nest Learning thermostat Installation And Configuration Manual page 22

Humidifiers and Dehumidifiers
The 2nd generation Nest Learning Thermostat supports whole-
home humidifiers and dehumidifiers. There are multiple Pro Setup
settings for the * wire to control a humidifier or dehumidifier. All
wire Types include a Fan setting to specify whether or not the
fan should be activated when humidification or
dehumidification is engaged.
Humidifier
Pro Setup will show an Activate setting to specify whether heat
needs to be activated for humidification to turn on. Bypass
humidifiers should use the Hum. + heat option while steam,
independent, or on-demand humidifiers should use the Hum. only
option.
Dehum with A/C
Also known as AC integrated dehumidifiers, these systems require cooling to be activated to turn on
dehumidification.
Dehum Standalone
These dehumidifiers can be engaged independently and do not require heating or cooling to be active. Pro
Setup will show a To dehum. setting that specifies whether the * wire should be energized or not to turn on
dehumidification.
Two Stage Heat Pumps
The 2nd generation Nest Learning Thermostat will use System Match
to recognize your system as a two-stage heat pump if you've
connected Y1, Y2, and O/B wires. Nest assumes that your Y1 wire
will be used to activate 1st stage heating and cooling for your
heat pump.
Using Pro Setup, you can configure the Y2 wire to be used for
2nd stage cooling and heating, 2nd stage cooling only, or 2nd
stage heating only as well as specify the delivery mechanism
(such as forced air or radiant).
Nest Learning Thermostat installation & configuration guide Page 20