Nest "GREEN LIGHT" FAILURE
We've had a 3rd Gen Learning Thermostat for about two years with no issues. Yesterday, when it was 9 below zero here, that changed.
The Green Light on the unit was flashing for several hours, and the internal battery ran itself down to the point that Green changed to Red. At no time did any indication of trouble appear on the unit or in the Nest App. No error messages or codes. OUR FURNACE HOWEVER STOPPED WORKING. Resetting the Nest and unplugging it from its backplate and re-plugging had no effect.
Relying on advice I uncovered elsewhere on the Internet, I unplugged the Nest again and connected it to a USB charger for about 45 minutes. I plugged it back into the backplate. No more Red or Green light, and, more to the point, the furnace kicked in. 24 hours later, all is still well.
EXCEPT that I've begun to wonder whether I can trust the Nest. This is a device that has to work. A heating failure in this climate can be catastrophic!
- It appears that "something" caused the Nest to lock up, stop controlling the furnace, and deplete its internal battery.
- What caused this, and what can I do to minimize the chance of a recurrence?
- The Nest did not fail to a safe state and announce the failure with notifications to the app, or error codes on the display. Is this an acceptable architecture for a device that controls critical infrastructure?
- Would the Nest maintain the Safety Temperature (45 deg F) even if experiencing a "green light" failure?
Fortunately we were home, and able to deal with the problem prior to the arrival of our heating contractor (who later said that his solution would have been to replace the Nest with a different thermostat.) We do have a SensaPhone, which would have transmitted a low temperature alarm before our pipes started to burst.
What does Nest have to say?
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UPDATE, 02-Feb-2022
It turns out that the problem was that the wiring on the furnace side was incorrect. The lesson learned here is: Don't assume that the wires on your thermostat are properly connected to the furnace! Go down to the furnace and check!
