I have a dual transformer system. At the AC/Blower unit, there are 'R_c', 'Y' and 'G' wires; there's 0VAC between the 'Y' and 'G' wires, and each shows 27VAC to the 'R_c' wire.
At the boiler, there are 'R_h' and 'W' wires, with 27VAC between the two.
Nest reports "N72 error: No equipment detected" on 'R_h', but heat is working fine.
The boiler is connected to my standby generator. When we lose power, the AC/Blower goes dark, and Nest suddenly reports it is unable to detect an HVAC system. The stat won't even let me navigate away to the heat menu, and quickly drains its battery. I'm wondering what my options are for using the Nest for heat during a power outage. I can run a 'C' wire up from the boiler, but my understanding is Nest (ridiculously) insists that the 'C' come from cooling. Appreciate any ideas.