Nest thermostat star connector

A Nest thermostat’s star connector or O/B connector can be used to control several different system functions, depending on what type of system you have. For example, you can use the star connector to connect a wire that controls a humidifier or dehumidifier, or emergency heat. 

• During setup, the app will give you a custom wiring diagram to install your Nest thermostat. You should only insert a wire into the thermostat’s star connector if the wiring diagram tells you to.
• If the app tells you to put a wire into the star connector during setup, your thermostat will ask you to tell it which wire it is, and what it controls. 
• No matter which Nest thermostat you have, you can only put one wire into the star connector, so only one function is supported at a time.

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 pro. To get a professional to help install and set up your Nest products, contact our installation partner, OnTech.

Differences between Nest thermostat star  connectors

Different wires and features are supported by Nest Learning Thermostats, the Nest Thermostat E, and the Nest Thermostat. 

2nd and 3rd generation Nest Learning Thermostat Nest Learning Thermostats have a standalone star connector on the base.
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 and Nest Thermostat The Nest Thermostat E and the Nest Thermostat’s star connector also have an O/B label because it can alternately be used as an O/B connector for heat pump systems.	Nest Thermostat E Nest Thermostat
System wire	Function
Y2	Stage 2 cooling
W2	Stage 2 heating
O/B	Heat pump heat/cool changeover valve