Posted by andre.julien on September 8, 2005, 10:00 pm
What are the thermal properties of PEX compared to copper?
I understand that they use PEX in heated floors now, so it must
transfer heat relatively well. I want to run the pipe to and from the
solar collector through PEX with some insulation around it where it
will be exposed to the cold. Ottawa winters can be very cold. From what
I can tell PEX is easier to work with.
Can I use PEX in the collector if I make my own?
Posted by Steve Spence on September 8, 2005, 10:48 pm
pex can't handle heat as well as copper, and doesn't transfer heat as
well, but it's cheaper, and easier to fix a leak. tends not to burst in
freezing temps as well.
Dir., Green Trust, http://www.green-trust.org
Contributing Editor, http://www.off-grid.net
Posted by Gary on September 9, 2005, 3:31 am
I just finished a test on a solar collector that was made from CPVC
pipe and uses extruded alum radiant floor heat spreaders for the fins.
CPVC is the high temperature version of PVC pipe, and while its
composition is different from PEX, its thermal properties are close.
Some construction pictures and the thermal performance test results
Its kind of a mixed bag. I would take a careful look at the results
before you decide. I suspect you would get similar results from PEX.
"Build It Yourself" Solar Projects
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Posted by Bert Menkveld on September 10, 2005, 11:53 am
What a great experiment! Thanks for sharing your results with us Gary.
I wonder if the large temperature drop from the fin to the circulating water
might not be caused by the poor thermal conductance of the CPVC material
itself. I found a spec for thermal conductance of CPVC at
This gives a thermal conductance of 0.9 BTU-in/ft^2-hr-F. Compare this with
aluminum at about 1500, or copper at 2700!
I think I found a value for PEX at
http://www.vanguardpipe.com/paltechdata.htm , which gives 2.1
BTU-in/ft^2-hr-F. That would seem to be substantially better than CPVC.
Hey Nick, are you reading this? What do you think?
Posted by nicksanspam on September 10, 2005, 8:02 pm
Bert Menkveld <bertATreentronicsDOTcom> wrote:
And limited contact area between the fin and the pipe.
http://www.boedeker.com/pvc_p.htm ... gives a thermal conductance of
We need to divide by the wall thickness (less then 1"), but copper is
better. I'd like to see somebody make "Big Fins" out of aluminum coil
stock. Home Depot sells rolls of 0.019"(?) Al painted white on one side
and dark brown on the other for 50 cents/ft^2. They might make fine
Big Fins with 4" wings if bent to a U shape with a brake and attached
to copper pipe with some hardware. They could easily nest for shipping.