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Posted by Jersey John on September 25, 2008, 4:44 pm
Thank you for the information folks. I can not get my head wrapped
around the rationale for the " 1/15th rule of thumb". Care to

My sketch calls for a panel roughly two inches thick, a plenum cross-
section of one inch (the other inch being polyiso). 1 x 24 x 144 = 24
cubic feet   With the plenum cross-section of 24 sq in, related to the
inlet/outlet of 14 sq in each, the inlet/outlets are 58% of plenum
cross-section, granted, perhaps not as efficient but passive. As Gary
noted, I will pay a 25-30 % penalty but it will be passive not active.

Will be taking pictures along the way and I may even attempt to make a
web page showing it all.
Did I miss something?

Posted by gary on September 28, 2008, 2:22 am

Hi John,
The 1/15 th "rule" is for thermosyphon collectors -- ones that operate
on natural convection and don't have a fan.
I believe that it came out of work done by Steve Baer years ago, and
is based on experiments with actual collectors.
The idea is that if you use the groundrule, you will have enough air
flow through the collector to make it relatively efficient.

It says that if the height of your collector is 8 ft (96 inches), than
the depth of the collector airflow passage should be 1/15th of that or
about 6 inches -- this is the depth of just the air flow passage, and
does not include insulation.

The depth of the airflow passage increases as the collector gets
taller because the collector glazed area goes up with height, and the
greater glazed area means that collector generates more heat, which
requires more airflow to carry the heat away and more collector depth
to handle that airflow.

So, your 1 inch deep air flow passage is way way to thin to meet this
guideline, and (I think) the airflow will be very low, and the
collector not very efficient.  You would get a large temperature rise,
but very little airflow, and the losses out the glazing would be
high.  But, the depth of the airflow passage is closer if you are
going to use a fan to move air through the collector.


Posted by gary on September 28, 2008, 2:04 am
Hi Bill,

I think the 2 layers of screen actually works out quite well for a
thermosyphon collector.
When you do the numbers, and take into account absorption at layer 1
on the way in, layer 2 on the way in, the back wall of the collector
absorbing, then layer 2 on the way out, and layer 1 on the way, very
little light escapes.
The big advantage of screen is that it has little flow resistance and
still has a lot of area, so you can get both good absorption and good
airflow, where the airflow is just as important to the heat output as
the temperature rise.

My collector has 5 identical bays, and I tried a side by side test
with 1 layer of screen, 2 layers of screen, and 3 layers of screen.
The heat output for 2 layers was more than 1 layer, but 2 layers and 3
layers were nearly tied.  So, it seems to me that while the 3rd layer
increased absorption a bit (a very small bit I think), it increased
flow resistance enough that the net result was no more heat output.


Posted by Bill Kreamer on September 29, 2008, 2:23 pm
 On Sep 27, 10:04pm, g...@builditsolar.com wrote:

Hey there Gary,

Your are right, of course, the increase in resistance caused by a
denser absorber does reduce the flow in a passive system. But you will
see that problem go away, and efficiency and output increase, if you
bite the bullet and decide to use a small quiet fan. Incidentally, as
soon as you do, the back wall makes a relatively small heat exchange
contribution (the denser absorber becomes the major contributor) and
so in this case (fan, w/denser absorber) it should be made reflective.

Please keep up the good work. Your website (builditsolar.com) is a


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