I need to add a C wire to use my wifi thermostat. I have a 2 wire thermostat now. There is a third wire in the wall which runs to the basement where it is cut off. I can easily extend it to the boiler. There are 2 zones in the house. I am connecting the thermostat to the main zone so I can monitor the house temperature while we're traveling. It appears to me that I should connect the C wire to the "2" terminal of the zone valve as shown on the attached wiring diagram. This diagram was drawn by installer of the zone valves when the second zone was added. The zone valves are White-Rodgers 1361's. The boiler is a Laars MiniTherm. I have traced the wire to a 24VAc transformer in the boiler.
The White-Rodgers 1361 zone valve has five terminals arranged Like this:
2 2/5 4 1 3
2/5 and 4 run the the thermostat. 2 and 1 run to the transformer. The 2 terminal wire is marked "neutral" on the diagram. 3 runs to the boiler controls.