Article Summary:
The 2nd and 3rd Nest Learning Thermostats include a * connector that can be used for a few different functions: stage 3 heating, emergency heat, humidification, or dehumidification.

With the 2nd and 3rd generation Nest Learning Thermostats, we’ve added support for the following heating and cooling system functions:

HVAC wire

Function

W₃

Stage 3 heating

E

Emergency heat

HUM

Whole-home humidification

DEHUM

Whole-home dehumidification

Important note: The Nest Thermostat connector can only support one of these functions. As an example, if you have emergency heat and whole-home humidification, you’ll need to select only one in order for your system to be compatible with the Nest Thermostat.

When you put a wire in the connector, the Nest Thermostat will ask which of the above functions the wire supports. What we ask depends on what type of system you have.

If you have a conventional system, the Nest Thermostat will ask you if the wire in the connector controls humidification, dehumidification, or stage 3 heating. If you have a heat pump or dual fuel system, the Nest Thermostat will ask you if the wire in the connector controls humidification, dehumidification, or emergency heat.

Conventional heating and cooling

Heat pump

For systems that have stage 3 heating or emergency heat, you’ll be able to configure the Nest Thermostat yourself. However, if you’ve connected a humidifier or dehumidifier to the connector, we strongly recommend that you get help from a professional installer.