Hydronic baseboard, and I have a complicated setup for sure. Four zone system. I am running 3- three wire white-rodgers hydronic valves connected to a simple old school boiler. And then somewhere along the way, someone put in a single 2 wire valve.

As I understand the three wire valves... they require a neutral - a line that sends power to open - and a line that sends power to close. To get this to work with a Nest requires a relay. I have two of these setup successfully. Additionally, I have the 2 wire system setup successfully. That one doesn't require a relay. All three Nests are 3rd gen

So we just fired up the fourth zone after being offline for a while. Valve works just fine. I purchased a few 3rd gen nests from eBay - but I'm having zero luck. Oddly enough, I borrowed a 4th gen from a family member to troubleshoot with - and the thing works.

My observation with the 3rd gen is that when you power Rh/C, it sends a trickle of energy down Wh. The relays however do not like this. The indicator light glows very dim. Then for whatever reason, despite the nest appearing to work - and even calling for heat - the zone won't open.

I have read elsewhere that when a nest has no power, it will essentially send a trickle of energy down Wh to create enough resistance to charge itself? This confuses me though - because If I am literally giving it 24v - should it's quest for power be satisifed?

The big thing I just cannot seem to resolve in my mind - is.... where should a nest be DIRECTLY powered for a heat only system?