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I attempted something similar, at least on the nest side of things. I quickly realized the nest wouldn’t recognize any attached equipment unless connected to a 24vac line. I always wondered how the nest derived DC power to charge its internal battery when a common line was not used. Maybe I’ll research this some more and try again.

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The Nest can sort of trickle charge itself through its relay outputs, but for reliability you really want to provide 24VAC between R<->C.

To attach equipment, you can connect a 24VAC-friendly load between (e.g.) Y<->C or W<->C.

Didn’t know about the trickle charging, neat!

Problem with attaching a load is that if you want to build a standalone unit with the nest, you’ll need some type of inverter when powered off DC, or perhaps a simple sine wave generator? I’m more of a digital, DC power guy.

Yeah, you need a sine inverter, either 24VAC, or 120VAC with a 24VAC transformer.

There might be a way to trick the Nest into running on something closer to DC, but I could only guess.

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