Is the thermal expansion/contraction of PEX a problem?
No. While PEX expands more than other plumbing materials, directional changes made with the tubing and some slack in the tubing during installation accommodate the expansion and contraction of the system if properly installed.
Question and answer courtesy of Plastic Pipe and Fittings Association (PPFA). More information is available at: http://www.ppfahome.org/pex/faqpex.html