Posted by nicksanspam on December 24, 2008, 3:46 pm
Better and cheaper. A layer of black window screen would reduce reradiation
loss to the glazing and might keep the air between the screen and the glazing
close to 70 F, so a 1'x1' box with R1 glazing would lose (70-30)1ft^2/R1
= 40 Btu/h on a 30 F day. With 90% transmission, it would gain 225 Btu/h
in full sun, for a 185 Btu/h net output.
A panel with 10 cfm of airflow behind it producing H Btu/h of net heat
with a max temp of 70+H/10 and average 70+H/20 and H = 225-glazingloss
= 225-(70+H/20-30)1ft^2/R1 makes H = 185/1.05 = 176, no?
This seems like a good time to make a "heat trap" like an igloo for
the (20 F) barn cats, with a sleeping platform that's above the top of
an entrance below it, so cat-heated air (which rises) can't escape
through the entrance. We might put a cardboard box upside down on top of
3 strawbales in a U-shape, with a 4" gap between 2 of them so the cats
can enter via the gap/tunnel and climb up to lie on the strawbales
under the box. We could pile strawbales all around and over the box
or drape it all round with rags on the outside.
A 2'x2'x1' tall box with 12 square feet of 1" R4 rag surface and 2
20 watt (68 Btu/h) cats could stay 70 F until the barn temp dropped
to 70-2x68x4/12 = 24.7 F... 2" of rags would make this minus 21.7 F.
Merry Christmas to all, including the kitties (who might prefer a few
creatures stirring, eg some tasty mice.)
Posted by schooner on December 22, 2008, 12:14 pm
Another similar system I found on Gary's site:
Posted by RF on December 22, 2008, 8:38 pm
Great job! Very impressive indeed. I could do with
one myself and I'm tempted.
1 What happens to the heat input when the sun is
2 The polycarb seems to be reflecting lots of rays
for certain solar positions. The surfaces are
probably very smooth. Possibly dimpled surfaces
would reflect less and transmit more energy or
would it cost a lot more, if available?
If I were making this I would make one change to
the frame: Replace the 1x6s that form the frame
with 1" thick plywood - more stable, less twisting
and bending. Kiln dried 1x6s would be better than
fresh lumber. What kind of strength does the
polyiso have? Is it plastic foam with two layers
Posted by schooner on December 22, 2008, 9:03 pm
This isn't our design, it is one I found on the net. This is ours: