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re: (diy) solar /radant  heating of a small room
6 jan 2003
jim  wrote:

>i have a 25 by 10 foot room to heat in the high desert of southern
>california... (35 degrees north lat. @3200 feet)...

sounds like an easy climate for solar heating. january is the worst-case
month in daggett (34.87 n at 1929'), with 1650 btu/ft^2 on a south wall
and 1000 on a level surface on an average 48.7 f day. 

>what i want to do is place 200 feet of  1/2" pex pipe in the slab and
>then pump water into collectors on the roof... from what i have gleaned
>so far i am going against the grain by designing a system with air in it.
>i think the no air edict comes from boilers. am i correct?

the no-air edict comes from a desire to use less-expensive pumps with
cast-iron vs bronze impellers. the slab can provide desirable thermal mass
for overnight heat storage, but with enough insulation and airtightness,
you might use baseboard radiators with low-temp water instead of tubing.

>my real question is how do i size my collector?  i would like to use
>copper tubes under glass on a steel or aluminum sheet.  on a roof 25by
>12 facing south with a 3/10 pitch

consider making a thomason-style collector by trickling water between
a dark green galvalume roof and a single layer of dynaglas polycarbonate
greenhouse roofing. a square foot of roof with 90% solar transmission and
a projected horizontal area of 0.958 ft^2 and 0.287 ft^2 of south-facing
surface would collect 0.9(0.958x1000+0.287x1650) = 1288 btu/day. at 50%
efficiency, that's 193k btu/day. 

with no south windows or internal heat gain from electrical energy use,
193k btu = 24h(65-48.7)g makes g = 494 btu/h-f... 810 ft^2 of wall and
ceiling surface would need a min r-value of 810/g = 1.6. not much :-)

nick




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