-1/4 = 75% solar-heated, 3 makes 1-1/8 = 88%, and so on. 

>...can one simply build a well insulated structure oriented to the south
>face with a large enough solar closet built onto the south side of any
>conventional home?

that can work. a solar closet is a glazed box that normally lives inside
a sunspace with its own glazing. the sunspace provides heat for the house
on an average day, and the solar closet provides heat on cloudy days. 

a 48'x48' house with 8 8'x24' 8" r32 sips for walls and a flattish roof
with 12 moree and 192 ft^2 of u0.32 windows with 50% solar transmission,
96 ft^2 on the south, 48 on the east, 24 on the north and 24 on the west
and 30 cfm of air leakage would have a thermal conductance of 192ft^2xu0.32
= 61 btu/h-f for windows + 1344ft^2/r32 = 42 for walls + 2304/32 = 72 for
roof + 30 for air leaks, a total of 205. it needs 24h(65-35.9)205 = 143.2k
btu of heat on an average january day. if 600 kwh/mo of indoor electrical
use provides 68.2k btu/day of that, it needs 143.2k - 68.2k = 75k btu/day
of "other heat." the windows would provide 0.5x24(4x1340 +2x570+590+230)
= 87.8k btu, more than enough to keep the house warm, with some thermal
mass (how much, if the house is 70 f at dusk and 60 at dawn?) near the
windows to store overnight heat, so we don't need a sunspace for average-
day heat. 

this house would need 5x75k = 375k btu of heat for 5 cloudy days in a row,
which might come from 375k/(120-80) = 9375 pounds or 1172 gallons or 146 ft^3
of water cooling from 120 to 80 f over 5 days. a 4'x8'x8' tall shelfbox 
with a 4'x8'x4' tall 128 ft^3 tank under 12 4'x8' shelves with 2" of water
in a poly film duct on top of each could provide cloudy-day heat and preheat
water for showers using a 42 gallon pressurized galvanized tank in the lower
tank and a simple greywater heat exchanger.


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