Learn about the Nest thermostat’s star connector

The star connector can be used for several different functions, depending on which Google Nest thermostat you have, and what type of system you have. For example, you can use the star connector for an extra stage of heating, emergency heat, humidification, or dehumidification.

Important: No matter which thermostat you have, you can only put one wire into the star connector, so only one function is supported at a time.

Your system’s wiring may or may not need to use the Nest thermostat’s star Star connector icon connector, always refer to the custom wiring diagram provided by the Nest app during setup before you connect any wires to the star connector.

Nest Learning Thermostats and the Nest Thermostat E use the star Star connector icon connector a bit differently and support different wires and system features.

Check if your system needs the star connector

Use the Nest app to get a custom wiring diagram to install your thermostat. The app has complete installation and setup instructions. You should only insert a wire into the Nest star Star connector icon connector if the wiring guide provided by the Nest app checker tells you to.

The star Star connector icon connector can only control one wire, so in a few cases the compatibility checker will ask you to choose a system feature. For example, if your system has a third stage of heating and a humidifier, you’ll need to pick one during installation to put into to the Nest thermostat’s star connector.

Tell your thermostat what's in the star connector

The Nest star connector is multifunctional, so the compatibility checker may tell you to put one of several different wires into it. If you put a wire into the star connector during initial setup, your thermostat will ask you to tell it which wire it is.

Important: If you’ve connected a humidifier, dehumidifier, or multi-speed fan wire to the star connector, we strongly recommend that you get help from a Nest Pro installer.

Differences between Nest thermostat star connectors

2nd and 3rd generation Nest Learning Thermostat Nest Learning Thermostats have a standalone star connector on the base. Different wires and features are supported by Nest Learning Thermostats and the Nest Thermostat E. Important: No matter which thermostat you have, you can only put one wire into the star connector, so only one function is supported at a time.
System wire	Function
W3	Stage 3 heating
E	Emergency heat
HUM	Whole-home humidification
DEHUM	Whole-home dehumidification
G3	3rd speed for multi-speed fans

Nest Thermostat E The Nest Thermostat E’s star connector also has an O/B label because it can also be used as an O/B connector for heat pumps systems. Different wires and features are supported by Nest Learning Thermostats and the Nest Thermostat E.
System wire	Function
Y2	Stage 2 cooling
W2	Stage 2 heating
O/B	Heat pump heat/cool changeover valve

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