Google Nest Learning Thermostat 4th Gen Installation And Configuration Manual page 77

If the Nest thermostat isn't getting enough power from W or Y and there is no Common ("C") wire
Understanding Power Sharing
even if there is not a common wire through a Patented Power Sharing process. When there is not a common wire,
the thermostat will draw a small amount of power from the Y and or W wire when they are active or inactive.
Nest Learning Thermostat (4th gen) has improved power sharing in a few ways. This thermostat has a built-in
battery like the Nest Learning Thermostat (3rd gen), but it uses less power to run the thermostat. It can also draw
more power over the Y wire without activating the equipment. These new Power Sharing "states" can be identified
and understood through the power readings on the next page.
We find that in the vast majority of homes, the Nest Learning Thermostat can charge its built-in battery by
power sharing from the regular heating and cooling wires. But in some cases, a common wire is needed to
deliver consistent power to the Nest Learning Thermostat while allowing normal operation of the heating and
cooling system. If there is a common (C) wire installed, the Nest Learning Thermostat will not power share.
If a system can't deliver consistent power to the thermostat to keep its battery charged or correctly control
heating and cooling, users may experience one or more of these symptoms:
Reduced thermostat battery life
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Thermostat display won't turn on
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Thermostat motion sensing is disabled
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Thermostat occasionally disconnects from Wi-Fi
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System unexpectedly turns on or off
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System is making strange noises: chattering, stuttering, clicking, or thumping
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Heating or cooling is always on, and won't turn off
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Heating or cooling repeatedly turns on and off in a short period
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A "Delayed" or "Starts in" message often appears on the thermostat's screen (for example: "Heating is
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delayed for 2:30 mins")
The system fan is always running or won't turn on
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The system fan turns on and off repeatedly in a short period
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User gets an N410 error
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See page 77 for detailed power requirements and how to interpret them.
There are four options to solve a power issue:
1. Use the Nest Power Connector to provide constant power.
2. Use a spare wire as a common wire.
3. Add a resistor to an existing Y or W wire.
4. Pull a new common wire.
See page 78 for details and instructions about these four solutions.
- The Nest Learning Thermostat (4th gen) can get power from equipment wires
Troubleshooting - Power Sharing
The
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