How to connect thermostat C wire to Weil-McLain CGa boiler?
According to the CGa-010 wiring diagram, you'll want to connect the C wire for the thermostat to the terminal pointed to in this image.



Notice the transformer is circled in red. The location where the transformer wiring terminates is circled in orange. The "common" side of the transformers secondary is pointed to by a large blue arrow.