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Interesting. The problems most people are reporting sound like the trouble I had when I installed my Nests without a common wire: Because my system is wired int

Interesting. The problems most people are reporting sound like the trouble I had when I installed my Nests without a common wire: Because my system is wired into an electronic zone box, there's minimal current on the on/off wires that the Nest normally leaches off of to charge the battery. Nest's tech support helped me find and install a common wire already in the wall; probably the most impressive tech support call I've ever been on, so it sucks they're being overwhelmed at the moment. My guess is that the new software update pushed the power requirements of the Nest beyond a significant fraction of home systems' ability to recharge it. Regardless, sounds like a detailed post mortem is in order.

pistle 13y ago
Why does it need to charge? Is there a benefit in breaking the traditional battery-as-backup-only model? If the home has power, the controller has power. If the home loses power, batteries can be put in to retain programs, etc. It seems like they created a problem unnecessarily.
- twoodfin 13y ago
  As a plug-in thermostat replacement, the Nest has to rely on the power that's already present in typical home thermostat wiring. That power is more than enough for a typical "dumb" digital thermostat, and should be enough for a Nest in most circumstances (especially since the display and WiFi don't have to be on all the time). The battery is there for power outages and buffering.
  - joezydeco 13y ago
    A dumb digital thermostat needs no power from the house, there's usually a small replaceable battery (like a 9V alkaline) to drive the relay and switch the control wires. The even dumber mechanical/mercury ones worked totally without power. Not every system provides the 24VAC red/common wire from the HVAC unit to the thermostat. There is a small amount of current between the hot wire and the control input, which is why you can't usually use solid state triacs/SSRs in a home thermostat. You need an old-fashioned contactor relay. Twoodfin is referring to the Nest parasitically drawing current from those control wires to charge the battery.

pistle 13y ago

Why does it need to charge? Is there a benefit in breaking the traditional battery-as-backup-only model? If the home has power, the controller has power. If the home loses power, batteries can be put in to retain programs, etc. It seems like they created a problem unnecessarily.

twoodfin 13y ago
As a plug-in thermostat replacement, the Nest has to rely on the power that's already present in typical home thermostat wiring. That power is more than enough for a typical "dumb" digital thermostat, and should be enough for a Nest in most circumstances (especially since the display and WiFi don't have to be on all the time). The battery is there for power outages and buffering.
- joezydeco 13y ago
  A dumb digital thermostat needs no power from the house, there's usually a small replaceable battery (like a 9V alkaline) to drive the relay and switch the control wires. The even dumber mechanical/mercury ones worked totally without power. Not every system provides the 24VAC red/common wire from the HVAC unit to the thermostat. There is a small amount of current between the hot wire and the control input, which is why you can't usually use solid state triacs/SSRs in a home thermostat. You need an old-fashioned contactor relay. Twoodfin is referring to the Nest parasitically drawing current from those control wires to charge the battery.