Wiring – HVAC: adding a C wire for Nest thermostat

hvacnestthermostatthermostat-c-wirewiring

Just installed a brand new gas boiler, want to keep using my existing Nest thermostat (they are compatible according to Nest).

  • Current system only has two wires: Rh and W

Nest is designed to steal current from Rh to charge, but here is my problem: the potential between Rh and W is only 22V, and the Nest requires at least 24V. I get a no power reading on the thermostat.

I know I can add a switching relay, as the furnace instructions state – but here's my idea and I hope somebody can tell me if this would work or not (I don't want to damage anything):

The potential between Rh and ground is about 24V; there is about 1.5V on the W wire. Could I simply run a C wire and connect it to ground (or the the C terminal on the transformer inside the boiler)? Or do I really need to spring for a switching relay.

Attaching the schematic from the manual on connecting thermostat.

enter image description here

Best Answer

I'm not sure if I have solved the problem or not; but I checked the two leads against earth ground with my multimeter and it turned out the wire I thought was Rh was in fact W – the polarity of the wires had been switched when the new boiler was installed.

I switched the wires and now my Nest seems happy, although it still claims low battery. At least I can control the heat now. Will post any updates if the problem comes back.

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