I am trying to replace some bad pipes on my boiler which is antique to say the least. The problem is I don't know how to figure out which psi rating relief valve to get. I know there are several factors in determining this but have no idea what they are. The old relief valve was not really functional so I can't start from there. Can someone walk me though the process of finding this out?

To avoid confusion:
Water from out side to fill the system > valve to stop water flow > small section of pipe > relief valve with run-off pipe > boiler