Posted By NRT.Rob on 09/29/2009 8:01 AM
NO, NO, NO... never use pex in a solar system. In the event of any kind of significant failure, a solar system can melt pex in a hurry, with any kind of panels or collectors on any kind of system. using pex on a solar loop is a horrible idea. horrible, bad, should never be done by anyone, ever, for any reason whatsoever.
there is NO SUCH THING as a failsafe that can guarantee that sort of failure can never happen. drainbacks can fail to drain back for various reasons. glycol systems obviously are an issue. none of the failsafes mentioned on that website are "fail safe". they are manual solutions or perhaps, if you're lucky, workable solutions as long as nothing critical fails. the website suggests covering your panels with a cloth when you get too hot!!! putting a cloth over your collectors???? really??? that's a safety feature??? you've got to be kidding me.
critical things will fail. when they do, what happens? for this problem, with copper, not much. worst case you'll blow a pressure relief valve. with pex, you melt the pipes and rain superhot liquid down from whereever it melts first. not a good scenario. do not do this.
It's not even clear if the surplus Craiglist panels in question can be operated in drainback mode, so many aspects of Gary Reysa's home-built systems may not apply. But tolerance for issues like melt-out risk tends to be a hallmark of DIY hackers & engineers like Gary.
Professionals would (should, could) NEVER take an approach that required more than the most minimal maintenance & operation by a disinterested homeowner, but those willing to manage it can do OK.
Vertically mounted panels with designed-in seasonal overhangs for minimizing summertime gain can have quite reasonable stagnation temps. (This should be measured, not just calculated, of course.) Movable shades can work well for the active & adventurous. (Something rigid & quite opaque, like OSB & sliding door track would be preferable to a flimsy fabric solution though.)
IIRC Reysa measured the stagnation temps at various mounting angles as a method of determining how much angle he could get away with to avoid melting the PEX, and by running the system at atmospheric pressure, the "rain of superhot liquid" in the event of a melt out is more likely to be dribble than an out & out storm. He makes up for the performance hit of higher mounting angles with more panel (since it's so cheap.) I've definitely seen scarier solar hacks than that, eh? :-) He still might put a float switch in his tank to stop his pump & limit the volume if the PEX leaks (from meltout or other).
Wirsbo claims to be at the 26 year point of an ongoing test @ 203F & 151psi for their PEX- 200F seems like a safe stagnation temp, if it can be brought that low. The actual melting point of PEX most often quote is "around 270F". Running it at atmospheric pressure in drainback mode you're unlikely to see a slug of 250F water the way you can in a pressurized loop, but you could conceivably see some 250F+ steam if you let the panels get hot enough in stagnation. Reysa limits his storage temp to 140F, so there's quite a bit of chillin' goin' on with the water entering a stagnating panel. But the fact that he's letting the panels stagnate whenever the storage is topped off @ 140F means he's probably not seeing PEX melting temps even in the panel (they're still there & not leaking internally- so far anyway... :-) ) I don't think his risk of scalding or home damage is great, but it's still non-zero. The fact that the PEX in the panel limits the location of a melt out leak pretty much to the panel. This would NOT be the case with a copper panel allowed to stagnate to temps as high as 270F or more.
Perhaps I wasn't as clear in the qualifications of my "qualified answer" as I should have been. Yes, you can use PEX
if you can verify by design & test that you can keep stagnation temps under control,
and are willing to manage, monitor & maintain the system carefully (if not as obsessively as Reysa, with his ongoing datalogging & system tweaking.)
With the surplus panels you should be able to get a good sense of the stagnation temps in a S-facing vertical mounting NOW, since the highest temps are likely to occur in the late-summer/early-fall when the midday sun is a bit lower and the days are still pretty warm, minimizing panel losses. Building overhangs to kill late-spring/early summer midday gain isn't rocket science either. If you decide you need more summer BTUs at the lower temps of a partially shaded vertical mount, if have the space, the cost of more surplus panels is also not very much, but separate DHW and space heating systems are probably still a good idea, since the temperature requirements are different enough.
Personally, I prefer pressurized systems with a glycol loops & HX, but I'd still go for the vertical mount & overhangs, even panel shades to limit heat-dumping requirements for the space heating panels, whether it's low enough temp to use PEX or not. But "can you", use PEX with solar plumbing well, yeah, kinda, if...