What type of piping is used on the pressure relief and drain system for a 2 inch pipe in a Type 4 system?

The correct choice is epoxy coated carbon steel schedule 40 for the pressure relief and drain system in a Type 4 system due to its specific attributes suited for this application.

Epoxy coating provides an extra layer of protection against corrosion, which is particularly important in systems where water and potentially corrosive materials are involved. The use of carbon steel ensures strength and durability while the schedule 40 specification relates to the wall thickness, which is adequate for the pressure ratings typically encountered in a Type 4 system. This combination makes it a reliable choice for both safety and longevity.

Using seamless steel pipes, whether schedule 40 or schedule 80, may enhance pressure handling capabilities but may not be necessary in a pressure relief and drain context where lower strength is sufficient alongside corrosion resistance. Epoxy coated carbon steel schedule 80 is more robust than necessary and might lead to unnecessary costs or complexities in the system without offering significant advantages over the schedule 40 option.