I'm glad you're getting a professional in to look at it. If it is low water, it's a perfect expamle why every boiler should have a low water cut off as is mandated in my state. If it is not low water, and correct me if I'm wrong, but I think it means that two controls have failed at the same time, or, the high limit failed some time ago & you didn't know it, and now the aquastat has failed also. At least the high pressure relief worked as planned! This is the type of thing that Harry Homeowner sees the relief dripping, solders a cap on it, and then.....then.... KA-BLOOMIE!! In one split second NASA has competition. Oh, wait, I forgot, NASA doesn't shoot off rockets.