Ian wrote:
I am installing a new NEST thermostat, replacing an old two wire mercury bulb furnace thermostat. There are only two unmarked wires. There's 24 V across the wires.

Can anyone tell me which letters on the NEST base (W, Y, O/B, AUX, E, G, C, etc) they will map to? It has to be the most basic of installations; open or close the circuit.

Thanks,

Ian

Hmm,
Why are you asking? Read the manual? No? You can download it.
Still have questions? Then ask.
2 wires? Where is another one for fan control? Maybe you are
future proofing but in this case NEST is way more than what you need.