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C-Wire for new T-stat

I have a new (old) home with the old Honeywell round t-stats. I would like to install a set-back t-stat that requires a c-wire. Looking at the zone valves in the basement, I can see that only two wires are used. The green wire is not connected. Obviously, the same is the case at the t-stat location.

When do I connect the green wire to provide the c-wire at the new thermostat? The boiler is a WM GV-5.


Denver, CO (previously Boston)
Steve from Denver, CO