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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
by 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 agoAs 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 agoA 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.