I'm trying to connect a 3rd gen Nest thermostat. I have a 2 zoned heat and 1 zone AC. The zone with this thermostat has a boiler wired Rw, W1 and HVAC wired Rc, G, Y1. It's summer so I'm only using the AC currently, but the thermostat seems to be drawing power from my 2 zone relay attached to my boiler even when the heat isn't on. It keeps triggering it on and off constantly. I called NEST support and they said I need a C wire that needs to come from the HVAC not the boiler.enter image description here

I'm looking at the wiring for the HVAC and I'm not sure terminal to attach the common wire to.

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Any advice would be greatly appreciated. Thanks

  • Is the hydronic boiler feeding radiators, a coil in the air handler, or both? Commented Aug 11, 2018 at 20:58
  • Also, is pulling a thermostat cable with more wires between the air handler and the thermostat an option? Commented Aug 11, 2018 at 21:05
  • 1
    @ThreePhaseEel, I'm confused with the first question. The boiler for heat is a separate unit in the basement. The air handler is only for AC. There is no connection between systems. Yes pulling a cable with more wires is an option. But I was hoping to use a FAST-STAT Common Maker Thermostat Wire Extender from amazon which would use my existing wire configuration without needing to pull a new one. But, I'd need to know what terminal to connect the common wire to at the hair handler.
    – Andy W
    Commented Aug 12, 2018 at 15:23

1 Answer 1


The "B" terminal is your common. You will definitely need to pull more wires. And YES you need a common for the nest to work.

To make your life much simpler, you can add a FAST-STAT Common Maker Thermostat Wire Extender (Adds a Common "C" Connection).

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