Nest thermostat star terminal

A Nest thermostat’s star terminal or O/B terminal can be used to control several different system functions, depending on what type of system you have. For example, you can use the star terminal 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 terminal if the wiring diagram tells you to.
• If the app tells you to put a wire into the star terminal 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 terminal, so only one function is supported at a time.
• If you upgrade to a new thermostat, you'll be asked what equipment is connected to the  terminal. If you're unsure, exit the setup flow and check your previous thermostat's wiring by going to Devices  your device's tile  Settings   Thermostat  Wiring.
  • You can also check the wiring from the thermostat by going to Settings Equipment.

Important: If you’ve connected a humidifier, dehumidifier, or multi-speed fan wire to the star terminal, we strongly recommend that you get help from a pro. Make an appointment with our installation partner OnTech to have a trained pro come to your home to install and set up your Nest products.

Differences between Nest thermostat star  terminals

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

Nest Learning Thermostat (2nd, 3rd, and 4th gen) Nest Learning Thermostats have a standalone star  terminal 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 terminal 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