If the Nest thermostat isn't getting enough power from W or Y and there is no Common (“C”) wire
While we find that in the vast majority of homes the Nest Thermostat can charge its built in battery by power
sharing from the regular heating and cooling wires. But in some rare cases a common wire is needed to
deliver consistent power to the Nest Thermostat while allowing normal operation of the heating and cooling
system. If there is a common (C) wire installed the Nest Thermostat will not power share.
Symptoms of a power sharing issue
In conventional heating and cooling or cooling only systems:
• Cooling is always on or on intermittently when the thermostat is not calling for cooling.
• Cooling does not activate when the thermostat calls for cooling.
• Cooling intermittently deactivates while the thermostat is still calling for cooling.
In conventional heating only systems:
• Heating is always on or on intermittently when the thermostat is not calling for heating.
• Heating does not activate when the thermostat calls for heating.
• Heating intermittently deactivates while the thermostat is still calling for heating.
• There is a unusual noise coming from the customer's heating system.
In heat pump systems:
• Heating or cooling is always on or on intermittently when the thermostat is not calling for it.
• Heating or cooling does not activate when the thermostat calls for it.
• Heating or cooling intermittently deactivates while the thermostat is still calling for it
In these situations, the thermostat can't successfully charge from the Y or W wires without interfering with
the normal operation of the heating and cooling system. There are three options:
Use a spare wire as common wire
The easiest way to solve the problem is by using a spare wire in the thermostat wire as a common wire.
Simply connect one end of the the unused wire to the Common (“C”) terminal in the HVAC controller and the
other end to the thermostat’s C connector.
Add a resistor to an existing Y or W wire
We've found that many Y and W wire circuits that cannot supply enough power can be strengthened by
bridging the Common terminal at the HVAC equipment to W or Y through a 220-ohm, 5W resistor.
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