I recently installed some Nest E thermostats for heating only usage. They’ve been great so far, but my HVAC guy warned me I would likely run into issues without a common wire. In the newer part of the house there is hot water baseboard heating. These wires come in to these two valves (for reference let’s call the closer valve Zone A and the further valve Zone B) :
These wires then run along to a switching relay and 24V transformer:
A wire goes from the switching relay
To this Honeywell box (which I believe is a zone control?)
And finally to this oil burner control where it meets the wiring that appears to go to the thermostat that controls the steam radiators (Zone C) in the older part of the house (I still need to fish a green/common wire through for this one).
So my question is - after I fish a common wire through for Zone C, how do I need to reconfigure the wiring so I am getting ~24V through my common wires?