I have a 20 yr old Weil McLain cgi gas boiler, 133,000 output. It's a converted gravity system with 524 sq. ft. radiation and 4" (I think) main supply and return lines, with the circulator on the return line and a separate Lochinvar water heater. The system was designed by Fremont Lobestall for those who knew him.
For several years the boiler has relieved out of the pressure relief valve
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