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Passive Solar Heater Box Design - Page 3

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Posted by Bill Kreamer on July 1, 2005, 5:31 am
I quit using "black painted fiberglass" absorbers for reasons of
smell/outgassing and troublesome preparation/mounting. I now use black
polyester dressmaking felt, which does not outgas or melt at collecting

Do use an intake fan - it pays generous dividends. Passive is not your
efficiency friend here.

Silicone glue it onto inch-wide aluminum-flashing side rails, with the rails
oriented on a diagonal, forcing a front-to-back air flow through the felt.
Seal around the edges of the felt absorber so the air must pass through the

Cooler room temperature air floods the front face of the collector before
passing through the absorber, and cools all areas of the absorber equally -
this is important for efficiency.

You can build your box of wood and line the inside with foil-faced urethane
or polyisocyanurate foam. This presents an aluminum surface to the

If you do not allow the collector to stagnate, you are safe using these
materials. It is best to mount the collector vertically.

You can email me requesting a PDF file of illustrated instructions for a
building a high performance solar hot-air heater using these principles.


Posted by livefree on July 1, 2005, 6:42 am
On Fri, 1 Jul 2005 01:31:32 -0400, "Bill Kreamer"

Does the felt fade quickly?  I have black painted fiberglass furnace
filter, and it has faded some after a couple years.  If I repaint,
I'll look for some better paint.  I haven't noticed any outgassing,
but my nose isn't very sensitive.

Mine is in a 2x6 stud wall using salvaged storm windows.  I draw air
from the bottom and use a fan.  My black filter material has 3 sloped
sections per window, but the air passes from back to front so I can
slope them toward the sun.  With a fan, (or without maybe) I don't see
the significance of pulling the cooler air through the hot side first,
which would seem to require either pulling hot air down (8' in my
case) or sloping the filter material downward (if mounted vertically).
Perhaps it is more significant with felt since furnace filter material
probably heats more uniformly through all the layers.


Posted by David L. Jones on July 1, 2005, 10:45 pm
 livefree@ordie.com wrote:

So (correct me if I am wrong), both of you guys don't use the
traditional(?) collector plate approach with a still air gap between
the collector plate and glass? You simply pump the air through the
whole box? I was under the impression that this approach leads to
excess heat loss back through the front glass/polycarb sheet.

Dave :)

Posted by Anthony Matonak on July 2, 2005, 1:54 am
 David L. Jones wrote:

Their collector plate IS the felt (or furnace filter painted black).
They pass the air through their collector plate and, since the felt
has a lot of surface area, it transfers heat very well to the air.
Since the heat is transfered from their collector material to the
air so well, it's not at as high a temperature and therefore will
radiate less heat back out through the front glass.


Posted by Gary on July 2, 2005, 2:58 am
 Anthony Matonak wrote:

Maybe this is obvious, but you have to take care to make sure that the
flow path for ALL of the air is through the collector.  Most of the
designs introduce the cool air from the room on the bottom of the
collector, and on the glass (south) side of the absorber. The air
flows upward and North through the absorber and exits from a vent that
is North of the absorber.  Some designs tilt the absorber slightly the
south at the top.  This makes flow path area larger at the bottom on
the glass side, and larger at the top on the exit vent side.

While I have great respect for Bill K's collector that uses felt as
the absorber, I think you might also consider using 3 layers of black
aluminum window screen as in the collector link below.  The window
screen is easy to buy, cheap, doesn't smell, lasts (more or less)
forever, and will withstand the temperature of a fully stagnated
collector pointed right at the sun.  It also has good efficiency
(based on some fairly careful testing).

Link for collector using metal screening as absorber:

Others have also used expanded metal lath, and reported that it works


"Build It Yourself" Solar Projects

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