The ratings should be printed on the pex itself. It works well for ng oil or other sources of ondemand heat. But I have been hesitant to use it in a coal/wood system. A temp/pressure relief is helpful with a source of fresh water. A tempering valve is helpful, etc.
But as mentioned above, whathappens ifyouhave a water line failure, the valve sticks, your circulator pump fails, etc?
A boiler can usuallymsurvive a lot of heat, but the pex gets soft as temps rise towards max.
After all, you don't want to be in a situation where youhave to dump the warp core...or emergency cool off to prevent majorhome damage by sacrificing your boiler.
That being said, I've read some who use pex for the last section of line, with everythingnnear the boiler being copper or iron.
Everything is a risk, even coal burning itself. PEX sure is nice with crimpers. I installed a ng boiler system for my fil and it works wonderfully, but I keep telling myself NOT to use it in my baseboard/radiator system (currently offline during a majormremodel, using forced air hand fired), even though it would be easier and faster in some places.
In floor is muc more common for PEX usage, which uses cooler temps, 140-170 typically.
Hope this helps. Sorry for typos, Android systems don't believe users need arrow keys