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re: ...geodesic dome questions
20 nov 2003
alec chiasson  wrote:

>is 4' as high as i can go with air heaters in the south wall, while keeping
>the system passive?

maybe not.

>noting the chimney formula cfm = 16.6 av sqrt(hdt), how do you figure out
>the size of the vents in the sunspace? are they sized proportionally to the
>sunspace volume to facilitate a moving stream of air?

you might use desired sunspace air temps and the amount of glazing and the
peak sun power. for instance, if 70 f living space air enters a sunspace on
a clear 30 f day and 100 f sunspace air returns to the living space and
a transpired mesh collector near the north wall keeps room air in the bulk
of the sunspace and near the glazing and makes 100 f sun-warmed air to the
north of it, and 250 btu/h of sun falls on each square foot of r2 sunspace
glazing with 80% solar transmission and 200 btu/h enters the sunspace and
the glazing loses (70-30)1ft^2/r2 = 20 btu/h to the outdoors, a sunspace
with 100 ft^2 of glazing would gain 18k btu/h, net. 

with 100 f air behind the mesh and 70 f air on the other side of the living
space wall, dt = 30 f and 18k btu/h = 30cfm, since 1 btu/h will heat a 1 cfm
airstream about 1 degree f. if h = 8' and 18k btu/h = 30x16.6avsqrt(8x30),
av = 2.3 ft^2.

>is it prudent to overbuild a solar system somewhat?

sure. you might do this to better ensure that it works on partly-cloudy days
or to make up for construction mishaps like insulation gaps or air leaks or
to keep framing on 4' dimensions. you might want to avoid cutting 4'x8' sheets
of plywood or foamboard or 4'x50' rolls of polycarbonate plastic or 10' or 20'
wide rolls of epdm rubber. 

nick




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