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I installed a nest thermostat a month ago. A very simple two wire install for a boiler. It worked fine without any issues for a month. This morning I went to adjust the temperature remotely and was told it was offline.

When I went down to the thermostat I found it had a low battery condition and had disabled the wifi antenna. I checked the voltage in the line, ~28V. The unit does appear to be communicating with the boiler as adjusting the temperature does cycle the boiler, it just appears to be no longer drawing energy for a charge.

  • I'd call/email/chat nest customer service if you have not. The are very good and efficient. – Tyson Dec 28 '17 at 15:24
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Nest customer service is probably your best bet, but I'm going to give you a guess.

In a two wire install the Nest charges itself when the boiler is off, using the difference in voltage between the two sides of the on/off relay inside the Nest itself. A small amount of current is allowed to flow through the charging circuit, and back to the power supply via the boiler's on/off circuitry. This current is small enough to charge the Nest but not signal the boiler to turn on. When the Nest wants the boiler on it closes an internal relay and shorts the two wires together. Enough current flows to signal the boiler to turn on, but there is no longer a difference in voltage to charge the Nest.

It has been very cold in the US recently. My guess is the Nest has to turn the boiler on for long periods of time, probably long enough that its small internal battery has not had enough time to recharge while the boiler is off.

If you are handy you might try tracing the thermostat wiring and look if it would be possible to install a new cable with more conductors so the Nest can have always on power available to it. If you can run the wire (available at most home improvement and hardware stores) a heating technician could switch over to the new cable in a single short visit.

  • Thanks, I'll give that a try. In regard to your last paragraph, the run would be rather difficult. currently I have a 3 wire cable. The third wire is not used, so in theory I could wire a common, but based on what I have read about installing common's I don't see a point. If their source originates at the same place as RH, the thermostat transformer, what is the point in running two wires? Overheating would be the only reason to do this, and with this small quantity or current, that won't be an issue. – mreff555 Dec 28 '17 at 19:04
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    If you have three wires you should make the third the common. You should have a red wire (which comes from one side of the power transformer, and a white wire, (which when powered from the red turns the boiler on). If you add a common (usually black but if you have a three wire cable the extra wire is probably green) this provides a return path to power the Nest when the boiler is on (rather than relying on running a small amount of current through the white wire). Power flows from one side of transformer via Red, through the Nest power supply and back to the transformer via black. – mfarver Dec 28 '17 at 22:37
  • That makes much more sense than any definition I have read. I was getting a deer in the headlights look when I would ask people if by common, they meant common ground. An expression I frequently heard which made no sense was common hot. Since nobody seemed to know exactly how it should be wired I concluded that it made no sense. However as a common ground I see why it would be useful and allow the nest to charge much faster. I guess I'll try that first and call nest if that doesn't get me anywhere. – mreff555 Dec 29 '17 at 1:48

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