c wire on a multi taco zone valve boiler
One of those transformer terminals is &quot;R&quot; the other is &quot;C&quot;
If the red wire joins to the thermostat &quot;R&quot;  wires then the beige wire is &quot;C&quot;
Thermostat wires should not be inside that junction box with the mains wiring,  get a new box for the thermostat wires.