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I've seen first hand what happens when you use pex on the hot side of a solar hot water collector, we used some to allow for some flex between the copper of the collector and the copper lines running to the house, about 3 months in to the system being up and running we got a call from the home owner saying the saw a blue stream of steam and water shooting 20 feet in the air, I collected some tools and went to check it out, the pex we used had melted, swelled and burst, sending a stream of boiling antifreeze/water mix shooting in to the air, if someone had been near it they would have been burned, maybe killed.
we figured that the one end of the panel with the pump controls had just enough shade that the pump didn't come on for a few minutes, while the rest of the panel got hot enough that the pex that was rated at over 280 degrees melted, normally in systems like this you have a temp release valve that opens at 260F to protect the pex that is in the concrete slab, and later systems instead of having a piece of pex to flex, they use radiator hose or hydrolec hose.
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