I have had pretty good luck this cold winter with my Evacuated tube system. But 2 things: 1. I mixed 50/50 with Prop. Glycol and did not initially put the Freeze Protection switch to the on position,,, thinking that I was good down to - 50 deg. C. But my system was charged at 30 psi and I am pretty sure I got some slush, The pressure spiked and some of the pipe insulation melted due to stagnation of the mix at the solar panel ( Just an aside, even when it is 15 deg F out the solar loop, on a sunny day, gets to 110 deg.) So did having the system charged at 30 psi in the winter add to the Slush formation.... Since I have switched the Freeze protection on this has not recurred. 2. This relates to # 1 in that I thought I had bought some higher temp closed cell foam insulation, but I guess it did not do so well with temps over 220 deg F, when there was some stagnation. Any good sites for buying hi-temp pipe insulation?

Not in any significant way.  It takes a LOT of pressure to raise the ice point.

Bruce