What if Nest isn't getting enough power from W or Y and the consumer doesn't have a Common
(“C”) wire installed?
While we find that in the vast majority of homes Nest can charge its built in battery power sharing from the
regular heating and cooling wires, a few installations require a common wire to bring power to Nest without
interfering with the normal operation of the heating and cooling system.
What are the symptoms of a power sharing issue?
In conventional heating and cooling or cooling only systems:
•
Cooling is always on or on intermittently when Nest is not calling for cooling.
•
Cooling does not activate when Nests calls for cooling.
•
Cooling intermittently deactivates while Nest is still calling for cooling.
In conventional heating only systems:
•
Heating is always on or on intermittently when Nest is not calling for heating.
•
Heating does not activate when Nests calls for heating.
•
Heating intermittently deactivates while Nest 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 Nest is not calling for it.
•
Heating or cooling does not activate when Nest calls for it.
•
Heating or cooling intermittently deactivates while Nest is still calling for it
In these situations, Nest 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; these changes should only be
made by professionals.
Use a spare wire as common wire
Ordinarily you can solve the problem by turning an unused wire in the thermostat wire bundle into a
common wire.
Simply connect one end of the the spare or unused wire to the Common (“C”) terminal in the
HVAC controller and the other end to Nest'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.
In a heat-only system, you need to bridge from common C to W. In a HVAC system with a Y wire, you must
bridge from common C to Y. (Nest can only charge from W when Y is not connected.)
Nest Learning Thermostat installation & configuration guide
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