Matty read post # 5, the heat obviously compromised the pex or the fittings would not be leaking, perhaps a lower temp relief valve or a second relief valve would have kept this from happening, without pressure the water would have a very hard time to go above 213 degrees much like boiling water in a pan on a gas stove, pressurize it and it will climb in temp very fast, remove the pressure and it will very quickly drop back to 212 degrees, It's called latent heat. NH, you posted your last comment before I finished, was there steam / water still coming from the boiler at that temp, if so then the damage to pipes and fittings possibly could have been avoided by a second relief valve, the boiler temp would certainly have still climbed in temp but the pressure and extreme temp of the water / steam would not have damaged the piping and fittings, would you rather that steam vent from a safe pressure relief valve or burst a fitting possibly in your home with your children in the basement playing, I am in no way bashing pressurized systems, all I'm saying is make it as safe as possible, steam is not something to be taken lightly, IT CAN KILL YOU OR YOUR LOVED ONES if done incorrectly!