Well, I tried an admittadly cheezy solar heating experiment. I painted a
piece of flashing flat black and backed it with a piece of corrugated
cardboard. I then mounted it in a garage-shop window, 2" from the insulated
panes in a double hung sash (roughly 24 x 44). A 2"opening on top and
another on the bottom and the thing will bring the temp up 4* in a 336 sq ft
area. Evening temps are in the teens and daytime temps in the low to mid
30's, mostly sunny, but as the days cloud over I'm slowly surrendering to
the thermal flywheel of the equipment. So, what's the consensus? Not
enough mass in the flashing? Too much glare from the glass (not low-E)? I
had planned on a thermosiphon protruding from the bottom of the window at an
angle about equal to our latitude (
Unfortunately, time, money and health have me doing this poor substitute.
TIA for your thoughts!
I was of the supposition that the angle of incidence of the sun rays to a
more perpendicular glass surface would reflect more of the light than if the
glass was inclined, causing the all of the light to pass straight through to
the black flashing.
it sounds like it is pretty effective, actually, considering you have less
than 8 s.f. in a vertical opening.
To usefully heat the space you'll probably need about 40 s.f. at the proper
This is hard to do in common structures, which is why you see various style
collectors on roofs.
The other problem with wall glazing is that it's a significant heat loss
when the sun is not shining.
If you're just heating a workshop during the day, it's not an issue.
Where I live near Phila, 40 ft^2 of R2 south windows with 80% solar
trasmission might admit 32K Btu and lose 6h(70-30)40ft^2/R2 = 4.8K on
an average 30 F January day, for a net gain of 27.2K Btu, enough to keep
336 ft^2 with 1000 ft^2 of exterior walls and ceiling 70 F for 24 hours if
27.2K = 24h(70-30)1000/Rv with Rv = 35 walls and ceiling, given night
insulation for the window/collectors and enough thermal mass.
No. Vertical south glazing is fine for winter heating.
Unlike a window, an air heater might have insulation behind the absorber
with no airflow nor heat loss at night :-)
That could work on an average day with R18 walls and ceiling.
In the north east, clear day radiation is worth ~64 btu per square foot
of south facing glass. now, figure my well insulated 1000sq ft great
room runs 40000 btus on a cold night, you do the math.
As far as sloping the collector, in the winter, with the sun low, it is
not that important. For max year round efficiency it is probably better.
What might be fun with the oil/gas so high is to build a little outside
collector, maybe a 4x8 plywood box with a little pump and some copper
coils to collect and a car radiator or somesuch on the inside and a
little fan. Might cost a hundred bucks to make, if you can scrounge some
stuff. math says it would put out more than one of those plug in 1500
watt heaters, free.
I am not arguing with you, just the numbers from the book 'from the
ground up' from some years ago. Also fits my seat of the pants for my
barn built then. 50 sq ft of south facing glass did not come close to
what 200 gallons of heating oil did in a season.
Again, I am not arguing with you, merely stating what my room needs, and
how unreasonable it is you expect that amount of heat from a small
Nice book. I can't find my copy, altho I've still got some copies of "From
the Walls In," having given away lots of them. If that's an accurage quote,
what did Charlie Wing mean by "64 Btu per square foot"? Full sun (AM2) is
about 250 Btu/h-ft^2. I've heard Wing's books and lectures and TV show and
Cornerstones school did such a good job teaching people how to save energy
that Exxon bought him out after a couple of years, and now he spends most
of his time sailing around the world on his large yacht :-)
Where I live near Phila, each $1 square foot of polycarbonate solar siding
might collect the heat equivalent of 1 gallon of oil per year...
Mon temp sun gain loss net
Oct 56.4 1150 1035 142 893 temp is the 24 hour vs daytime average.
Nov 46.4 990 891 202 689 sun is average Btu/ft^2-day on a south wall
Dec 35.8 900 810 265 545 gain is 0.9xsun
Jan 30.4 1000 900 298 602 loss is 6h(80-temp)1ft^2/R1
Feb 33.0 1080 972 282 690 net is gain - loss
Mar 42.4 1070 963 226 737
Apr 52.4 950 855 166 689
4224 Btu/day x 30 days = 126720 Btu/year.
I did find my copy, and the number was more like 60, and it is actually
more like the average net available btus, IOW, taking into account
cloudyness etc, not sunny day as I had stated. It is however the number
one would use to calculate heating. Also it was for Boston/Portland ME,
Philly is somewhat further south, thus more available sun.
I did not have my reading glasses, nor the energy to fight thru the
actual math, but there is a bunch of it there.
On 13 Dec 2005 11:48:38 -0500, email@example.com wrote:
Just build a greenhouse/lean-to against the entire south side
of the house, circulate the air when it's colder in the
house than both (A) the greenhouse, and (B) you want it.
In the daytime, when it's helping. you can use the greenhouse
as extra living space, and even when it's too cold for that,
it's still keeping that side of the house warmer than it would
7' is all you need. If you're going to invest in a big expanse
of glass, you might as well enclose living space in it.
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