HVAC: adding a C wire for Nest thermostat
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.