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My house has separate thermostats for heat and cool, each located in a different part of the house. It's a single-zone forced air system handling both heat and cool. The existing thermostat wiring is minimal: 2 wires going to the heat thermostat (R, W) and 3 wires going to the cool thermostat (R, Y and G). No spare wires are available in the walls for a C-wire connection.

I want to replace both existing thermostats with separate Nest thermostats. During setup of the first replacement the Google Home app stops and tells me I need a Nest Power Connector since there is no C-wire. Since there will be two thermostats, I believe I need a separate Power Connector for each thermostat to provide each with its own virtual C-wire connection.

First question: Is a dual Power Connector arrangement actually necessary in this case? I couldn't see another way to get continuous power to both thermostats without re-wiring them altogether, so I think the answer is yes.

Second question: Is the following proposed wiring correct, and not likely to cause problems?

I fear I may be asking for trouble by attempting this upgrade, and would appreciate informed opinions about that. Proposed Dual Nest Power Connector Wiring

Thanks! --Doug

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Yes. Each thermostat needs a C-wire because each requires power.

Your existing thermostats act just as switches, and each requires an independent return path in common.

Your wiring diagram looks to be correct! But I am unsure whether the nest common adders will work in this configuration. If they don't, I recommend you look at a common-maker like Fast-Stat. I have no affiliation but I used multiple when upgrading my old 2-wire hydronic system.

Think about it like this: The red wire delivers 24v to each thermostat. Thermostats need constant power so they complete their circuit on the common wire.

If you power the white wire that's a call for heat, the yellow wire a call for cooling, the green is the fan.

The nest common makers use a clever system of resistors to allow the nest to draw power and return it on the white/yellow wires but not enough to trigger the HVAC.

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  • I tried to find a schematic of the Power Connector internals, but was unsuccessful. I suspected it was a network of passive components (because what else could it be?), but I suppose it's a proprietary design. Your explanation is helpful. You say you are unsure about it working in this configuration...could you speculate on possible failure modes I might encounter?
    – DougSlug
    Commented Dec 6, 2023 at 16:15
  • @DougSlug I would be curious if the HVAC transformer would have any issues powering them both using its resistors. Putting two resistors in parallel can have unexpected behavior for "phantom power draw"
    – Matthew
    Commented Dec 6, 2023 at 23:22
  • Yes, that is what I was concerned about...this is where having a better understanding of what's inside the Power Connector would really help. But I'm thinking that the resistance in question would involve the W/Y line (rather than between R and C), and since they are using two separate loops, they wouldn't be in parallel with each other, especially if only one or the other is ever in use. Could there be some diodes in there somewhere to prevent unwanted current in certain paths? It's also possible that I'm missing something obvious here.
    – DougSlug
    Commented Dec 7, 2023 at 5:04
  • @DougSlug if it doesn't work with the nest common devices, get two Fast-Stat common makers. I can almost guarantee that will work because they use devices on both sides and don't rely on phantom power
    – Matthew
    Commented Dec 7, 2023 at 6:22

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