Pressure relief valve materials

[a guest]

For the over flow tub on the hot water tank , Can this be pex plumbing ?

[Michael]

Presently, we only care that there is a drip leg that reaches to within 18 inches of the floor, we do not judge the materials it is made of – that is a local code issue that can vary by locale.

There's a good article on this topic at https://www.nachi.org/tpr-valves-discharge-piping.htm