As has been emphasized elsewhere, collectors using PEX (or CPVC) tubing for the absorber must be protected from from overheating when its sunny and there is not water flow through the collector.
If a PEX collector is exposed to full sun with no water flowing through the collector to cool it, the PEX can be damaged beyond repair.
The most straight forward and reliable protection method is to install the the collector either vertical or at a steep tilt angle (70 degs or more). The combination of high collector tilt angle and high elevation summer sun results in a very high incidence angle of the sun on the collector and a lower collector temperature.
The high tilt angle also has the advantage of more direct solar radiation of the lower winter sun on the collector during the winter when colder temperatures and more cloudy weather make collection more difficult.
The PEX will not be damaged as long as water from the storage tank is being pumped through it, but pump or controller failures or setting a maximum temperature for the storage tank make it hard to insure that water will always be circulating.
I also believe that it is not a good idea to use double glazing on a PEX collector as this will increase collector internal temperatures.
PEX should only be used in systems where the pressure in the collector plumbing is low -- like drain back systems.
Of course, using copper tubing instead of PEX is another method.
Here is an example of a PEX collector which was mounted at a tilt angle of 45 degrees -- the PEX was damaged by exposure to high temperatures when no water was flowing.
Thanks to Antal for sending these in.
June 29, 2014