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Join Date: Jan 2012
Posts: 12
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01-22-12, 06:37 PM   #9 (permalink)  
Nest is "compatible" with 2 wire 24v systems because it can charge itself using the AC supply for the heat signal while the heat is on. It then utilizes its battery until the heat is on again, at which point it again charges itself during the time the room/building is being heated.[1] How well that actually works in practice is another question, but it IS supported by Nest, and you can verify this by using their compatibility checker.

As for how I know I have a millivolt system... there are two reasons:

1) We had a power outage, and the thermostat continued to work, even though literally *nothing* else had power, and there is no battery backup in the existing thermostat.

2) I traced the wires from the thermostat all the way to the furnace, and noticed that they end at a small box right next to the pilot light. There is no other wiring running into or out of the furnace.

[1] Undocumented Nest incompatibility with single-stage wiring