"C" For nest g4cv thermostat

Weil McLain pcg-4 series 5…old thermostat was a Honeywell digital with batteries..I have 4 wires but only 2 were used red – R & green – W. It’s a 2 zone hydronic system. There is a multi tap transformer S84-19 and it shows a c terminal so I tapped off that terminal and when I did setup with the nest app & it detected R & W but not the c wire. This furnace is heat only and no circuit board.