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) :

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These wires then run along to a switching relay and 24V transformer:

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A wire goes from the switching relay

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To this Honeywell box (which I believe is a zone control?)

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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).

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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?

1 Answer 1


First, if the Nest isn't reporting problems from lack of a C wire, then you don't have problems, so don't make problems.

If you do need a common wire, I gather by the grand tour of your system that you have no idea where C can be found. The answer is the transformer has 2 terminals on the Low voltage side. They are R and C.

Hypothetically, if you ran a C wire up to your thermostat and accidentally attached it to the R wire on the transformer, absolutely nothing would happen. (The Nest would be trying to power itself off the voltage difference between R and R, which is 0 volts.)

You see where this gets pretty easy.

You can also tap "C" at any place it goes, e.g. Any solenoid, relay or actuator.

  • C is where the green wires are attached. I’ve gathered that much and know that they are not the green wires that come to my thermostats, but that these are the green wires from the valve which are spliced with the red wires from the thermostats. So I guess I’m a bit confused as to what my hvac guy was saying because It looks to me like my R wire is already attached to common except maybe in the case of the Zone C thermostat because I have no idea what’s going on in the switch.
    – Jordan
    Dec 12, 2018 at 23:49

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