Burnham Boiler Overheating 260F

jldaddy

I have a cast iron gas fired Burnham boiler with one Taco 007 circulator pump and 3 Taco zone valves. Boiler was installed in 1990. Never had an issue until two weekends ago when I got woken up to pipes banging and water alarm going off. Boiler relief valve was shooting out steam. I turned the boiler off and called a plumber. He changed the aqua stat and the expansion tank. Boiler had issues heating up the house after these repairs, plumber came back and bled the air out of the system. Boiler ran for about a week, heating the main and second floor with no issues, basement would not heat up and reach setpoint. This morning, i woke up to the same loud banging noise and water alarm, boiler was reading 260F and 22 PSI. I immediately turned off the boiler. What can be wrong??