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Solar DHW: Copper or PEX

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Posted by Steve Shantz on October 18, 2007, 2:30 am
I'm helping put up a DHW system for a friend.  His yard is arranged in
a way that prevents the collectors from being mounted on his roof.
The best place in the yard is about 130' from his house.  He wants to
use PEX to and from the collectors.  I'm strongly recommending copper
(3/4").  But that much copper =$$$.  Can a compromise be reached where
the supply line is PEX, and the return is copper?

The system will comprise of 3 - 4x10 used flat plate collectors, with
glycol in the collector loop.  A grid powered circ pump will move the
glycol around, controlled by a differential temperature controller.
My fear is that sooner or later, his system will stagnate and this
might cause catastrophic problems with the PEX (especially the return
line) if the system is restarted hot.

Also, I'm thinking about using an STSS non-pressurized tank, 250 - 350
gallons in size.  This will help ride through the frequent cloudy
periods here in North Central Indiana.  My own system has similar over
sized storage, and I love it!  Has anybody used STSS tanks and the
heat exchangers that STSS provides?  It seems like a very inexpensive
way to store that much water.

Any help here would be appreciated.


Posted by Mark Schofield on October 18, 2007, 1:00 pm
I have an old Grumman Sunstream system. Al collectors. The install
instructions specified CPVC plastic piping. Some inside the house, some
outside. No problem with the CPVC after 3 years.

Posted by gary on October 18, 2007, 2:43 pm
I think PEX would be OK.

I have a system that uses 240 sqft of collector about 120 ft from the
There is a 410 gallon tank near the collectors, and a CPVC line from
the tank to the house.
There is certainly no problem with PEX or CPVC in this situation,
since the tank temperature never go above 170F.

Details here:
You might take a look at how the pipe was insulated on the run from
the tank/collectors to the house.  I think its pretty cheap and
effective.  Good insulation on this long an underground run is

Another example system that uses PEX:


Posted by nicksanspam on October 18, 2007, 6:55 pm

STSS has a new tank heat exchanger that's mostly PEX, after some copper
at the beginning. Less copper, since it has become a lot more expensive.
Sven says the copper is still needed for the initial burst of high temp
water when the collector pump starts.


Posted by Loren Amelang on October 18, 2007, 8:41 pm


I know from experience that you can easily end up venting serious
steam from an array that size, even with the circuit pressurized. I've
arranged a completely independent DC powered backup pump with its own
backup control thermostat, because it is such a pain to have your
glycol vented all over the yard. A big expansion tank is your friend,
but if the collectors begin making continuous steam it won't have a
chance of coping.

I keep imagining an opaque cover, rolled up around a length of pipe
just above the face of the collectors, and retained by latches that
would release it if the panels exceeded normal operating temperature.
It rolls down and shades the panels until you have corrected the
overheat problem. Haven't built one yet, though...  Something like
that could make your friend's installation safe for Pex.

Depending on your desired output temperature, you might exceed the Pex
limit even during normal operation. I'm looking at the STSS tanks you
mentioned, and the only temperature I see there is 120F. With that
much storage and that low a temperature, the cold feed lines to your
collectors could probably be Pex. I need 160F in my tank to feed my
radiant floor effectively, so I'm much closer to the edge. I really
prefer copper (but my panels are on my house).


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