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I'm concerned about @ThreePhaseEel's comment on how the Nest will respond to the switching 24VDC that the diode bridge will provide. Although I'm confident that would be an appropriate location to pull a common wire from, I'm going to construct my own circuit from more standard parts.


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It seems that non-standard colors and mismatching connections are everywhere, so confirm everything and don't rely on color just because that's what the wiring diagrams say. From all of your diagrams, it appears there are several locations that a common wire should be accessible. On the simplified diagram posted first, the second wire running to your A/C is ...


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T is common, confirmed by reading 24VAC across T and V (24VAC should also exist across V and ground to confirm V as hot). A/C may need a few minutes to kick on.


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On dual transformer systems, Nest expects the common wire to come from the cool side (Nest Pro Guide, bottom of page 20); look towards your A/C not your boiler. If there is a common terminal on your A/C control board and 18/5 wire bundle - use one of the extra wires to connect the common terminal to your Nest 18/3 wire bundle Use Venstar Add-A-Wire (or ...


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Your smart thermostat requires a common wire so it can get power. It doesn't "use" the common wire for anything and your furnace or the new cooler will not need anything to happen with the common wire. This is a simplification, but think of the R wire as positive and the C wire as negative. The thermostat sends the positive voltage from R to the other ...


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Formerly, your Nest worked by leaking power through the furnace or air conditioner's operating relay. The relay was old and brutish enough that it flowed enough current for it to work. Now, you have an electronic control furnace that does not have those smarts. This is similar to the problem people have with lighted switches or dimmers when they switch ...


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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 ...


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