I'm in the process of installing some smart thermostats (Nest Thermostat E) to replace my old ones. I have a boiler radiator heating system with three zones. Two zones are straightforward heat only (main floor and basement). The third zone (upstairs) also controls AC and uses an air controller for forced air. The air actually blows over a radiator coil (I think this is the right term?) to supply the heat in this zone. In other words, there are no traditional radiators upstairs, just the forced air system blowing air over this radiator coil inside the ductwork.

I want to replace all three thermostats with Nests. None of the current thermostats are using a C wire. I understand it's not strictly necessary to have a C wire for the Nests, but they will function better if they have one. I'd like to wire them with C-wires and I think it's doable with my system. After studying up on all of this and researching what I have, I think I understand what I need to do, but I want to double-check and see if anyone can point out anything I'm overlooking or not understanding correctly.


  • Here is the diagram I put together trying to map out how everything connects together in the current state. A few notes:

  • Bottom left transformer, the boiler (controlled by Hydrostat 3200-Plus), and the three zone values are all in the basement.

  • The bottom right transformer and the air controller unit are in an upstairs closet.

  • "Air Controller #1" and "Air Controller #2" are both components that are located inside of my air controller unit. One seems to be controlled by the W1 wire from the thermostat, and the other from the G wire from the thermostat. So it seems like the W1 wire triggers the boiler as well as the air controller. I'm not sure why the G wire doesn't just power the air controller in the event of both calls for heating and cooling though?

  • I didn't see any labels for the bottom right transformer, but I'm assuming what makes sense is that the red wiring coming off the transforming is the "hot" (R) and the green wire coming off is the "neutral" (C). That seems to make the rest of the diagram wiring make sense in my mind at least. FYI this transformer is a Supco T45.

Adding C-wire

I think what I need to do is:

  • For main floor and basement zones, make a connection between C coming off of the transformer (bottom left, where green wire meets the two orange wires) and an unused wire in both of the wires going to the main floor and basement thermostats. There are spare wires to be used in these.

  • For the upstairs zone, make a connection between the splice that joins the 5 wires (4 red, 1 green coming off the transformer) which is located in the air controller, and the thermostat. There's no extra wires available going to the thermostat, but it'll be easy to add something here since they are close. Again, looking for confirmation that this 5-wire splice is where my "C wire" should originate from.

Does this seem like the correct interpretation and approach to take?

Other Questions

  • Should I worry about the number of items on a transformer? For example, the transformer in the basement (bottom left in diagram) is currently powering two zone valves. If it is also powering two Google Nest Thermostat E's, will that be too much? I can't find a VA rating on this thermostat, nor any model or labels at all other than the R and C terminals.

  • In reading up on adding C-wires, I saw mention a few times of isolation relays (for example, this thread). My interpretation is that these are necessary sometimes when you need to add an external transformer, and that they separate the circuit that is supplying the C wire to the thermostat from the circuit running within the boiler controller. I'm assuming I'm OK in my situation because the zone valves are "isolating" the circuits involving the external transformer (bottom left) from the circuit within the Hydrostat boiler controller. Is this correct?

1 Answer 1


Your wiring plan looks good and you seem to have a correct understanding of the wiring and how it should work. That bundle with the one green and 4 reds is the "C" wire. The color makes it hard to follow in your diagram, but I guess you used real world colors, so that makes sense.

The 24v transformer will be AC power (alternating current), so it doesn't really matter which terminal is R and which is C. One wire goes to the R terminal on the thermostat and the other one is C.

The W wire controlling the fan is sometimes done for safety to make it impossible to call for heat without the fan also being on. That can also be done to get away with using one less wire, but either way that happens and I wouldn't worry about "fixing" that unless you have a strong need for turning the fan on by itself.

The transformer only powers a small coil in a relay to turn the devices on and off. It might also supply a small amount of power to the control boards, and after the update it will supply power to the Nest. These are all so low power that you shouldn't need to worry about it. You can check the voltage of the transformer while the system is running to make sure it's not dropping low, but I don't see an issue with what you are planning.

Isolation relay is used if you want to power the thermostat with a different transformer than the equipment. So, for instance if it was convenient for you to power the upstairs thermostat from the transformer that powers the basement unit, that would be a problem. The thermostat must be powered by the same transformer as the equipment, which is what you plan on doing. No addition relays should be needed.

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