My son in law just called and said that his pressure relief valve just blew on his boiler. We've been out of town for 4 days and he set the temp down to 60. When he got home, he fired up the boiler, then he said about ten minutes later, the pressure valve blew and he couldn't shut it off. I tried asking him over the phone if there was a shut off valve for the main water supply to the boiler and he could not find one. Water kept spewing out the valve so he shut off the main water supply from the well. There could be a number of questions here, but mainly, what controls the boiler pressure. Is it the expansion tank's pressure or something else. And are there boilers that would not have a shut off valve. He said this is a Stewart Warner system and probably fairly old.