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.

  • Hello, and welcome to Home Improvement. I've edited your post to be clearer; please make sure it's still what you meant. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Nov 16 '19 at 14:23
  • Are you open to getting a different thermostat? What make and model are the air-handler and furnace involved? Why do you have a separate air-handler and furnace, even? – ThreePhaseEel Nov 16 '19 at 16:43
  • I apologize - I meant to say boiler rather than furnace. Unfortunately I already have 2 Nests connected so it would be a pricey change. The air handler is relatively new, while the boiler is likely early 90's - converted to natural gas from oil. – AlmostSurely Nov 17 '19 at 12:29
  • The air handler is a Ruud RCHL-48A1G. – AlmostSurely Nov 17 '19 at 12:35
  • I take it the air-handler is merely one hydronic zone out of several, no? – ThreePhaseEel Nov 19 '19 at 12:45

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