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Hey I know this is a long time after the post but this isn't a clear question. They don't tell you what type of boiler it is or ,although the gauges are correct, is there one reading higher then the others? The easy answer is that the safety valve has malfunctioned. Either the spring is weak, dirt is preventing the valve from seating etc. the tricky answer uses various situations to troubleshoot boiler. Is the boiler over firing and isolated from the header they all share ? Has there been an upset in operation that could cause this like a drop in demand creating a sudden pressure rise in one boiler? Has something failed in the steam line (control valve ) causing sudden over pressure? You really could go on and on. I think the question is designed this way to get you troubleshooting. Less