Oversized AND undersized...

Danny Scully

Well, the boiler was oversized and the header was undersized. As usually, fail by the original installer.

Danny Scully

Not a problem for @Ezzyt, my brother and I. We removed the undersized header and installed a new property sized drop header. To address the oversized boiler, we installed a 2-stage gas valve to more closely match the connected radiation. Additional we cleaned up the condensate zone piping and installed the only circulator for that job, a bronze B & G 100.

The Steam Whisperer

If I understand venting correctly, the boiler and water heater are also not vented correctly. The smaller appliance must enter the common vent before the larger. Instead of using a high/ low gas valve, why not just set the thing up at low fire?