Posted by pjo on November 18, 2008, 4:28 am
I made my first passive heating panel last week. Spent a lot of time on the
Desoto site (lots of good info there). It is small (about 3.5 sq ft) but
fit in an existing "hole" in my wall. I've got it in my pump room which is
attached to the garage/basement. Our pressure tank from the well is in
there and it freezes once or twice a year when we get the -20 F stretch (I'm
in southern WI). Anyway I'm hoping that it pumps out just enough to keep
the room from freezing. This was a couple night project to see what it can
actually do. Unfortunately it's been cloudy and crappy out since I put it
in, hopefully this weekend will be sunny so I can see if it actually pumps
out any heat.
My collector is the "type 3" variety, basically it has a S channel to help
prevent drafts and provide a little extra insulation. Glazing is .08
acrylic, inside of the panel is 1/2" plywood painted w/ hi temp BBQ paint.
Also I included two layers of black metal window screening. The screening
is pretty much in the center of the channel, not flat against the "black
wall". I offset it with a couple of wood spacers.
My main questions are (being a newb):
What strategies do folks employ to capture UV wavelength light? The
spectrum has a lot more area under the curve than the visible light. It's
been awhile since I've been in my physics classes but I would think the flat
black paint mostly absorbs visible light.
Does the thickness of the wood in the panel really have any affect? I was
thinking that if I have fixed width to work with, it my be better to use
1/4" plywood to maximize the width of the air channels than 1/2" which has
more rigidity and a higher R value (well, that may be negligible)
Has anyone tried using insulated glass panels (bascially what they use in
window and doors)? I would think if non LoE glazing was used, you would
retain pretty much the same transparency as a single glazing but gain a lot
in insulation value (basically what a normal window would be).
Anyone this has turned into quite the novel so I'll cut it off here. I'm
sure I'll have more questions for ya'll.
Thanks in advance,
Posted by Morris Dovey on November 18, 2008, 7:49 am
First of all, thanks for the kind words!
It's tiny, but will put out /some/ heat. A thermometer should give you
some strong clues as to how well it'll do the job. It should be putting
out measurable heat even in full overcast.
See http://www.iedu.com/DeSoto/Astro/Absorber.html for a rough look at
solar output (atmospheric filtering carves notches into this) and note
that UV is a relatively small component compared to visible and lower
This is a Good Thing - it lets us use UV blockers to protect
UV-vulnerable glazings without major losses.
The thickness of the wood in the plenum dividers isn't particularly
important. The depth(s) of the plenum(s) is considerably more important.
In terms of "bang for the buck", you're better off spending for 6mm
twinwall polycarbonate - and it's a lot less prone to hail/kid damage.
DeSoto, Iowa USA
Posted by pjo on November 19, 2008, 12:48 am
I meant infrared, not UV. Had a brain fart last night.
Is yellowing a concern w/ polycarbonate? Also what about
expansion/contraction vs glass?
Posted by Morris Dovey on November 19, 2008, 1:10 am
Yellowing hasn't been a problem with any of the stuff I've used. I still
have a scrap from my first twinwall project (almost thirty years ago!)
and it shows no signs of yellowing. All of the stuff I've bought has
been UV-protected on one side, and I've been careful to keep that side out.
I've only built one panel with glass - interestingly, it was a 3'x4'
panel installed in an uninsulated 16'x16' milkhouse on my Minnesota
farmstead - to keep my well head pressure tank and water line from
freezing. That glass was secured with points and putty just like a
window, and expansion wasn't a problem.
Expansion/contraction hasn't been a problem with the polycarbonate in
temperatures ranging from -30F to +180F.
DeSoto, Iowa USA
Posted by nicksanspam on November 19, 2008, 10:35 am
My 4'x16'x10 mil double-glazed HP92W Lexan panels have a 3.75x10^-5F/F
thermal expansion coefficient, so they become 16(1+3.75x10^-5(100)) = 16.06'
long (0.72" longer) as the temperature rises from 0 to 100 F, which could
let them flop and fatigue in the wind and bulge about sqrt(96.36^2-96^2)
= 8.3" in the middle, were they not slightly inflated to 0.1" H20.