Can I use ProPEX for Pressure Relief and Steam Generator Drain?
The code requirement in most areas for the P&amp;T relief plumbing is that it must be a material suitable for potable water plumbing - which in turn typically means it must allow a working pressure of 100psi at 180f. This might seem like an inadequate set of requirements for something intended to drain boiling water or steam, well over 180f, but the key term there is working - that is, it needs to remain durable when exposed to that pressure and temperature over long terms. The relief drain plumbing does not need to support pressures that high, and will only briefly be exposed to high temperatures so long term reliability is not required.

You may notice that PEX tubing is marked as complying with ASTM F877. ASTM F877 standards require that the tubing is able to endure 150psi at 210F for 30 days straight.