The pex I use is rated for 200* but I was told by my supplier it is tested to 350* .The solar water systems I set up can get to 300* at the collector ,I use 2 insulation wraps ( ac pipe insulation x 2 ) and have never had a problem.In the bus is the same pex for in floor heat but I used 3/4 auto heater hose in and out at the proheat for 12' to the manifold then to pex .I used the heater hose as I was unsure about constant vibration from road,motor . Over 6 years and counting no problems dave