I'm connecting solar panels, running glycol (closed loop). The loop has to be copper as the panels & fluid can reach up to 300F in the unlikely event there's a power failure at sunny noon time. If power comes back the system will kick on sending up to 300F fluid through the lines. Pex is out. I can't find anything if flexible copper would be okay or not, it would be a heck of a lot easier to run it in the attic than soldering stretches of rigid pipe plus glycol is thinner than water, and will leak in places water will not so the fewer joints likely the better. Know if there's any downfall to flexible copper?