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I have a Lennox wall mounted boiler (gwm 075ie), I am trying to connect a "c" wire so I can install nest thermostats. I have 3 zones, currently running 2 wires (r&w). There is an extra wire behind each thermostat, that runs down close to the boiler, none of those wires are connected to anything. My current transformer is rated at 40va.

Unsure of the best way to do this. I have taco valves (z075c2-1) each valve has a "c" connector. Could I just run a wire to each of those connectors to the thermostat? Is the transformer able to handle 3 zones and 3 smart thermostats? Taco says a 40va transformer can run 12 valves.

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Yes, that C connector on the valves is what you want. The nest thermostats use very little power, so that shouldn't be an issue for your transformer.

There are two wires coming from the transformer. On a typical HVAC system, the red wire is used for control, and the other wire is what gets used as the "common" or C-wire. In your diagram it looks like white is being used for control and red is going to the C terminal of the valves.

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  • My estimate of how much a Nest uses is in the 5VA range -- does that seem correct to you? – ThreePhaseEel Jul 27 at 23:14
  • @ThreePhaseEel they are very low power. I tried to put an AC clamp meter on the C-Wire of my ecobee and I could only come up with 0.08A. That's on the low end of accuracy, but yea, I think 5VA would be a high limit for a thermostat like that. – JPhi1618 Jul 29 at 21:19

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